Digestion of proteins. General pathways of amino acids transformation.

 

Proteins are essential nutrients for the human body. They are one of the building blocks of body tissue, and can also serve as a fuel source. As fuel, proteins contain 4 kcal per gram, just like carbohydrates and unlike lipids, which contain 9 kcal per gram.

Proteins are polymer chains made of amino acids linked together by peptide bonds. In nutrition, proteins are broken down in the stomach during digestion by enzymes known as proteases into smaller polypeptides to provide amino acids for the body, including the essential amino acids that cannot be biosynthesized by the body itself.

Amino acids can be divided into three categories: essential amino acids, non-essential amino acids and conditional amino acids. Essential amino acids cannot be made by the body, and must be supplied by food. Non-essential amino acids are made by the body from essential amino acids or in the normal breakdown of proteins. Conditional amino acids are usually not essential, except in times of illness, stress or for someone challenged with a lifelong medical condition.

Essential amino acids are leucine, isoleucine, valine, lysine, threonine, tryptophan, methionine, phenylalanine and histidine. Non-essential amino acids include alanine, asparagine, aspartic acid and glutamic acid. Conditional amino acids include arginine, cysteine, glutamine, glycine, proline, serine, and tyrosine.

Amino acids are found in animal sources such as meats, milk, fish and eggs, as well as in plant sources such as whole grains, pulses, legumes, soy, fruits, nuts and seeds. Vegetarians and vegans can get enough essential amino acids by eating a variety of plant proteins.

Protein functions in body

Protein is a nutrient needed by the human body for growth and maintenance. Aside from water, proteins are the most abundant kind of molecules in the body. Protein can be found in all cells of the body and is the major structural component of all cells in the body, especially muscle. This also includes body organs, hair and skin. Proteins also are utilized in membranes, such as glycoproteins. When broken down into amino acids, they are used as precursors to nucleic acid, co-enzymes, hormones, immune response, cellular repair and molecules essential for life. Finally, protein is needed to form blood cells.

Proteins are very important molecules in our cells. They are involved in virtually all cell functions. Each protein within the body has a specific function. Some proteins are involved in structural support, while others are involved in bodily movement, or in defense against germs. Proteins vary in structure as well as function. They are constructed from a set of 20 amino acids and have distinct three-dimensional shapes. Below is a list of several types of proteins and their functions.

Antibodies - are specialized proteins involved in defending the body from antigens (foreign invaders). They can travel through the blood stream and are utilized by the immune system to identify and defend against bacteria, viruses, and other foreign intruders. One way antibodies counteract antigens is by immobilizing them so that they can be destroyed by white blood cells.

Contractile Proteins - are responsible for movement. Examples include actin and myosin. These proteins are involved in muscle contraction and movement.

Enzymes - are proteins that facilitate biochemical reactions. They are often referred to as catalysts because they speed up chemical reactions. Examples include the enzymes lactase and pepsin. Lactase breaks down the sugar lactose found in milk. Pepsin is a digestive enzyme that works in the stomach to break down proteins in food.

Hormonal Proteins - are messenger proteins which help to coordinate certain bodily activities. Examples include insulin, oxytocin, and somatotropin. Insulin regulates glucose metabolism by controlling the blood-sugar concentration. Oxytocin stimulates contractions in females during childbirth. Somatotropin is a growth hormone that stimulates protein production in muscle cells.

Structural Proteins - are fibrous and stringy and provide support. Examples include keratin, collagen, and elastin. Keratins strengthen protective coverings such as hair, quills, feathers, horns, and beaks. Collagens and elastin provide support for connective tissues such as tendons and ligaments.

Storage Proteins - store amino acids. Examples include ovalbumin and casein. Ovalbumin is found in egg whites and casein is a milk-based protein.

Transport Proteins - are carrier proteins which move molecules from one place to another around the body. Examples include hemoglobin and cytochromes. Hemoglobin transports oxygen through the blood. Cytochromes operate in the electron transport chain as electron carrier proteins.

Protein function in exercise

Proteins are one of the key nutrients for success in terms of sports. Amino acids, the building blocks of proteins, are used for building tissue, including muscle, as well as repairing damaged tissues. Proteins usually only provide a small source of fuel for the exercising muscles, being used as fuel typically only when carbohydrates and lipid resources are low.

Sources

Animal sources of protein.

 

A wide range of foods are a source of protein. The best combination of protein sources depends on the region of the world, access, cost, amino acid types and nutrition balance, as well as acquired tastes. Some foods are high in certain amino acids, but their digestibility and the anti-nutritional factors present in these foods make them of limited value in human nutrition. Therefore, one must consider digestibility and secondary nutrition profile such as calories, cholesterol, vitamins and essential mineral density of the protein source. On a worldwide basis, plant protein foods contribute over 60 percent of the per capita supply of protein, on average. In North America, animal-derived foods contribute about 70 percent of protein sources.

Meat, eggs and fish are sources of complete protein. Milk and milk-derived foods are also good sources of protein.

Whole grains and cereals are another source of proteins. However, these tend to be limiting in the amino acid lysine or threonine, which are available in other vegetarian sources and meats. Examples of food staples and cereal sources of protein, each with a concentration greater than 7 percent, are (in no particular order) buckwheat, oats, rye, millet, maize (corn), rice, wheat, spaghetti, bulgar, sorghum, amaranth, and quinoa.

Vegetarian sources of proteins include legumes, nuts, seeds and fruits. Legumes, some of which are called pulses in certain parts of the world, have higher concentrations of amino acids and are more complete sources of protein than whole grains and cereals. Examples of vegetarian foods with protein concentrations greater than 7 percent include soybeans, lentils, kidney beans, white beans, mung beans, chickpeas, cowpeas, lima beans, pigeon peas, lupines, wing beans, almonds, Brazil nuts, cashews, pecans, walnuts, cotton seeds, pumpkin seeds, sesame seeds, and sunflower seeds.

Plant sources of protein.

 

Food staples that are poor sources of protein include roots and tubers such as yams, cassava and sweet potato. Plantains, another major staple, are also a poor source of essential amino acids. Fruits, while rich in other essential nutrients, are another poor source of amino acids per 100 gram consumed. The protein content in roots, tubers and fruits is between 0 and 2 percent. Food staples with low protein content must be complemented with foods with complete, quality protein content for a healthy life, particularly in children for proper development.

A good source of protein is often a combination of various foods, because different foods are rich in different amino acids. A good source of dietary protein meets two requirements:

                    The requirement for the nutritionally indispensable amino acids (histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine) under all conditions and for conditionally indispensable amino acids (cystine, tyrosine, taurine, glycine, arginine, glutamine, proline) under specific physiological and pathological conditions

                    The requirement for nonspecific nitrogen for the synthesis of the nutritionally dispensable amino acids (aspartic acid, asparagine, glutamic acid, alanine, serine) and other physiologically important nitrogen-containing compounds such as nucleic acids, creatine, and porphyrins.

Healthy people eating a balanced diet rarely need protein supplements. Except for a few amino acids, most are readily available in human diet. The limiting amino acids are lysine, threonine, tryptophan and sulfur-containing amino acids.

The table below presents the most important food groups as protein sources, from a worldwide perspective. It also lists their respective performance as source of the commonly limiting amino acids, in milligrams of limiting amino acid per gram of total protein in the food source. The green highlighted cells represent the protein source with highest density of respective amino acid, while the yellow highlighted cells represent the protein source with lowest density of respective amino acid. The table reiterates the need for a balanced mix of foods to ensure adequate amino acid source.

Protein milkshakes, made from protein powder (center) and milk (left), are a common bodybuilding supplement.

Protein powders such as casein, whey, egg, rice and soy are processed and manufactured sources of protein. These protein powders may provide an additional source of protein for bodybuilders. The type of protein is important in terms of its influence on protein metabolic response and possibly on the muscle's exercise performance. The different physical and/or chemical properties within the various types of protein may affect the rate of protein digestion. As a result, the amino acid availability and the accumulation of tissue protein is altered because of the various protein metabolic responses.

Protein quality

Different proteins have different levels of biological availability (BA) to the human body. Many methods have been introduced to measure protein utilization and retention rates in humans. They include biological value, net protein utilization, and PDCAAS (Protein Digestibility Corrected Amino Acids Score) which was developed by the FDA as an improvement over the Protein efficiency ratio (PER) method. These methods examine which proteins are most efficiently used by the body. The PDCAAS rating is a fairly recent evaluation method; it was adopted by the US Food and Drug Administration (FDA) and the Food and Agricultural Organization of the United Nations/World Health Organization (FAO/WHO) in 1993 as "the preferred 'best'" method to determine protein quality. These organizations have suggested that other methods for evaluating the quality of protein are inferior.

Dietary requirements

An education campaign launched by the United States Department of Agriculture about 100 years ago, on cottage cheese as a lower-cost protein substitute for meat.

Considerable debate has taken place regarding issues surrounding protein intake requirements. The amount of protein required in a person's diet is determined in large part by overall energy intake, the body's need for nitrogen and essential amino acids, body weight and composition, rate of growth in the individual, physical activity level, individual's energy and carbohydrate intake, as well as the presence of illness or injury. Physical activity and exertion as well as enhanced muscular mass increase the need for protein. Requirements are also greater during childhood for growth and development, during pregnancy or when breast-feeding in order to nourish a baby, or when the body needs to recover from malnutrition or trauma or after an operation.

If enough energy is not taken in through diet, as in the process of starvation, the body will use protein from the muscle mass to meet its energy needs, leading to muscle wasting over time. If the individual does not consume adequate protein in nutrition, then muscle will also waste as more vital cellular processes (e.g. respiration enzymes, blood cells) recycle muscle protein for their own requirements.

According to US & Canadian Dietary Reference Intake guidelines, women aged 1970 need to consume 46 grams of protein per day, while men aged 1970 need to consume 56 grams of protein per day to avoid a deficiency. The American and Canadian guidelines recommend a daily protein dietary allowance, measured as intake per kilogram body weight, is 0.8 g/kg. However, this recommendation is based on structural requirements, but disregards use of protein for energy metabolism. This requirement is for a normal sedentary person.

Several studies have concluded that active people and athletes may require elevated protein intake (compared to 0.8 g/kg) due to increase in muscle mass and sweat losses, as well as need for body repair and energy source. Suggested amounts vary between 1.6 g/kg and 1.8 g/kg, while a proposed maximum daily protein intake would be approximately 25% of energy requirements i.e. approximately 2 to 2.5 g/kg. However, many questions still remain to be resolved.

The result of limited synthesis and normal rates of protein degradation is that the balance of nitrogen intake and nitrogen excretion is rapidly and significantly altered. Normal, healthy adults are generally in nitrogen balance, with intake and excretion being very well matched. Young growing children, adults recovering from major illness, and pregnant women are often in positive nitrogen balance. Their intake of nitrogen exceeds their loss as net protein synthesis proceeds. When more nitrogen is excreted than is incorporated into the body, an individual is in negative nitrogen balance. Insufficient quantities of even one essential amino acid is adequate to turn an otherwise normal individual into one with a negative nitrogen balance. The biological value of dietary proteins is related to the extent to which they provide all the necessary amino acids. Proteins of animal origin generally have a high biological value; plant proteins have a wide range of values from almost none to quite high. In general, plant proteins are deficient in lysine, methionine, and tryptophan and are much less concentrated and less digestible than animal proteins. The absence of lysine in low-grade cereal proteins, used as a dietary mainstay in many underdeveloped countries, leads to an inability to synthesize protein (because of missing essential amino acids) and ultimately to a syndrome known as kwashiorkor, common among children in these countries.

 

Protein deficiency

 

A child in Africa suffering from kwashiorkor one of the three protein energy malnutrition ailments afflicting over 10 million children in developing countries.

Protein deficiency and malnutrition can lead to variety of ailments including mental retardation and kwashiorkor. Symptoms of kwashiorkor include apathy, diarrhea, inactivity, failure to grow, flaky skin, fatty liver, and edema of the belly and legs. This edema is explained by the action of lipoxygenase on arachidonic acid to form leukotrienes and the normal functioning of proteins in fluid balance and lipoprotein transport.

Although protein energy malnutrition is more common in low-income countries, children from higher-income countries are also affected, including children from large urban areas in low socioeconomic neighborhoods. This may also occur in children with chronic diseases, and children who are institutionalized or hospitalized for a different diagnosis. Risk factors include a primary diagnosis of mental retardation, cystic fibrosis, malignancy, cardiovascular disease, end stage renal disease, oncologic disease, genetic disease, neurological disease, multiple diagnoses, or prolonged hospitalization. In these conditions, the challenging nutritional management may get overlooked and underestimated, resulting in an impairment of the chances for recovery and the worsening of the situation.

Deficiency of protein leads to following:

1. Shortage of protein leads to retardation of growth and in extreme cases failure of growth. This is manifested as marasmus and kwashiorkor among infants and children.

2. Protein deficiency affects the intestinal mucosa and the gland that secret digestive enzymes. This results in the failure to digest and absorb the food, consequently leading to diarrhea and loss of fluid and electrolyte.

3. The normal structure and function of liver is disturbed leading fat accumulation and fatty livers. Liver fails to synthesis plasma albumin thus leading to Oedema.

4. Muscle wasting and anemia due to the shortage of hemoglobin are common feature due to the deficiency of protein.

5. In case there is a deficiency of protein in life, the possibility of mental malfunction increases.

6. The amino acids presents in the protein help in tissue synthesis during growth period e.g. infancy childhood and adolescence. The body goes into negative N2 -balance due to the shortage of protein in the diet. This results in muscle wastage

7. Proteins from important constituents of hormones. How-ever the deficiency of proteins leads to no marked and characteristic changes in the functioning of endocrine glands.

8. Proteins furnish 10-12per cent of calories required daily. However the major part of proteins is essentially for body-building purposes only.

 

 

Nitrogen balance

The nitrogen balance index (NBI) is used to evaluate the amount of protein used by the body in comparison with the amount of protein supplied from daily food intake. The body is in the state of nitrogen (or protein) equilibrium when the intake and usage of protein is equal. The body has a positive nitrogen balance when the intake of protein is greater than that expended by the body. In this case, the body can build and develop new tissue. Since the body does not store protein, the overconsumption of protein can result in the excess amount to be converted into fat and stored as adipose tissue.

Blood urea nitrogen can be used in estimating nitrogen balance.

A positive value is often found during periods of growth, tissue repair or pregnancy.

 

This means that the intake of nitrogen into the body is greater than the loss of nitrogen from the body, so there is an increase in the total body pool of protein.

The body has a negative nitrogen balance when the intake of protein is less than that expended by the body. In this case, protein intake is less than required, and the body cannot maintain or build new tissues.

A negative nitrogen balance represents a state of protein deficiency, in which the body is breaking down tissues faster than they are being replaced. The ingestion of insufficient amounts of protein, or food with poor protein quality, can result in serious medical conditions in which an individual's overall health is compromised. The immune system is severely affected; the amount of blood plasma decreases, leading to medical conditions such as anemia or edema; and the body becomes vulnerable to infectious diseases and other serious conditions. Protein malnutrition in infants is called kwashiorkor, and it poses a major health problem in developing countries, such as Africa, Central and South America, and certain parts of Asia. An infant with kwashiorkor suffers from poor muscle and tissue development, loss of appetite, mottled skin, patchy hair, diarrhea, edema, and, eventually, death (similar symptoms are present in adults with protein deficiency). Treatment or prevention of this condition lies in adequate consumption of protein-rich foods.

A negative value can be associated with burns, fevers, wasting diseases and other serious injuries and during periods of fasting. This means that the amount of nitrogen excreted from the body is greater than the amount of nitrogen ingested.

It can be used in the evaluation of malnutrition.

The difference between the total nitrogen intake by an organism and its total nitrogen loss. A normal, healthy adult has a zero nitrogen balance.

 

Essential and nonessential amino acids.

 

THE TWENTY AMINO ACIDS

 

Name

Abbreviation

Linear structure formula (atom composition and bonding)

SOURCE: Institute for Chemistry

Alanine

ala

CH3-CH(NH2)-COOH

Arginine

arg

HN=C(NH2)-NH-(CH2)3-CH(NH2)-COOH

Asparagine

asn

H2N-CO-CH2-CH(NH2)-COOH

Aspartic acid

asp

HOOC-CH2-CH(NH2)-COOH

Cysteine

cys

HS-CH2-CH(NH2)-COOH

Glutamine

gln

H2N-CO-(CH2)2-CH(NH2)-COOH

Glutamic acid

glu

HOOC-(CH2)2-CH(NH2)-COOH

Glycine

gly

NH2-CH2-COOH

Histidine

his

NH-CH=N-CH=C-CH2-CH(NH2)-COOH |____________|

Isoleucine

ile

CH3-CH2-CH(CH3)-CH(NH2)-COOH

Leucine

leu

(CH3)2-CH-CH2-CH(NH2)-COOH

Lysine

lys

H2N-(CH2)4-CH(NH2)-COOH

Methionine

met

CH3-S-(CH2)2-CH(NH2)-COOH

Phenylalanine

phe

Ph-CH2-CH(NH2)-COOH

Proline

pro

NH-(CH2)3-CH-COOH |__________|

Serine

ser

HO-CH2-CH(NH2)-COOH

Threonine

thr

CH3-CH(OH)-CH(NH2)-COOH

Tryptophan

trp

Ph-NH-CH=C-CH2-CH(NH2)-COOH |_________|

Tyrosine

tyr

HO-Ph-CH2-CH(NH2)-COOH

Valine

val

(CH3)2-CH-CH(NH2)-COOH

consist of a sufficient and balanced supply of both essential and nonessential amino acids in order to ensure high levels of protein production.

Essential vs. Nonessential Amino Acids

Nonessential

Essential

Alanine

Arginine*

Asparagine

Histidine

Aspartate

Isoleucine

Cysteine

Leucine

Glutamate

Lysine

Glutamine

Methionine*

Glycine

Phenylalanine*

Proline

Threonine

Serine

Tryptophan

Tyrosine

Valine

The amino acids arginine, methionine and phenylalanine are considered essential for reasons not directly related to lack of synthesis. Arginine is synthesized by mammalian cells but at a rate that is insufficient to meet the growth needs of the body and the majority that is synthesized is cleaved to form urea. Methionine is required in large amounts to produce cysteine if the latter amino acid is not adequately supplied in the diet. Similarly, phenyalanine is needed in large amounts to form tyrosine if the latter is not adequately supplied in the diet.

The quality of protein depends on the level at which it provides the nutritional amounts of essential amino acids needed for overall body health, maintenance, and growth. Animal proteins, such as eggs, cheese, milk, meat, and fish, are considered high-quality, or complete, proteins because they provide sufficient amounts of the essential amino acids. Plant proteins, such as grain, corn, nuts, vegetables and fruits, are lower-quality, or incomplete, proteins because many plant proteins lack one or more of the essential amino acids, or because they lack a proper balance of amino acids. Incomplete proteins can, however, be combined to provide all the essential amino acids, though combinations of incomplete proteins must be consumed at the same time, or within a short period of time (within four hours), to obtain the maximum nutritive value from the amino acids. Such combination diets generally yield a high-quality protein meal, providing sufficient amounts and proper balance of the essential amino acids needed by the body to function.

Digestion of proteins in stomach and small intestine.

Breakdown of Food

Breakdown of Food

 

Digestion in the stomach

 

Chemical digestion begins in the stomach. The stomach is a large, hollow, pouched-shaped muscular organ. Food in the stomach is broken down by the action of gastric juice, which contains hydrochloric acid and pepsin (an enzyme that digests protein). The stomach begins its production of gastric juice while food is still in the mouth. Nerves from the cheeks and tongue are stimulated and send messages to the brain. The brain in turn sends messages to nerves in the stomach wall, stimulating the secretion of gastric juice before the arrival of food. The second signal for gastric juice production occurs when food arrives in the stomach and touches the lining.

Gastric juice is secreted from the linings of the stomach walls, along with mucus that helps to protect the stomach lining from the action of the acid. Three layers of powerful stomach muscles churn food into a thick liquid called chyme (pronounced KIME). From time to time, chyme is passed through the pyloric sphincter, the opening between the stomach and the small intestine.

Protein digestion begins when the food reaches the stomach and stimulates the release of hydrochloric acid (HCl) by the parietal cells located in the gastric mucosa of the GI (gastrointestinal) tract. Hydrochloric acid provides for a very acidic environment, which helps the protein digestion process in two ways:

(1)  through an acid-catalyzed hydrolysis reaction of breaking peptide bonds (the chemical process of breaking peptide bonds is referred to as a hydrolysis reaction because water is used to break the bonds); and (2) through conversion of the gastric enzyme pepsinogen (an inactive precursor) to pepsin (the active form). Pepsinogen is stored and secreted by the "chief cells" that line the stomach wall. Once converted into the active form, pepsin attacks the peptide bonds that link amino acids together, breaking the long polypeptide chain into shorter segments of amino acids known as dipeptides and tripeptides.

File:Protein digestion.PNG

These protein fragments are then further broken down in the duodenum of the small intestines.

(2)  The brush border enzymes, which work on the surface of epithelial cells of the small intestines, hydrolyze the protein fragments into amino acids.

Digestion and absorption in the small intestine

The small intestine is a long, narrow tube running from the stomach to the large intestine. The small intestine is greatly coiled and twisted. Its full length is about 20 feet (6 meters). The small intestine is subdivided into three sections: the duodenum (pronounced do-o-DEE-num), the jejunum (pronounced je-JOO-num), and the ileum (pronounced ILL-ee-um).The cells of the small intestine actively absorb the amino acids through a process that requires energy.

The duodenum is about 10 inches (25 centimeters) long and connects with the lower portion of the stomach. When chyme reaches the duodenum, it is further broken down by intestinal juices and through the action of the pancreas and gall bladder. The pancreas is a large gland located below the stomach that secretes pancreatic juice into the duodenum through the pancreatic duct. There are three enzymes in pancreatic juice that break down carbohydrates, fats, and proteins. The gall bladder, located next to the liver, stores bile produced by the liver. While bile does not contain enzymes, it contains bile salts that help to dissolve fats. The gall bladder empties bile into the duodenum when chyme enters that portion of the intestine.

The amino acids travel through the hepatic portal vein to the liver, where the nutrients are processed into glucose or fat (or released into the bloodstream). The tissues in the body take up the amino acids rapidly for glucose production, growth and maintenance, and other vital cellular functioning. For the most part, the body does not store protein, as the metabolism of amino acids occurs within a few hours.

Amino acids are metabolized in the liver into useful forms that are used as building blocks of protein in tissues. The body may utilize the amino acids for either anabolic or catabolic reactions. Anabolism refers to the chemical process through which digested and absorbed products are used to effectively build or repair bodily tissues, or to restore vital substances broken down through metabolism. Catabolism, on the other hand, is the process that results in the release of energy through the breakdown of nutrients, stored materials, and cellular substances. Anabolic and catabolic reactions work hand-in-hand, and the energy produced in catabolic processes is used to fuel essential anabolic processes. The vital biochemical reaction of glycolysis (in which glucose is oxidized to produce carbon dioxide, water, and cellular energy) in the form of adenosine triphosphate, or ATP, is a prime example of a catabolic reaction. The energy released, as ATP, from such a reaction is used to fuel important anabolic processes, such as protein synthesis.

The chyme first enters duodenum, which is a part of small intestine. It is a C-shaped structure about 25 centimeters long. The chyme is very acidic but here it mixes with an alkaline secretion and becomes neutral. Pancreas secrete digestive enzyme, trypsin and chymotrypsin, which reach the duodenum through bloodstream and aid in the breakdown of proteins. They break the complex protein molecules into its constituents, amino acids. They accomplish this task of breaking down by hydrolysis, described above. The walls of the small intestine are covered with numerous finger like projections, known as villi. They increase the surface area of the small intestine by about 600 times. Each villus contains a network of blood capillaries and lymph vessels. The amino acids pass through the capillary walls, and get carried away by the blood flowing through the network. In this manner, the amino acids thus produced get absorbed, reach different body parts and finally get converted to human proteins. The human body uses proteins for building and maintaining its structures, sometimes for energy generation as well.

Trypsin is a serine protease found in the digestive system of many vertebrates, where it hydrolyses proteins. Trypsin is produced in the pancreas as the inactive proenzyme trypsinogen. Trypsin cleaves peptide chains mainly at the carboxyl side of the amino acids lysine or arginine, except when either is followed by proline. It is used for numerous biotechnological processes. The process is commonly referred to as trypsin proteolysis or trypsinisation, and proteins that have been digested/treated with trypsin are said to have been trypsinized.

Function

In the duodenum, trypsin catalyzes the hydrolysis of peptide bonds, breaking down proteins into smaller peptides. The peptide products are then further hydrolyzed into amino acids via other proteases, rendering them available for absorption into the blood stream. Tryptic digestion is a necessary step in protein absorption as proteins are generally too large to be absorbed through the lining of the small intestine.

Trypsin is produced in the pancreas, in the form of the inactive zymogen trypsinogen. When the pancreas is stimulated by cholecystokinin, it is then secreted into the first part of the small intestine (the duodenum) via the pancreatic duct. Once in the small intestine, the enzyme enteropeptidase activates it into trypsin by proteolytic cleavage. Auto catalysis does not happen with trypsin since trypsinogen is a poor substrate for trypsin. This activation mechanism is common for most serine proteases, and serves to prevent autodegradation of the pancreas.

Mechanism

The enzymatic mechanism is similar to that of other serine proteases. These enzymes contain a catalytic triad consisting of histidine-57, aspartate-102, and serine-195. These three residues form a charge relay that serves to make the active site serine nucleophilic. This is achieved by modifying the electrostatic environment of the serine. The enzymatic reaction that trypsins catalyze is thermodynamically favorable but requires significant activation energy (it is "kinetically unfavorable"). In addition, trypsin contains an "oxyanion hole" formed by the backbone amide hydrogen atoms of Gly-193 and Ser-195, which serves to stabilize the developing negative charge on the carbonyl oxygen atom of the cleaved amides.

The aspartate residue (Asp 189) located in the catalytic pocket (S1) of trypsins is responsible for attracting and stabilizing positively charged lysine and/or arginine, and is, thus, responsible for the specificity of the enzyme. This means that trypsin predominantly cleaves proteins at the carboxyl side (or "C-terminal side") of the amino acids lysine and arginine except when either is bound to a C-terminal proline., although large-scale mass spectrometry data suggest cleavage occurs even with proline. Trypsins are considered endopeptidases, i.e., the cleavage occurs within the polypeptide chain rather than at the terminal amino acids located at the ends of polypeptides.

Properties

Trypsins has an optimal operating pH of about 7.5-8.5 and optimal operating temperature of about 37C.

The activity of trypsins is not affected by the inhibitor tosyl phenylalanyl chloromethyl ketone, TPCK, which deactivates chymotrypsin. This is important because, in some applications, like mass spectrometry, the specificity of cleavage is important.

Trypsins should be stored at very cold temperatures (between −20C and −80C) to prevent autolysis, which may also be impeded by storage of trypsins at pH 3 or by using trypsin modified by reductive methylation. When the pH is adjusted back to pH 8, activity returns.

Clinical significance

Activation of trypsin from proteolytic cleavage of trypsinogen in the pancreas can lead to a series of events that cause pancreatic self-digestion, resulting in pancreatitis. One consequence of the autosomal recessive disease cystic fibrosis is a deficiency in transport of trypsin and other digestive enzymes from the pancreas. This leads to the disorder termed meconium ileus. This disorder involves intestinal obstruction (ileus) due to overly thick meconium, which is normally broken down by trypsins and other proteases, then passed in faeces.

Trypsin is available in high quantity in pancreases, and can be purified rather easily. Hence it has been used widely in various biotechnological processes.

In a tissue culture lab, trypsins are used to re-suspend cells adherent to the cell culture dish wall during the process of harvesting cells. Some cell types have a tendency to "stick" - or adhere - to the sides and bottom of a dish when cultivated in vitro. Trypsin is used to cleave proteins bonding the cultured cells to the dish, so that the cells can be suspended in fresh solution and transferred to fresh dishes.

Trypsin can also be used to dissociate dissected cells (for example, prior to cell fixing and sorting).

Trypsins can be used to break down casein in breast milk. If trypsin is added to a solution of milk powder, the breakdown of casein will cause the milk to become translucent. The rate of reaction can be measured by using the amount of time it takes for the milk to turn translucent.

Trypsin is commonly used in biological research during proteomics experiments to digest proteins into peptides for mass spectrometry analysis, e.g. in-gel digestion. Trypsin is particularly suited for this, since it has a very well defined specificity, as it hydrolyzes only the peptide bonds in which the carbonyl group is contributed either by an Arg or Lys residue.

Trypsin can also be used to dissolve blood clots in its microbial form and treat inflammation in its pancreatic form.

 

Rotting of proteins in a large intestine, products.

 

Large Intestine Overview

The large intestine performs the following functions:

- reabsorbs water and maintains the fluid balance of the body

                     - absorbs certain vitamins

                     - processes undigested material (fibre)

                     - stores waste before it is eliminated.

The large intestine is wider and heavier than the small intestine. However, it is much shorteronly about 5 feet (1.5 meters) long. It rises up on the right side of the body (the ascending colon), crosses over to the other side underneath the stomach (the transverse colon), descends on the left side, (the descending colon), then forms an s-shape (the sigmoid colon) before reaching the rectum and anus. The muscular rectum, about 6 inches (16 centimeters) long, expels feces (stool) through the anus, which has a large muscular sphincter that controls the passage of waste matter.

The large intestine removes water from the waste products of digestion and returns some of it to the bloodstream. Fecal matter contains undigested food, bacteria, and cells from the walls of the digestive tract. Millions of bacteria in the large intestine help to produce certain B vitamins and vitamin K. These vitamins are absorbed into the bloodstream along with the water.

Caecum

The caecum is the first part of the large intestine. Shaped like a small pouch and located in the right lower abdomen, it is the connection between the small intestine and the colon.

The caecum accepts and stores processed material from the small intestine and moves it towards the colon. As the processed food approaches the end of the small intestine, a valve separating the small and large intestines opens, the caecum expands and the material enters. At this stage, the mixture normally contains:

                     undigested food (fibre)

                     a little bit of water

                     some vitamins

                     some minerals or salts

The metabolism of amino acids can be understood from the dynamic catabolic and anabolic processes. In the process referred to as deamination, the nitrogen-containing amino group (NH2) is cleaved from the amino acid unit. In this reaction, which requires vitamin B6 as a cofactor, the amino group is transferred to an acceptor keto-acid, which can form a new amino acid. Through this process, the body is able to make the nonessential amino acids not provided by one's diet. The keto-acid intermediate can also be used to synthesize glucose to ultimately yield energy for the body, and the cleaved nitrogen-containing group is transformed into urea, a waste product, and excreted as urine.

Proteins are vital to basic cellular and body functions, including cellular regeneration and repair, tissue maintenance and regulation, hormone and enzyme production, fluid balance, and the provision of energy.

THE KINDS OF GASTRIC JUICE ACIDITY.

Gastric acid is a digestive fluid, formed in the stomach. It has a pH of 1.5 to 3.5 and is composed of hydrochloric acid (HCl) (around 0.5%, or 5000 parts per million) as high as 0.1 N, and large quantities of potassium chloride (KCl) and sodium chloride (NaCl). The acid plays a key role in digestion of proteins, by activating digestive enzymes, and making ingested proteins unravel so that digestive enzymes break down the long chains of amino acids.

Gastric acid is produced by cells lining the stomach, which are coupled to systems to increase acid production when needed. Other cells in the stomach produce bicarbonate, a base, to buffer the fluid, ensuring that it does not become too acidic. These cells also produce mucus, which forms a viscous physical barrier to prevent gastric acid from damaging the stomach. Cells in the beginning of the small intestine, or duodenum, further produce large amounts of bicarbonate to completely neutralize any gastric acid that passes further down into the digestive tract.

Gastric acid is produced by parietal cells (also called oxyntic cells) in the stomach. Its secretion is a complex and relatively energetically expensive process. Parietal cells contain an extensive secretory network (called canaliculi) from which the gastric acid is secreted into the lumen of the stomach. These cells are part of epithelial fundic glands in the gastric mucosa. The pH of gastric acid is 1.35 to 3.5 [2] in the human stomach lumen, the acidity being maintained by the proton pump H+/K+ ATPase. The parietal cell releases bicarbonate into the blood stream in the process, which causes a temporary rise of pH in the blood, known as alkaline tide.

The resulting highly acidic environment in the stomach lumen causes proteins from food to lose their characteristic folded structure (or denature). This exposes the protein's peptide bonds. The chief cells of the stomach secrete enzymes for protein breakdown (inactive pepsinogen and rennin). Hydrochloric acid activates pepsinogen into the enzyme pepsin, which then helps digestion by breaking the bonds linking amino acids, a process known as proteolysis. In addition, many microorganisms have their growth inhibited by such an acidic environment, which is helpful to prevent infection.

Secretion

There are three phases in the secretion of gastric acid:

1.                The basal phase: A small amount of acid is always being secreted into the stomach. The three following phases increase the secretion rate in order to digest a meal.

2.                The cephalic phase: Thirty percent of the total gastric acid secretions to be produced is stimulated by anticipation of eating and the smell or taste of food. This signalling occurs from higher centres in the brain through the Vagus Nerve. It activates parietal cells to release acid and ECL cells to release histamine. The Vagus nerve also releases Gastrin Releasing Peptide onto G cells. Finally, it also inhibits somatostatin release from D cells.

3.                The gastric phase: About fifty percent of the total acid for a meal is secreted in this phase. Acid secretion is stimulated by distension of the stomach and by amino acids present in the food.

4.                The intestinal phase: The remaining 10% of acid is secreted when chyme enters the small intestine, and is stimulated by small intestine distension and by amino acids. The duodenal cells release entero-oxyntin which acts on parietal cells without affecting gastrin.

There is also a small continuous basal secretion of gastric acid between meals of usually less than 10 mEq/hour.

Regulation of secretion

Diagram depicting the major determinants of gastric acid secretion, with inclusion of drug targets for peptic ulcer disease (PUD) and gastroesophageal reflux disease (GERD).

Gastric acid production is regulated by both the autonomic nervous system and several hormones. The parasympathetic nervous system, via the vagus nerve, and the hormone gastrin stimulate the parietal cell to produce gastric acid, both directly acting on parietal cells and indirectly, through the stimulation of the secretion of the hormone histamine from enterochromaffine-like cells (ECL). Vasoactive intestinal peptide, cholecystokinin, and secretin all inhibit production.

The production of gastric acid in the stomach is tightly regulated by positive regulators and negative feedback mechanisms. Four types of cells are involved in this process: parietal cells, G cells, D cells and enterochromaffine-like cells. Besides this, the endings of the vagus nerve (CN X) and the intramural nervous plexus in the digestive tract influence the secretion significantly.

Nerve endings in the stomach secrete two stimulatory neurotransmitters: acetylcholine and gastrin-releasing peptide. Their action is both direct on parietal cells and mediated through the secretion of gastrin from G cells and histamine from enterochromaffine-like cells. Gastrin acts on parietal cells directly and indirectly too, by stimulating the release of histamine.

The release of histamine is the most important positive regulation mechanism of the secretion of gastric acid in the stomach. Its release is stimulated by gastrin and acetylcholine and inhibited by somatostatin.

Neutralization

In the duodenum, gastric acid is neutralized by sodium bicarbonate. This also blocks gastric enzymes that have their optima in the acid range of pH. The secretion of sodium bicarbonate from the pancreas is stimulated by secretin. This polypeptide hormone gets activated and secreted from so-called S cells in the mucosa of the duodenum and jejunum when the pH in duodenum falls below 4.5 to 5.0. The neutralization is described by the equation:

HCl + NaHCO3 → NaCl + H2CO3

The carbonic acid rapidly equilibrates with carbon dioxide and water through catalysis by carbonic anhydrase enzymes bound to the gut epithelial lining,[6] leading to a net release of carbon dioxide gas within the lumen associated with neutralisation. In the absorptive upper intestine, such as the duodenum, both the dissolved carbon dioxide and carbonic acid will tend to equilibrate with the blood, leading to most of the gas produced on neutralisation being exhaled through the lungs.

Role in disease

In hypochlorhydria and achlorhydria, there is low or no gastric acid in the stomach, potentially leading to problems as the disinfectant properties of the gastric lumen are decreased. In such conditions, there is greater risk of infections of the digestive tract (such as infection with Vibrio or Helicobacter bacteria).

In ZollingerEllison syndrome and hypercalcemia, there are increased gastrin levels, leading to excess gastric acid production, which can cause gastric ulcers.

In diseases featuring excess vomiting, patients develop hypochloremic metabolic alkalosis (decreased blood acidity by H+ and chlorine depletion).

 

Your stomach is where the food you eat is broken down into smaller pieces. This action is called digestion.

Stomach (gastric) juice is the term used to describe the chemicals that break down food in the stomach. These include hydrochloric acid and an enzyme called pepsin. Gastric juice is sometimes referred to as stomach acid, although not all of the substances in gastric juice are acidic.

 

 

Hypoaciditas, hyperaciditas, anaciditas, hypochlorhydria, hyperchlorhydria, achlorhydria.

 

HYPOCHLORHYDRIA (Obvious) is the lack of adequate production of Hydrochloric Acid (HCL) by the Stomach Parietal Cells.

Many people, in the process of aging, develop various stages of Hypochlorhydria; however, it is not confined to this aging group.  Many young people also develop this problem. Bear in mind that the presence of HCL in the Stomach generally inhibits (slows down or stops) the reflex of rapid-dumping of foods out of the Stomach, rendering the critical First Stage of Digestion partially or totally incomplete. Also, HCL performs a natural sterilization of the foods that we swallow. This is quite important, because nothing that we eat is sterile. In the pre-digestion phase of the Stomach, HCL, Pepsin, certain Enzymes, plus the Intrinsic Factor, which is essential for the absorption of Vitamin B-12, play key rolls in the conversion processes of Proteins to Amino Acids and Starches to Sugars that can be utilized by our bodies (in conjunction with the Duodenal, 2nd Phase of Digestion).

ACHLORHYDRIA

ACHLORHYDRIA is the total absence of HCL production in the stomach.

 Patients with Achlorhydria May Have a form of Pernicious Anemia. This will also show in a routine Blood test. When the Anemia is corrected the Stomachs Parietal Function will generally return to Normal.

HYPERCHLORHYDRIA

HYPERCHLORHYDRIA, The above Graph indicates the excess production of Hydrochloric Acid (HCL). This condition may cause Delayed, or Marked-delayed, emptying time of the Stomach's contents. In many cases, Patients with Delayed and Marked-delayed emptying, will retain food in their Stomachs for 6 to 24 hours, or much longer in many cases.

Absorption of Amino Acids and Peptides

 

Dietary proteins are, with very few exceptions, not absorbed. Rather, they must be digested into amino acids or di- and tripeptides first. In previous sections, we've seen two sources secrete proteolytic enzymes into the lumen of the digestive tube:

                    the stomach secretes pepsinogen, which is converted to the active protease pepsin by the action of acid.

                    the pancreas secretes a group of potent proteases, chief among them trypsin, chymotrypsin and carboxypeptidases.

Through the action of these gastric and pancreatic proteases, dietary proteins are hydrolyzed within the lumen of the small intestine predominantly into medium and small peptides (oligopeptides).

The brush border of the small intestine is equipped with a family of peptidases. Like lactase and maltase, these peptidases are integral membrane proteins rather than soluble enzymes. They function to further the hydrolysis of lumenal peptides, converting them to free amino acids and very small peptides. These endproducts of digestion, formed on the surface of the enterocyte, are ready for absorption.

 

Absorption of Amino Acids

The mechanism by which amino acids are absorbed is conceptually identical to that of monosaccharides. The lumenal plasma membrane of the absorptive cell bears at least four sodium-dependent amino acid transporters - one each for acidic, basic, neutral and amino acids. These transporters bind amino acids only after binding sodium. The fully loaded transporter then undergoes a conformational change that dumps sodium and the amino acid into the cytoplasm, followed by its reorientation back to the original form.

Thus, absorption of amino acids is also absolutely dependent on the electrochemical gradient of sodium across the epithelium. Further, absorption of amino acids, like that of monosaccharides, contributes to generating the osmotic gradient that drives water absorption.

The basolateral membrane of the enterocyte contains additional transporters which export amino acids from the cell into blood. These are not dependent on sodium gradients.

 

Absorption of Peptides

There is virtually no absorption of peptides longer than four amino acids. However, there is abundant absorption of di- and tripeptides in the small intestine. These small peptides are absorbed into the small intestinal epithelial cell by cotransport with H+ ions via a transporter called PepT1.

Once inside the enterocyte, the vast bulk of absorbed di- and tripeptides are digested into amino acids by cytoplasmic peptidases and exported from the cell into blood. Only a very small number of these small peptides enter blood intact.

 

Absorption of Intact Proteins

As emphasized, absorption of intact proteins occurs only in a few circumstances. In the first place, very few proteins get through the gauntlet of soluble and membrane-bound proteases intact. Second, "normal" enterocytes do not have transporters to carry proteins across the plasma membrane and they certainly cannot permeate tight junctions.

One important exception to these general statements is that for a very few days after birth, neonates have the ability to absorb intact proteins. This ability, which is rapidly lost, is of immense importance because it allows the newborn animal to acquire passive immunity by absorbing immunoglobulins in colostral milk.

In constrast to humans and rodents, there is no significant transfer of antibodies across the placenta in many animals (cattle, sheep, horses and pigs to name a few), and the young are born without circulating antibodies. If fed colostrum during the first day or so after birth, they absorb large quantities of immunoglobulins and acquire a temporary immune system that provides protection until they generate their own immune responses.

The small intestine rapidly loses the capacity to absorb intact proteins - a process called closure - and consequently, animals that do not receive colostrum within the first few days after birth will likely die due to opportunistic infections.

Intestinal barrier function regulates transport and host defense mechanisms at the mucosal interface with the outside world. Transcellular and paracellular fluxes are tightly controlled by membrane pumps, ion channels and tight junctions, adapting permeability to physiological needs. Permeability greatly defines the absorbance of compounds in the intestine. In case of high permeability compounds it is unlikely that the absorption will be modified by transporters, yet the test compound might be involved in transporter mediated drug-drug interactions. The absorption of medium and low permeability compounds can be affected by membrane transporters located at the endothelial cells of the intestinal barrier.

In the intestine transporters are localized in the brush border membrane and on the basolateral side of intestinal cells. Four major ABC efflux transporters have been shown to localize at the apical/luminal membrane of enterocytes, and thus are thought to form a barrier to intestinal absorption of substrate drugs.

 

General pathways of amino acids transformation

Deamination of amino acids, it kinds. The role of vitamins in deamination of amino acids.

 

 

Oxidative Deamination Reaction

Deamination is also an oxidative reaction that occurs under aerobic conditions in all tissues but especially the liver. During oxidative deamination, an amino acid is converted into the corresponding keto acid by the removal of the amine functional group as ammonia and the amine functional group is replaced by the ketone group. The ammonia eventually goes into the urea cycle.

Oxidative deamination occurs primarily on glutamic acid because glutamic acid was the end product of many transamination reactions.

The glutamate dehydrogenase is allosterically controlled by ATP and ADP. ATP acts as an inhibitor whereas ADP is an activator.

 

Central Role for Glutamic Acid:

Apparently most amino acids may be deaminated but this is a significant reaction only for glutamic acid. If this is true, then how are the other amino acids deaminated? The answer is that a combination of transamination and deamination of glutamic acid occurs which is a recycling type of reaction for glutamic acid. The original amino acid loses its amine group in the process. The general reaction sequence is shown on the left.

 

Synthesis of New Amino Acids:

The same reaction works in reverse for the synthesis of amino acids. In this situation alpha-ketoglutaric acid first uses transamination of a different amino acid to make glutamic acid, which then reacts with a keto acid to make a new amino acid. In effect, the interconversion of alpha-ketoglutaric acid and glutamic acid lies at the very heart of nitrogen metabolism. These molecules serve as the "collection and receiving agent" for nitrogen. The subsequent fate of the amino group is in new amino acids, any nitrogen bases, or any other nitrogen containing compounds.

 

Transamination of amino acids, mechanism, role of enzymes and coenzymes.

 

Transamination as the name implies, refers to the transfer of an amine group from one molecule to another. This reaction is catalyzed by a family of enzymes called transaminases. Actually, the transamination reaction results in the exchange of an amine group on one acid with a ketone group on another acid. It is analogous to a double replacement reaction.

Pyridoxine is an example of a vitamin which is required for the synthesis of a coenzyme. Other examples which you have met in this course are listed in the table below.

vitamin

cofactor

example enzymes

pyridoxine (vit b6)

pyridoxal phosphate

transaminases

niacin (nicotinamide)

NAD+/ NADP+

malate dehydrogenase

riboflavin (vit b2)

FAD

succinate dehydrogenase

thiamine (vit b1)

TPP

pyruvate dehydrogenase

pantothenate

Coenzyme A

fatty acid metabolism

The most usual and major keto acid involved with transamination reactions is alpha-ketoglutaric acid, an intermediate in the citric acid cycle. A specific example is the transamination of alanine to make pyruvic acid and glutamic acid.

Other amino acids which can be converted after several steps through transamination into pyruvic acid include serine, cysteine, and glycine.

Other Transamination Reactions:

Aspartic acid can be converted into oxaloacetic acid, another intermediate of the citric acid cycle. Other amino acids such as glutamine, histidine, arginine, and proline are first converted into glutamic acid.

Glutamine and asparagine are converted into glutamic acid and aspartic acid by a simple hydrolysis of the amide group.

All of the amino acids can be converted through a variety of reactions and transamination into a keto acid which is a part of or feeds into the citric acid cycle. The interrelationships of amino acids with the citric acid cycle are illustrated in the graphic on the left.

Amino Acids in Overall Metabolism:

Once the keto acids have been formed from the appropriate amino acids by transamination, they may be used for several purposes. The most obvious is the complete metabolism into carbon dioxide and water by the citric acid cycle.

However, if there are excess proteins in the diet those amino acids converted into pyruvic acid and acetyl CoA can be converted into lipids by the lipogenesis process. If carbohydrates are lacking in the diet or if glucose cannot get into the cells (as in diabetes), then those amino acids converted into pyruvic acid and oxaloacetic acids can be converted into glucose or glycogen.

The hormones cortisone and cortisol from the adrenal cortex stimulate the synthesis of glucose from amino acids in the liver and also function as antagonists to insulin

Synthesis of New Amino Acids:

In addition to the catabolic function of transamination reactions, these reactions can also be used to synthesize amino acids needed or not present in the diet. An amino acid may be synthesized if there is an available "root" ketoacid with a synthetic connection to the final amino acid. Since an appropriate "root" keto acid does not exist for eight amino acids, (lys, leu, ile, met, thr, try, val, phe), they are essential and must be included in the diet because they cannot be synthesized.

Glutamic acid usually serves as the source of the amine group in the transamination synthesis of new amino acids. The reverse of the reactions mentioned earlier are the most obvious methods for producing the amino acids alanine and aspartic acid.

Several nonessential amino acids are made by processes other than transamination. Cysteine is made from methionine, and serine and glycine are synthesized from phosphoglyceric acid - an intermediate of glycolysis

 

The correlation between transamination and deamination.

Pathways

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Decarboxylisation of amino acids, role of enzymes and co-enzymes.

Decarboxylation is any chemical reaction in which a carboxyl group (-COOH) is split off from a compound as carbon dioxide (CO2).

Common biosynthetic decarboxylations of amino acids to amines are:

Other decarboxylation reactions from the citric acid cycle include:

to succinyl-CoA.

Enzymes that catalyze decarboxylations are called decarboxylases or, more formally, carboxy-lyases . Carboxy-lyases, also known as decarboxylases, are carbon-carbon lyases that add or remove a carboxyl group from organic compounds. These enzymes catalyze the decarboxylation of amino acids, beta-keto acids and alpha-keto acids

Heating or pyrolysis of Δ9-Tetrahydrocannabinolic acid yields the psychoactive compound

 

Main bioactive amines, their source and role in organism.

 

 


Fig. 1. The formation of biogenic amines with an example of tyramine production.

Biogenic amines are basic nitrogenous low molecular weight compounds with biological activity that may be formed or catabolised during the normal metabolism of animals, plants and micro-organisms. Biogenic amines are derived mainly from amino acids through substrate-specific decarboxylase enzymes. Amines may be formed by yeasts during the alcoholic fermentation (mostly putrazine); by lactic acid bacteria (LAB) during malolactic fermentation (MLF. Biogenic amines can also be present in the must, just as putrescine in grapes is associated with potassium deficiencies in the soil. The main biogenic amines are histamine, tyramine, putrescine, cadaverine and phenylethylamine. Microorganisms decarboxylise amino acids in order to provide the cell with energy and to protect the cell against acidic environments by increasing the pH.

Biogenic amines are important because they contain a health risk for sensitive individuals. Symptoms include nausea, respiratorial discomfort, hot flushes, cold sweat, palpitations, headaches, red rash, high or low blood pressure. Alcohol and acetaldehyde have been found to increase the sensitivity to biogenic amines.

Mast cells release histamine when an allergen is encountered.

The histamine response can produce sneezing, itching, hives and watery eyes.

 

Allergies.
Some antigens, termed allergens trigger the release of IgE antibodies from specialized B cells.  The antibodies are inserted into the membrane of leucocytes called mast cells.  If the allergen is again presented to an activated mast cell, it binds to the cell surface of the mast cell.  This triggers the cell to release histamine and leucotrines that cause allergic reactions

Serotonin

5-Hydroxytryptophan is decarboxylated to serotonin (5-hydroxytryptamine or 5-HT) by the enzyme aromatic-L-amino-acid decarboxylase with the help of vitamin B6. This reaction occurs both in nervous tissue and in the liver. 5-HTP crosses the bloodbrain barrier, while 5-HT does not. Excess 5-HTP, especially when administered with Vitamin B6, is thought to be metabolized and excreted.

In animals including humans, serotonin is synthesized from the amino acid L-tryptophan by a short metabolic pathway consisting of two enzymes: tryptophan hydroxylase (TPH) and amino acid decarboxylase (DDC). The TPH-mediated reaction is the rate-limiting step in the pathway. TPH has been shown to exist in two forms: TPH1, found in several tissues, and TPH2, which is a brain-specific isoform.

Serotonin taken orally does not pass into the serotonergic pathways of the central nervous system because it does not cross the blood-brain barrier. However, tryptophan and its metabolite 5-hydroxytryptophan (5-HTP), from which serotonin is synthesized, can and do cross the blood-brain barrier. These agents are available as dietary supplements and may be effective serotonergic agents. One product of serotonin breakdown is 5-Hydroxyindoleacetic acid (5 HIAA), which is excreted in the urine. Serotonin and 5 HIAA are sometimes produced in excess amounts by certain tumors or cancers, and levels of these substances may be measured in the urine to test for these tumors.

The main functions of serotonin are: the regulation of mood, appetite, sleep, muscle contraction, and some cognitive functions including memory and learning. Modulation of serotonin at synapses is thought to be a major action of several classes of pharmacological antidepressants.

Serotonin is a monoamine neurotransmitter synthesized in serotonergic neurons in the central nervous system (CNS) and enterochromaffin cells in the gastrointestinal tract. Serotonin is synthesized extensively in the human gastrointestinal tract (about 90%), and the major storage place is platelets in the blood stream.

In the central nervous system, serotonin is believed to play an important role in the regulation of body temperature, mood, sleep, vomiting, sexuality, and appetite. Low levels of serotonin have been associated with several disorders, notably clinical depression, migraine, irritable bowel syndrome, tinnitus, fibromyalgia, bipolar disorder, and anxiety disorders.

The pharmacology of 5-HT is extremely complex, with its actions being mediated by a large and diverse range of 5-HT receptors. At least seven different receptor "families" are known to exist, each located in different parts of the body and triggering different responses. As with all neurotransmitters, the effects of 5-HT on the human mood and state of mind, and its role in consciousness, are very difficult to ascertain. Serotonin (5-HT) receptors are also used by other psychoactive drugs, including LSD, DMT, and psilocybin, the active ingredient in psychedelic mushrooms.

Serotonin uptake

The body sometimes deliberately prolongs the transmission of a signal across a synapse by slowing the destruction of neurotransmitters. It does this by releasing into the synapse special long-lasting chemicals called neuromodulators. Some neuromodulators aid the release of neurotransmitters into the synapse; others inhibit the reabsorption of neurotransmitters so that they remain in the synapse; still others delay the breakdown of neurotransmitters after their reabsorption, leaving them in the tip to be released back into the synapse when the next signal arrives.

Mood, pleasure, pain, and other mental states are determined by particular groups of neurons in the brain that use special sets of neurotransmitters and neuromodulators. Mood, for example, is strongly influenced by the neurotransmitter serotonin. Many researchers think that depression results from a shortage of serotonin. Prozac, the worlds bestselling antidepressant, inhibits the reabsorption of serotonin, thus increasing the amount in the synapse.

Drugs alter transmission of impulses across the synapse.

Depression can result from a shortage of the neurotransmitter serotonin. The antidepressant drug Prozac works by blocking reabsorption of serotonin in the synapse, making up for the shortage.

Dopamine - an important transmitter

 


Arvid Carlsson performed a series of pioneering studies during the late 1950's, which showed that dopamine is an important transmitter in the brain. It was previously believed that dopamine was only a precursor of another transmitter, noradrenaline. Arvid Carlsson developed an assay that made it possible to measure tissue levels of dopamine with high sensitivity. He found that dopamine was concentrated in other areas of the brain than noradrenaline, which led him to the conclusion that dopamine is a transmitter in itself. Dopamine existed in particularly high concentrations in those parts of the brain, called the basal ganglia, which are of particular importance for the control of motor behavior.

L-DOPA (L-3,4-dihydroxyphenylalanine) is a chemical that is made and used as part of the normal biology of some animals and plants. Some animals including humans make it via biosynthesis from the amino acid L-tyrosine. L-DOPA is the precursor to the neurotransmitters dopamine, norepinephrine (noradrenaline), and epinephrine (adrenaline) collectively known as catecholamines. L-DOPA can be manufactured and in its pure form is sold as a psychoactive drug with the INN levodopa; trade names include Sinemet, Parcopa, Atamet, Stalevo, Madopar, Prolopa, etc.). As a drug it is used in the clinical treatment of Parkinson's disease and Dopamine (abbreviated as DA), a simple organic chemical in the catecholamine family, is a monoamine neurotransmitter and hormone, which has a number of important physiological roles in the bodies of animals. In addition to being a catecholamine and a monoamine, dopamine may be classified as a substituted phenethylamine. Its name derives from its chemical structure, which consists of an amine group (NH2) linked to a catechol structure, called dihydroxyphenethylamine, the decarboxylated form of dihydroxyphenylalanine (acronym DOPA). In the brain, dopamine functions as a neurotransmittera chemical released by nerve cells to send signals to other nerve cells.dopamine-responsive dystonia.

Dopamine is synthesized in the body from within cells (mainly by neurons and cells in the medulla of the adrenal glands) and can be created from any one of the following three amino acids:

L-Phenylalanine (PHE)

L-Tyrosine (L-4-hydroxyphenylalanine; TYR)

L-DOPA (L-3,4-dihydroxyphenylalanine; DOPA)

These amino acids are provided from natural sources such as the ingestion of various kinds of food, with L-tyrosine being the most common of the three. Although dopamine itself is also commonly found in many types of food, unlike the amino acids that form it, it is incapable of crossing the protective blood-brain-barrier (BBB), which severely restricts its functionality upon consumption. It must be formed from within the walls of the BBB to properly perform its cognitive duties, though not its peripheral actions. Dopamine itself is also used in the synthesis of the following related catecholamine neurotransmitters:

Norepinephrine (β,3,4-trihydroxyphenethylamine; Noradrenaline; NE, NA)

Epinephrine (β,3-dihydroxy-N-methylphenethylamine; Adrenaline; EPI, ADR)

This is the complete metabolic pathway:

L-Phenylalanine → L-Tyrosine → L-DOPA → Dopamine → Norepinephrine → Epinephrine

L-Phenylalanine is converted into L-tyrosine by the enzyme phenylalanine hydroxylase (PAH) with molecular oxygen (O2) and tetrahydrobiopterin (THB) as cofactors. L-Tyrosine is converted into L-DOPA by the enzyme tyrosine hydroxylase (TH) with tetrahydrobiopterin (THB), O2, and ferrous iron (Fe2+) as cofactors. L-DOPA is converted into dopamine by the enzyme aromatic L-amino acid decarboxylase (AAAD; also known as DOPA decarboxylase (DDC)) with pyridoxal phosphate (PLP) as the cofactor. The reactions are illustrated as follows:

PAH: L-Phenylalanine + THB + O2 + Fe2+ → L-Tyrosine + DHB + H2O + Fe2+

TH: L-Tyrosine + THB + O2 + Fe2+ → L-DOPA + DHFA + H2O + Fe2+

AAAD: L-DOPA + PLP → Dopamine + PLP + CO2

Other decarboxylation reactions from the citric acid cycle include:

to succinyl-CoA.

Drugs against Parkinson's disease
Arvid Carlsson realized that the symptoms caused by reserpine were similar to the syndrome of Parkinson's disease. This led, in turn, to the finding that Parkinson patients have abnormally low concentrations of dopamine in the basal ganglia. As a consequence L-dopa was developed as a drug against Parkinson's disease and today still is the most important treatment for the disease. During Parkinson's disease dopamine producing nerve cells in the basal ganglia degenerate, which causes tremor, rigidity and akinesia. L-dopa, which is converted to dopamine in the brain, compensates for the lack of dopamine and normalizes motor behavior.

Antipsychotic and antidepressive drugs
Apart from the successful treatment of Parkinson's disease Arvid Carlsson's research has increased our understanding of the mechanism of several other drugs. He showed that antipsychotic drugs, mostly used against schizophrenia, affect synaptic transmission by blocking dopamine receptors. The discoveries of Arvid Carlsson have had great importance for the treatment of depression, which is one of our most common diseases. He has contributed strongly to the development of selective serotonin uptake blockers, a new generation of antidepressive drugs.

 

Illustration



 

 Dopamine nerve pathways in the brain. Arvid Carlsson showed that there were particularly high levels of the chemical transmitter dopamine in the so called basal ganglia of the brain, which are of major importance for instance for the control of our muscle movements. In Parkinson's disease those dopamine producing nerve cells whose nerve fibers project to the basal ganglia die. This causes symptoms such as tremor, muscle rigidity and a decreased ability to move about.

 

Illustration



 

A message from one nerve cell to another is transmitted with the help of different chemical transmitters. This occurs at specific points of contact, synapses, between the nerve cells. The chemical transmitter dopamine is formed from the precursors tyrosine and L-dopa and is stored in vesicles in the nerve endings. When a nerve impulse causes the vesicles to empty, dopamine receptors in the membrane of the receiving cell are influenced such that the message is carried further into the cell. In the treatment of Parkinson's disease, the drug L-dopa is given, and is converted to dopamine in the brain. This compensates for the patient's lack of dopamine.

Sedatives

Sedatives (SED-uh-tivz), sometimes called as tranquilizers (TRANK-will-LY-zerz) or sleeping pills, include barbiturates or "downers." They are drugs that produce a calming effect or sleepiness. Physicians prescribe them to relieve anxiety, promote sleep, and treat seizures. When they are abused or taken at high doses, however, many of these drugs can lead to loss of consciousness or even death. Combining sedatives with alcohol is particularly dangerous. Possible effects of sedative abuse include poor judgment, slurred speech, staggering, poor coordination, and slow reflexes.

 

Cocaine is a mood-altering drug that interferes with normal transport of the neurotransmitter dopamine, which carries messages from neuron to neuron. When cocaine molecules block dopamine receptors, too much dopamine remains active in the synaptic gaps between neurons, creating feelings of excitement and euphoria.

 

GABA - \gamma-aminobutyric acid

The term GABA refers to the simple chemical substance \gamma-aminobutyric acid (NH2CH2CH2 CH2COOH). It is the major inhibitory neurotransmitter in the central nervous system. Its presence in the brain first was reported in 1950 (Roberts and Frankel, 1950a).

For several years the presence of GABA in brain remained a biochemical curiosity and a physiological enigma. It was remarked in the first review written on GABA that Perhaps the most difficult question to answer would be whether the presence in the gray matter of the central nervous system of uniquely high concentrations of \gamma-aminobutyric acid and the enzyme which forms it from glutamic acid has a direct or indirect connection to conduction of the nerve impulse in this tissue . However, later that year, the first suggestion that GABA might have an inhibitory function in the vertebrate nervous system came from studies in which it was found that topically applied solutions of GABA exerted inhibitory effects on electrical activity in the brain. In 1957, the suggestion was made that indigenously occurring GABA might have an inhibitory function in the central nervous system from studies with convulsant hydrazides.

Also in 1957, suggestive evidence for an inhibitory function for GABA came from studies that established GABA as the major factor in brain extracts responsible for the inhibitory action of these extracts on the crayfish stretch receptor system. Within a brief period the activity in this field increased greatly, so that the research being carried out ranged all the way from the study of the effects of GABA on ionic movements in single neurons to clinical evaluation of the role of the GABA system in epilepsy, schizophrenia, mental retardation, etc. This surge of interest warranted the convocation in 1959 of the first truly interdisciplinary neuroscience conference ever held, at which were present most of the individuals who had played a role in opening up this exciting field.

Schematic representation of the gamma-aminobutyric acid (GABAA) receptor

Schematic representation of the gamma-aminobutyric acid (GABAA) receptor. The functional receptor consists of five proteins, or subunits--most likely two α subunits, one β subunit, and two γ subunits. (Question marks indicate that the identity of these subunits has not been confirmed.) The proposed binding sites for GABA (α and β subunits), benzodiazepines (adjacent α and γ subunits), barbituates (unidentified subunit), and alcohol α, β, and γ subunits) are indicated. P's represent phosphate groups attached to the receptor that regulate the receptor's activity and sensitivity to alcohol.

The trace amine tryptamine is produced in the brain and possibly other tissues by the decarboxylation of L-tryptophan. tryptamine is believed to function as a neurotransmitter or neuromodulator in the CNS of mammals. It is catabolized by monoamine oxidase A catalyzed oxidation to indole acetaldehyde. This aldehyde can then be oxidized to indole-3-acetate by an undetermined isoform of aldehyde dehydrogenase , or reduced to indole-3-ethanol (tryptophol) by aldehyde reductase in the presence of NADPH. It was previously suggested that the cytochrome P450 isozyme CYP2D6 mediates the oxidative deamination of tryptamine, but later work showed that monoamine oxidase A was responsible for this activity. An additional metabolite of tryptamine has been identified as (4R)-2-(3-indolylmethyl)-1,3-thiazolidine-4-carboxylate, a condensation product of indole acetaldehyde and free L-cysteine present in brain tissue. Its formation in vitro is spontaneous and its bioligical function is unknown.

 

Histamine

 

Histamine [2-(4-Imidazolyl)-ethylamine] is an important mediator of many biological processes including inflammation, gastric acid secretion, neuromodulation, and regulation of immune function. Due to its potent pharmacological activity even at very low concentrations, the synthesis, transport, storage, release and degradation of histamine have to be carefully regulated to avoid undesirable reactions. Histamine is also generated by microbiological action in the course of food processing and it is therefore present in substantial amounts in many fermented foodstuffs and beverages.

Histamine is formed by decarboxylation of the amino acid L-histidine in a reaction catalyzed by the enzyme histidine decarboxylase. The major routes of histamine inactivation in mammals are methylation of the imidazole ring, catalyzed by histamine N-methyltransferase, and oxidative deamination of the primary amino group, catalyzed by diamine oxidase.

 

 

Histidine decarboxylase (HDC, EC 4.1.1.22) generates histamine by catalyzing the removal of the carboxyl group from the amino acid L-histidine.

Mammalian HDC enzymes utilize pyridoxal-phosphate as an active-site cofactor. HDC is synthesized as a 74 kDa precursor consisting of 662 amino acid residues that is processed to a 54 kDa active form that apparently forms homodimers. Although the details of HDC processing have yet to be worked out the active enzyme appears to be a cytosolic protein. In mammals, the enzyme is encoded by a single gene with twelve exons designated HDC. The human HDC gene is located on chromosome 15q21-22. HDC is an unstable protein that is synthesized only when a cell needs to make histamine and the enzyme is immediately degraded when sufficient histamine has been generated. Therefore, HDC is detectable only in cells actively synthesizing histamine. In contrast, the histamine degrading enzymes HMT and DAO are constitutively produced and are present in relatively constant amounts. Mast cells, basophils, enterochromaffin-like cells in the gastric mucosa and histaminergic neurons synthesize considerable amounts of histamine and store the mediator in special storage granula inside the cell. Upon appropriate stimulation, these cells can rapidly release relatively large amounts of histamine and thereby efficiently activate suitable effector mechanisms. In recent years, it became clear that apart from these histamine-storing cell types, many other cells including epithelial cells and lymphocytes can express HDC and synthesize histamine. In these cells, histamine is not stored but appears to be made short-term in small amounts and immediately released.

Key Concepts:

Histamine mediates a wide variety of physiological and pathological responses, such as inflammation, gastric acid secretion, neurotransmission and immune modulation.

Histamine is synthesised through decarboxylation of lhistidine by histidine decarboxylase and is catabolised through oxidative deamination or methylation.

Tissue histamine levels are transiently increased by degranulation of mast cells and basophils while they are upregulated by de novo synthesis in gastric enterochromaffinlike cells, and neurons.

Histamine exerts its functions by acting on its specific receptors consisting of H1, H2, H3 and H4 receptors.

Histamine is a paracrine mediator, of which actions are generally limited in the local microenvironment.

Histamine H1 receptor antagonists have brought successful therapeutic approaches for immediate allergy, because histamine evokes vasodilation and increased vascular permeability by acting on the H1 receptor.

In the central nervous system, histamine is involved in awakening, appetite, maintenance of circadian rhythm, learning and memory, which are regulated by the H1 and H3 receptors.

Histamine stimulates parietal cells to induce gastric acid secretion by acting on the H2 receptor, of which antagonists drastically improved therapeutic approaches for peptic ulcer.

Presynaptic histamine H3 receptor regulates release of various neurotransmitters including histamine itself, and is expected as a potential drug target for the treatment of cognitive dysfunctions.

Histamine H4 receptor is expressed exclusively in blood cells and mediates their chemotaxis in response to histamine.

 

Diagnostic role of determination of AlAT and AsAT

Aspartate transaminase

Aspartate transaminase (AST), also called aspartate aminotransferase (AspAT/ASAT/AAT) or serum glutamic oxaloacetic transaminase (SGOT), is a pyridoxal phosphate (PLP)-dependent transaminase enzyme. AST catalyzes the reversible transfer of an α-amino group between aspartate and glutamate and, as such, is an important enzyme in amino acid metabolism. AST is found in the liver, heart, skeletal muscle, kidneys, brain, and red blood cells, and it is commonly measured clinically as a marker for liver health.

Function

Aspartate transaminase catalyzes the interconversion of aspartate and α-ketoglutarate to oxaloacetate and glutamate.

Aspartate (Asp) + α-ketoglutarate ↔ oxaloacetate + glutamate (Glu)

 

Reaction catalyzed by aspartate aminotransferase

As a prototypical transaminase, AST relies on PLP as a cofactor to transfer the amino group from aspartate or glutamate to the corresponding ketoacid. In the process, the cofactor shuttles between PLP and the pyridoxamine phosphate (PMP) form. The amino group transfer catalyzed by this enzyme is crucial in both amino acid degradation and biosynthesis. In amino acid degradation, following the conversion of α-ketoglutarate to glutamate, glutamate subsequently undergoes oxidative deamination to form ammonium ions, which are excreted as urea. In the reverse reaction, aspartate may be synthesized from oxaloacetate, which is a key intermediate in the citric acid cycle.

Isozymes

Two isoenzymes are present in a wide variety of eukaryotes. In humans:

                    GOT1/cAST, the cytosolic isoenzyme derives mainly from red blood cells and heart.

                    GOT2/mAST, the mitochondrial isoenzyme is present predominantly in liver.

These isoenzymes are thought to have evolved from a common ancestral AST via gene duplication, and they share a sequence homology of approximately 45%.

AST has also been found in a number of microorganisms, including E. coli, H. mediterranei, and T. thermophilus. In E. coli, the enzyme is encoded by the aspCgene and has also been shown to exhibit the activity of an aromatic-amino-acid transaminase.

Structure

 

Structure of aspartate transaminase from chicken heart mitochondria

 

X-ray crystallography studies have been performed to determine the structure of aspartate transaminase from various sources, including chicken mitochondria, pig heart cytosol, and E. coli. Overall, the three-dimensional polypeptide structure for all species is quite similar. AST is dimeric, consisting of two identical subunits, each with approximately 400 amino acid residues and a molecular weight of approximately 45 kD. Each subunit is composed of a large and a small domain, as well as a third domain consisting of the N-terminal residues 3-14; these few residues form a strand, which links and stabilizes the two subunits of the dimer. The large domain, which includes residues 48-325, binds the PLP cofactor via an aldimine linkage to the ε-amino group of Lys258. Other residues in this domain Asp 222 and Tyr 225 also interact with PLP via hydrogen bonding. The small domain consists of residues 15-47 and 326-410 and represents a flexible region that shifts the enzyme from an "open" to a "closed" conformation upon substrate binding.

The two independent active sites are positioned near the interface between the two domains. Within each active site, a couple arginine residues are responsible for the enzymes specificity for dicarboxylic acid substrates: Arg386 interacts with the substrates proximal (α-)carboxylate group, while Arg292 complexes with the distal (side-chain) carboxylate.

In terms of secondary structure, AST contains both α and β elements. Each domain has a central sheet of β-strands with α-helices packed on either side.

Mechanism

Aspartate transaminase, as with all transaminases, operates via dual substrate recognition; that is, it is able to recognize and selectively bind two amino acids (Asp and Glu) with different side-chains. In either case, the transaminase reaction consists of two similar half-reactions that constitute what is referred to as a ping-pong mechanism. In the first half-reaction, amino acid 1 (e.g., L-Asp) reacts with the enzyme-PLP complex to generate ketoacid 1 (oxaloacetate) and the modified enzyme-PMP. In the second half-reaction, ketoacid 2 (α-ketoglutarate) reacts with enzyme-PMP to produce amino acid 2 (L-Glu), regenerating the original enzyme-PLP in the process. Formation of a racemic product (D-Glu) is very rare.

The specific steps for the half-reaction of Enzyme-PLP + aspartate Enzyme-PMP + oxaloacetate are as follows (see figure); the other half-reaction (not shown) proceeds in the reverse manner, with α-ketoglutarate as the substrate.

 

Reaction mechanism for aspartate aminotransferase

1.     Internal aldimine formation: First, the ε-amino group of Lys258 forms a Schiff base linkage with the aldehyde carbon to generate an internal aldimine.

2.     Transaldimination: The internal aldimine then becomes an external aldimine when the ε-amino group of Lys258 is displaced by the amino group of aspartate. This transaldimination reaction occurs via a nucleophilic attack by the deprotonated amino group of Asp and proceeds through a tetrahedral intermediate. As this point, the carboxylate groups of Asp are stabilized by the guanidinium groups of the enzymes Arg386 and Arg 292 residues.

3.     Quinonoid formation: The hydrogen attached to the a-carbon of Asp is then abstracted (Lys258 is thought to be the proton acceptor) to form a quinonoid intermediate.

4.     Ketimine formation: The quinonoid is reprotonated, but now at the aldehyde carbon, to form the ketimine intermediate.

5.     Ketimine hydrolysis: Finally, the ketimine is hydrolyzed to form PMP and oxaloacetate.

This mechanism is thought to have multiple partially rate-determining steps.[15] However, it has been shown that the substrate binding step (transaldimination) drives the catalytic reaction forward.

Clinical significance

AST is similar to alanine transaminase (ALT) in that both enzymes are associated with liver parenchymal cells. The difference is that ALT is found predominantly in the liver, with clinically negligible quantities found in the kidneys, heart, and skeletal muscle, while AST is found in the liver, heart (cardiac muscle), skeletal muscle, kidneys, brain, and red blood cells. As a result, ALT is a more specific indicator of liver inflammation than AST, as AST may be elevated also in diseases affecting other organs, such as myocardial infarction, acute pancreatitis, acute hemolytic anemia, severe burns, acute renal disease, musculoskeletal diseases, and trauma.

AST was defined as a biochemical marker for the diagnosis of acute myocardial infarction in 1954. However, the use of AST for such a diagnosis is now redundant and has been superseded by the cardiac troponins.

AST (SGOT) is commonly measured clinically as a part of diagnostic liver function tests, to determine liver health.

 

Patient type

Reference ranges

Female

6 - 34 IU/L

Male

8 - 40 IU/L

 

Alanine transaminase

 

Alanine transaminase or ALT is a transaminase It is also called serum glutamic pyruvic transaminase (SGPT) or alanine aminotransferase (ALAT).

ALT is found in serum and in various bodily tissues, but is most commonly associated with the liver. It catalyzes the two parts of the alanine cycle.

Function

It catalyzes the transfer of an amino group from alanine to α-ketoglutarate, the products of this reversible transamination reaction being pyruvate and glutamate.

glutamate + pyruvate α-ketoglutarate + alanine

 

Alanine transaminase

ALT (and all transaminases) require the coenzyme pyridoxal phosphate, which is converted into pyridoxamine in the first phase of the reaction, when an amino acid is converted into a keto acid.

Clinical significance

It is commonly measured clinically as a part of a diagnostic evaluation of hepatocellular injury, to determine liver health. When used in diagnostics, it is almost always measured in international units/liter (U/L). While sources vary on specific normal range values for patients, 10-40 U/L is the standard normal range for experimental studies. Alanine transaminase shows a marked diurnal variation.

Elevated levels

Patient type

Reference ranges

Female

538 IU/L

Male

1050 IU/L

 

Significantly elevated levels of ALT(SGPT) often suggest the existence of other medical problems such as viral hepatitis, diabetes, congestive heart failure, liver damage, bile duct problems, infectious mononucleosis, or myopathy. For this reason, ALT is commonly used as a way of screening for liver problems. Elevated ALT may also be caused by dietary choline deficiency. However, elevated levels of ALT do not automatically mean that medical problems exist. Fluctuation of ALT levels is normal over the course of the day, and ALT levels can also increase in response to strenuous physical exercise.

When elevated ALT levels are found in the blood, the possible underlying causes can be further narrowed down by measuring other enzymes. For example, elevated ALT levels due to liver-cell damage can be distinguished from biliary duct problems by measuring alkaline phosphatase. Also, myopathy-related ALT levels can be ruled out by measuring creatine kinase enzymes. Many drugs may elevate ALT levels, including Zileuton, omega-3-acid ethyl esters (Lovaza), anti-inflammatory drugs, antibiotics, cholesterol medications, some antipscyhotics such as Risperidone, and anti-convulsants.

For years, the American Red Cross used ALT testing as part of the battery of tests to ensure the safety of its blood supply by deferring donors with elevated ALT levels. The intent was to identify donors potentially infected with Hepatitis C because there was no specific test for that disease at the time. Prior to July 1992, widespread blood donation testing in the USA for Hepatitis C was not carried out by major blood banks. With the introduction of second-generation ELISA antibody tests for Hepatitis C, the Red Cross changed the ALT policy. As of July 2003[update], donors previously disqualified for elevated ALT levels and no other reason may be reinstated as donors by contacting the donor counseling department of their regional Red Cross organization.

Enhanced cardiac enzyme profile.

Abstract

A protocol for an enhanced Cardiac Enzyme Profile is proposed based on an admission, or initial, serum specimen and a second specimen 16 hours after onset of symptoms as minimal baseline serum samples in order to accomplish several simultaneous goals: 1. Detecting CK2MB at its average peak for maximal assurance of diagnosis when release is small and for prognosis in all cases of increased serum CK2MB 2. Detection of laboratory evidence of myocardial injury when admission is delayed after onset by the collection of an admission sample for declining CK2MB, and for assays of other enzymes with longer time curves after myocardial injury such as LD isoenzymes and ASAT/ALAT activities and ratio 3. Establishment of decision limits and criteria for the determination of laboratory evidence of myocardial injury 4. Providing cost-effective procedures other than limitation of the number of samples; these include establishing thresholds and criteria for total CK, total LD, and ASAT so that isoenzymes and ALAT are only performed when thresholds are exceeded and criteria are met; performing only CK and, if the threshold is exceeded, CK isoenzymes on the 16-hour sample; collecting additional samples after the first two only when indicated by positive or suspicious (borderline) results and only on routine morning or afternoon rounds rather than specifically timed specimens (except in cases involving thrombolytic therapy); and termination of the protocol once peak positive CK2MB activity and requisite diagnostic consensus confirmation (such as positive LD isoenzymes) is obtained whether or not thrombolytic therapy is involved. Tissue localization of the enzymes has been outlined in some detail with particular reference to the amount of CK2MB in skeletal muscle. Pathophysiological factors discussed in more depth in a previous article have been amplified here with particular reference to the role of increased synthesis as a response to myocardial injury by surrounding prehypertrophic and hypertrophic myocardium as a possible major source of increased serum enzymes in myocardial infarction. ASAT and especially the ASAT/ALAT ratio are useful tests in the protocol, particularly in cases tested late after onset of symptoms when CK2MB has declined into the borderline or usual range, and ASAT/ALAT may be helpful in evaluating LD isoenzyme results. Codes for interpretive comments are provided to serve as guidelines.

Use of enzymes for the diagnosis of alcohol-related organ damage.

Abstract

Elevated levels of serum enzymes are frequently associated not only with alcohol-related organ damage but also with excessive alcohol consumption and alcoholism without significant tissue injury. However, both in the early detection of alcoholism as well as also in the diagnosis of alcohol-related diseases the sensitivities and specificities of these enzyme markers vary considerably. They may be influenced by nonalcohol-related diseases, enzyme-inducing drugs, nutritional factors, metabolic disorders, age, smoking, etc. Consequently, we have neither a single laboratory test--enzyme marker--nor a test combination that is reliable enough for the exact diagnosis between alcohol- and nonalcohol-related organ damage. In most cases it is possible to determine the tissue from which the elevated enzyme is derived, but only occasionally enzyme changes reflect the quantity of the tissue injury. Gamma-glutamyltransferase (GGT) is the most widely used laboratory marker of alcoholism and heavy drinking, detecting 34-85% of problem drinkers and alcoholics. However, the unspecificity of increased serum GGT limits its use for general screening purposes. Its value in the follow-up of various treatment programs, however, is well established. An elevated level of serum aspartate aminotransferase (ASAT) and alanine aminotransferase (ALAT) in an alcoholic or a heavy consumer indicates alcohol-induced organ damage. The use of test combinations significantly improves the information received with single serum enzyme determinations. An ASAT/ALAT ratio greater than 1.5 can be considered as highly suggestive for the alcoholic etiology of the liver injury. Still better discrimination between alcoholic and nonalcoholic origin of the liver disease may be achieved by the determination of the ratio of GGT to alkaline phosphatase. If this ratio exceeds 1.4 the specificity of the finding in favor for alcoholic liver injury is 78%. The determination of the mitochondrial isoenzyme of ASAT also improves the diagnostic value of ASAT determination. The ratio of mitochondrial isoenzyme to total over 4 is highly suggestive for alcohol-related liver injury. In general, however, the determination of serum activities of other enzymes such as ornithine carbamyl transferase, lactate dehydrogenase, isocitrate dehydrogenase, sorbitol dehydrogenase, alcohol dehydrogenase, guanase, aldolase, alkaline phosphatase or glutathione S-transferase do not significantly improve the diagnostic information obtained with more conventional laboratory markers of liver injury.