Medicine

Themes:

BIOCHEMICAL FUNCTION OF LIVER. POEPHYRINS AND BILE PIGMENTS. PATHOBIOCHEMISTRY OFJAUNDICE. METABOLISM OF XENOBIOTICS IN THE LIVER: MICROSOMAL OXIDATION, CYTOCHROME Р-450

Liver’s functions:

• It is responsible for the production of bile which is stored in the gallbladder and released when required for the digestion of fats.

• The liver stores glucose in the form of glycogen which is converted back to glucose again when needed for energy.

• It also plays an important role in the metabolism of protein and fats. It stores the vitamins A, D, K, B12 and folate and synthesizes blood clotting factors.

Role of the liver in carbohydrate metabolism.

From intestine glucose pass into the liver, where most part of it undergone the phosphorillation. Glucose-6-phosphate formed in result of this reaction, which catalyzed by two enzymes – hexokinase and glucokinase. When level of glucose in blood of v. porta and in the hepatocytes is normal activity of glucokinase is low. After eating activity of this enzyme increase and blood level of glucose decrease because glucose-6-phosphate cannot pass through membrane.

Fructose and galactose also transformed into glucose-6-phosphate in the liver.

Glucose-6-phosphate is a key product of carbohydrates metabolism. In the liver this substance can metabolized into different ways depend of liver’s and whole organism’s necessity.

1. Synthesis of glicogen. Content in the liver – 70-100g. After eating amount of glicogen in the liver increase up to 150g. After 24 hours of starvation content of glicogen in the liver decreases to zero and glukoneogenesis started.

2. Glucose-6-phosphatase catalize dephosphorillation of glucose-6-phosphate and free glucose formed. This enzyme is present in the liver, kidney and small intestine. This process keep normal level of glucose in the blood.

3. Excess of glucose-6-phosphate, which not used for synthesis of glicogen and forming of free glucose, decomposites in glycolysis for pyruvate and for acetyl-CoA, which are used for fatty acids synthesis.

4. Glucose-6-phosphate decomposites for H2O and CO2, and free energy for hepatocytes formed.

5. Part of glucose-6-phosphate oxidized in pentosophosphate cycle. This way of glucose decomposition supplyes reducted NADPH, which is necessary in fatty acid synthesis, cholesterin synthesis, and also pentosophosphates for nucleic acids. Near 1/3 of glucose in liver used for this pathway, another 2/3 – for glycolisis.

 

 

Role of the liver in lipid metabolism.

 

In the liver all processes of lipid metabolism take place. Most important of them are following:

1. Lipogenesis (synthesis of fatty acids and lipids). Substrate for this process – acetyl-CoA, formed from glucose and amino acids, which are not used for another purposes. This process is very active when the person eats a lot of carbohydrates. Liver more active than another tissues synthesizes saturated and monounsaturated fatty acids. Fatty acids then used for synthesis of lipids, phospholipids, cholesterol ethers. Glycerol-3-phosphate, which is necessary for lipids synthesis, formed in liver in result of two processes: from free glycerol under influence of glycerolkinase, or in reducing of dioxiacetone phosphate under influence of glycerolphosphate dehydrogenase. Active form of fatty acids interact with glycerol-3-phosphate and phosphatidic acid formed, which used for synthesis of triacylglycerines and glycerophospholipids.

2. Liver play a central role in synthesis of cholesterin, because near 80 % of its amount is synthesized there. Biosynthesis of cholesterin regulated by negative feedback. When the level of cholesterin in the meal increases, synthesis in liver decreases, and back to front. Besides synthesis regulated by insulin and glucagon. Cholesterin used in organism for building cell membranes, synthesis of steroid hormones and vitamin D. Excess of cholesterin leads out in the bile to the intestine. Another part of cholesterin used for bile acids synthesis. This process regulated by reabsorbed bile acids according to negative feedback principles.

3. Liver is a place of ketone bodies synthesis. These substances formed from fatty acids after their oxidation, and from liver transported to another tissues, first of all to the heart, muscles, kidneys and brain. These substances are main source of energy for many tissues of our organism excepting liver in normal conditions (heart) and during starvation (brain).

 

Role of the liver in protein metabolism.

 

Liver has full set of enzymes, which are necessary for amino acids metabolism. Amino acids from food used in the liver for following pathways:

1. Protein synthesis.

2. Decomposition for the final products.

3. Transformation to the carbohydrates and lipids.

4. Interaction between amino acids.

5. Transformation to the different substances with amino group.

6. Release to the blood and transport to another organs and tissues.

The high speed of protein synthesis and decomposition is typical for the liver. Hepatocytes catch different protein from blood (from hemolysated RBC, denaturated plasma proteins, protein and peptide hormones) and decomposite them to the free amino acids which used for new synthesis. When organism does not get necessary quantity of amino acids from food, liver synthesizes only high necessary proteins (enzymes, receptors).

Liver syntesizes 100 % of albumines, 90 % of α1-globulines, 75 % of α2-globulines, 50 % of β-globulines, blood clotting factors, fibrinogen, protein part of blood lipoproteins, such enzyme as cholinesterase. The speed of these processes is enough high, for example, liver synthesizes 12-16g of albumines per day.

Amino acids, which are not used for protein synthesis, transformed to another substances. Oxidative decomposition of amino acids is main source of energy for liver in normal conditions.

Liver can synthesize non-essential amino acids.

Liver synthesizes purine and pyrimidine nucleotides, hem, creatin, nicotinic acid, cholin, carnitin, polyamines.

The decomposition of hemoglobin in tissues, bile pigments formation.

After a life span of about 120 days the erythrocytes die. The dead erythrocytes are taken up by the phagocytes of the reticuloendothelial system of the body. About 7 gram of Hb is released daily from these phagocytosed erythrocytes. The Hb molecule is broken down into 3 parts:

1.        The protein (globin) part is utilized partly as such or along with other body proteins.

2.        The iron is stored in the reticuloendothelial cells and is reused for the synthesis of Hb and other iron containing substances of the body.

3.        The porphyrin part is converted to bile pigment, i.e. bilirubin which is excreted in bile.

The several stages, which are involved in the formation of bile pigment from Hb and the farther fate of this pigment, are given below:

1. Hemoglobin dissociates into heme and globin.

2. Heme in the presence of the enzyme, heme oxygenase, loses one molecule of CO and one atom of iron in Fe3+ form producing biliverdin. In this reaction, the porphyrin ring is cleaved by oxidation of the alpha methenyl bridge between pyrrole rings. The enzyme needs NADPH+H+ and O2.

Biliverdin which is green in color is the first bile pigment to be produced; it is reduced to the yellow-colored bilirubin, the main bile pigment, by the enzyme biliverdin reductase requiring NADPH+H+.

Bilirubin is non-polar, lipid soluble but water insoluble. Bilirubin is a very toxic compound. For example, it is known to inhibit RNA and protein synthesis and carbohydrate metabolism in brain. Mitochondria appear to be especially sensitive to its effect. Bilirubin formed in reticuloendothelial cells then is associated with plasma protein albumin to protect cells from the toxic effects. As this bilirubin is in complex with plasma proteins, therefore it cannot pass into the glomerular filtrate in the kidney; thus it does not appear in urine, even when its level in the blood plasma is very high. However, being lipid soluble, it readily gets deposited in lipid-rich tissues specially the brain.

This bilirubin is called indirect bilirubin or free bilirubin or unconjugated bilirubin.

The detoxication of indirect bilirubin takes place in the membranes of endoplasmatic reticulum of hepatocytes. Here bilirubin interact with UDP-glucuronic acid and is converted to the water soluble form -bilirubin mono- and diglucoronids. Another name of bilirubin mono- and diglucoronids is conjugated bilirubin or direct bilirubin or bound bilirubin. This reaction is catalized by UDP-glucoroniltransferase.

Conjugated bilirubin is water soluble and is excreted by hepatocytes to the bile. Conjugated (bound) bilirubin undergoes degradation in the intestine through the action of intestinal microorganisms. Bilirubin is reduced and, mesobilirubin is formed. Then mesobilirubin is reduced again and mesobilinogen is formed. The reduction of mesobilinogen results in the formation of stercobilinogen (in a colon). Stercobilinogen is oxidized and the chief pigment (brown color) of feces stercobilin is formed. A part of mesobilinogen is reabsorbed by the mucous of intestine and via the vessels of vena porta system enter liver. In hepatocytes mesobilinogen is splitted to pyrol compounds which are excreted from the organism with bile. If the liver has undergone degeneration mesobilinogen enter the blood and is excreted by the kidneys. This mesobilinogen in urine is called urobilin, or true urobilin. Thus, true urobilin can be detected in urine only in liver parenchyma disease.

Another bile pigment that can be reabsorbed in intestine is stercobolinogen. Stercobolinogen is partially reabsorbed in the lower part of colon into the haemorroidal veins. From the blood stercobolinogen pass via the kidneys into the urine where it is oxidized to stercobilin. Another name of urine stercobilin is false urobilin.

As mentioned above, the conversion of bilirubin to mesobilirubin occurs under the influence of intestinal bacteria. These bacteria are killed or modified when broad-spectrum antibiotics are administered. The gut is sterile in the newborn babies. Under these circumstances, bilirubin is not-converted to urobilinogen, and the feces are colored yellow due to bilirubin. The feces may even become green because some bilirubin is reconverted to green-colored biliverdin by oxidation.

The total bilirubin content in the blood serum is 1,7-20,5 micromol/l, indirect (unconjugated) bilirubin content is 1,7-17,1 micromol/l and direct (conjugated) bilirubin content is 0,86-4,3 micromol/l.

 

 

 

 Differentiation between unconjugated and conjugated bilirubin. Direct and indirect bilirubin.

Diazo reagent which is a mixture of sulfanilic acid, HCI and NaN02 is added to the serum. The conjugated bilirubin gives a reddish violet color with it and the maximum color intensity is obtained within 30 seconds; this is called direct test.

The unconjugated bilirubin does not give the direct test; however, it gives indirect test in which alcohol or caffeine is also added which sets free the bilirubin frum its complex with plasma proteins. Due to this difference in the type of diazo reaction given by these two forms of bilirubin, the term direct and indirect forms of bilirubin are also used to describe conjugated and unconjugated forms of bilirubin.

Some other differences between these two forms of bilirubin are given below:

The mechanism of jaundice development, their biochemical characteristic.

 

Jaundice or icterus is the orange-yellow discoloration of body tissues which is best seen in the skin and conjunctivae; it is caused by the presence of an excess of bilirubin in the blood plasma and tissue fluids. Depending upon the cause of an increased plasma bilirubin level, jaundice can be classified as

1)        pre-hepatic,

2)        hepatic and

3)        post-hepatic

Pre-hepafic jaundice. This type of jaundice is due to a raised plasma level of unconjugated bilirubin. It is due to an excessive breakdown of red cells which leads to an increased production of uncongugated bilirubin; it is also called haemolytic jaundice. As the liver is not able to excrete into the bile all the bilirubin reaching it, the plasma bilirubin level rises and jaundice results. This type of jaundice was in the past called acholuric jaundice because the unconjugated bilirubin, being bound to plasma proteins, is not excreted in the urine despite its high level in the plasma; the urine is also without bile salts. Prehepatic jaundice is also seen in neonates (physiological jaundice) especially in the premature ones because the enzyme UDP-glucuronyl transferase is deficient. Moreover relatively more bilirubin is produced in-the neonates because of excessive breakdown of red blood cells.

Hepatic jaundice.This is typically seen in viral hepatitis. Several viruses are responsible for viral hepatitis and include hepatitis A, B, C and D viruses. The liver cells are damaged: inflammation produces obstruction of bile canaliculi due to swelling around them. This cholestasis causes the bile to regurgitate into the blood through bile canaliculi. The blood contains abnormally raised amount both of conjugated and unconjugated bilirubin and bile salts which are excreted in the urine.

Post hepatic jaundice. This results when there is extrahepatic cholestasis due to an obstruction in the biliary passages outside the liver. In this way, the bile cannot reach the small intestine and therefore the biliary passages outside as well as inside the liver are distended with bile. This leads to damage to the liver and bile regurgitates into the blood. The conjugated bilirubin and the bile salt levels of the blood are thus greatly raised and both of these are excreted in the urine. Liver function tests will vary according to the degree of obstruction, i.e complete or incomplete. If the obstruction is complete, the stools become pale or clay-colored and the urine does not have any stercobilin. The absorption of fat and fat soluble vitamins also suffers due to a lack of bile salts. Excess of bile salts in the plasma produces severe pruritus (itching).

Hemolytic jaundice is characterized by

1.        Increase mainly of unconjugated bilirubin in the blood serum.

2.        Increased excretion of urobilinogen with urine.

3.        Dark brown colour of feces due to high content of stercobilinogen.

 Hepatic jaundice is characterized by

1.Increased levels of conjugated and unconjugated bilirubin in serum.

2.Dark coloured urine due to the excessive excretion of bilirubin and urobilinogen.

3.Pale, clay coloured stools due to the absence of  stercobilinogen.

4.Increased activities of alanine and aspartate transaminases.

 

 

Obstructive (post hepatic ) jaundice is characterized by

1.Increased concentration mainly of conjugated bilirubin in serum.

2.Dark coloured urine due to elevated excretion of bilirubin and clay coloured feces due to absence of stercobilinogen.

Role of the liver in detoxification processes.

A xenobiotics is a compound that is foreign to the body. The principal classes of xenobiotics of medical relevance are drugs, chemical cancerogens, and various compounds that have found their way into our environment by one route or another (insecticides, herbicides, pesticides, food additions, cosmetics, domestic chemical substances). Most of these compounds are subject to metabolism (chemical alteration) in the human body, with the liver being the main organ involved; occasionally a xenobiotics may be excreted unchanged.

Some internal substances also have toxic properties (for example, bilirubin, free ammonia, bioactive amines, products of amino acids decay in the intestine). Moreover, all hormones and mediatores must be inactivated.

Reactions of detoxification take place in the liver. Big molecules like bilirubin excreted with the bile to intestine and leaded out with feces. Small molecules go to the blood and excreted via kidney with urine.

The metabolism of xenobiotics has 2 phases:

In phase 1, the major reaction involved is hydroxylation, catalyzed by members of a class of enzymes referred to as monooxygenases or cytochrome P-450 species. These enzymes can also catalyze deamination, dehalogenation, desulfuration, epoxidation, peroxidation and reduction reaction. Hydrolysis reactions and non-P-450-catalyzed reactions also occur in phase 2.

In phase 2, the hydroxylated or other compounds produced in phase 1 are converted by specific enzymes to various polar metabolites by conjugation with glucuronic acid, sulfate, acetate, glutathione, or certain amino acids, or by methylation.

The overall purpose of metabolism of xenobiotics is to increase their water solubility (polarity) and thus facilitate their excretion from the body via kidney.Very hydrophobic xenobiotics would persist in adipose tissue almost indefinitely if they were not converted to more polar forms.

In certain cases, phase 1 metabolic reaction convert xenobiotics from inactive to biologically active compounds. In these instances, the original xenobiotics are referred to as prodrugs or procarcinogens. In other cases, additional phase 1 reactions convert the active compounds to less active or inactive forms prior to conjugation. In yet other cases, it is the conjugation reactions themselves that convert the active product of phase 1 to less active or inactive species, which are subsequently excreted in the urine or bile. In a very few cases, conjugation may actually increase the biologic activity of a xenobiotics.

Hydroxylation is the chief reaction involved in phase 1. The responsible enzymes are called monooxygenases or cytochrome P-450 species. The reaction catalyzed by a monooxygenase is:

RH + O2 + NADPH + H+ → R-OH + H2O + NADP

RH above can represent a very widee variety of drugs, carcinogens, pollutants, and certain endogenous compounds, such as steroids and a number of other lipids. Cytochrome P-450 is considered the most versatile biocatalyst known. The importance of this enzyme is due to the fact that approximately 50 % of the drugs that patients ingest are metabolized by species of cytochrome P-450. The following are important points concerning cytochrome P-450 species:

 

1. Like hemoglobin, they are hemoproteins.

2. They are present in highest amount in the membranes of the endoplasmic reticulum (ER) (microsomal fraction) of liver, where they can make up approximately 20 % of the total protein. Thay are also in other tissues. In the adrenal, they are found in mitochondria as well as in the ER; the various hydroxylases present in that organ play an important role in cholesterol and steroid biosynthesis.

3. There are at least 6 closely related species of cytochrome P-450 present in liver ER, each with wide and somewhat overlapping substrate specificities, that act on a wide variety of drugs, carcinogens, and other xenobiotics in addition to endogenous compounds such as certain steroids.

4. NADPH, not NADP, is involved in the reaction mechanism of cytochrome P-450. The enzyme that uses NADPH to yield the reduced cytochrome P-450 is called NADPH-cytochrome P-450 reductase.

5. Lipids are also components of the cytochrome P-450 system. The preferred lipid is phosphatidylcholine, which is the major lipid found in membranes of the ER.

6. Most species of cytochrome P-450 are inducible. For instance, the administration of phenobarbital or of many other drugs causes a hypertrophy of the smooth ER and a 3- to 4-fold increase in the amount of cytochrome P-450 within 4-5 days. Induction of this enzyme has important clinical implications, since it is a biochemical mechanism of drug interaction.

7. One species of cytochrome P-450 has its characteristic absorption peak not at 450 nm but at 448 nm. It is often called cytochrome-448.This species appears  to be relatively specific for the metabolism of polycyclic aromatic hydrocarbons (PAHs) and related molecules; for this reason it is called aromatic hydrocarbon hydroxylase (AHH). This enzyme is important in the metabolism of PAHs and in carcinogenesis produced by this agents.

8. Recent findings have shown that individual species of cytochrome P-450 frequently exist in polymorphic forms, some of which exhibit low catalytic activity. These observation are one important explanation for the variations in drug responses noted among many patients.

In phase 1 reactions, xenobiotics are generally converted to more polar, hydroxylated derivates. In phase 2 reactions, these derivates are conjugated with molecules such as glucuronic acid, sulfate, or glutatione. This renders them even more water-soluble, and they are eventually excreted in the urine or bile.

 

There are at least 5 types of phase 2 reactions:

 

1. Glucuronidation. UDP-glucuronic acid is the glucuronyl donor, and a variety of glucuronyl transferases, present in both the ER and cytosol, are the catalysts. Molecules such as bilirubin, thyroxin, 2-acetylaminofluorene (a carcinogen), aniline, benzoic acid, meprobromate (a tranquilizer), phenol, crezol, indol and skatol, and many steroids are excreted as glucuronides. The glucuronide may be attached to oxygen, nitrogen, or sulfur groups of substrates. Glucuronidation is probably the most frequent conjugation reaction.

Glucuronidation, the combining of glucuronic acid with toxins, requires the enzyme UDP-glucuronyl transferase (UDPGT). Many of the commonly prescribed drugs are detoxified through this pathway. It also helps to detoxify aspirin, menthol, vanillin (synthetic vanilla), food additives such as benzoates, and some hormones. Glucuronidation appears to work well, except for those with Gilbert's syndrome--a relatively common syndrome characterized by a chronically elevated serum bilirubin level (1.2-3.0 mg/dl). Previously considered rare, this disorder is now known to affect as much as 5% of the general population. The condition is usually without serious symptoms, although some patients do complain about loss of appetite, malaise, and fatigue (typical symptoms of impaired liver function). The main way this condition is recognized is by a slight yellowish tinge to the skin and white of the eye due to inadequate metabolism of bilirubin, a breakdown product of hemoglobin. The activity of UDPGT is increased by foods rich in the monoterpene limonene (citris peel, dill weed oil, and caraway oil). Methionine, administered as SAM, has been shown to be quite beneficial in treating Gilbert's syndrome.

2. Sulfation. Some alcohols, arylamines, and phenols are sulfated. The sulfate donor in these and other biologic sulfation reactions is adenosine 3´-phosphate-5´-phosphosulfate (PAPS); this compound is called active sulfate.

Sulfation is the conjugation of toxins with sulfur-containing compounds. The sulfation system is important for detoxifying several drugs, food additives, and, especially, toxins from intestinal bacteria and the environment. In addition to environmental toxins, sulfation is also used to detoxify some normal body chemicals and is the main pathway for the elimination of steroid and thyroid hormones. Since sulfation is also the primary route for the elimination of neurotransmitters, dysfunction in this system may contribute to the development of some nervous system disorders.

Many factors influence the activity of sulfate conjugation. For example, a diet low in methionine and cysteine has been shown to reduce sulfation. Sulfation is also reduced by excessive levels of molybdenum or vitamin B6 (over about 100 mg/day). In some cases, sulfation can be increased by supplemental sulfate, extra amounts of sulfur-containing foods in the diet, and the amino acids taurine and glutathione.

Sulfoxidation is the process by which the sulfur-containing molecules in drugs and foods are metabolized. It is also the process by which the body eliminates the sulfite food additives used to preserve many foods and drugs. Various sulfites are widely used in potato salad (as a preservative), salad bars (to keep the vegetables looking fresh), dried fruits (sulfites keep dried apricots orange), and some drugs. Normally, the enzyme sulfite oxidase metabolizes sulfites to safer sulfates, which are then excreted in the urine. Those with a poorly functioning sulfoxidation system, however, have an increased ratio of sulfite to sulfate in their urine. The strong odor in the urine after eating asparagus is an interesting phenomenon because, while it is unheard of in China, 100% of the French have been estimated to experience such an odor (about 50% of adults in the U.S. notice this effect). This example is an excellent example of genetic variability in liver detoxification function. Those with a poorly functioning sulfoxidation detoxification pathway are more sensitive to sulfur-containing drugs and foods containing sulfur or sulfite additives. This is especially important for asthmatics, which can react to these additives with life-threatening attacks. Molybdenum helps asthmatics with an elevated ratio of sulfites to sulfates in their urine because sulfite oxidase is dependent upon this trace mineral.

3. Conjugation with Glutathione. Glutathione (γ-glutamylcysteinylglycine) is a tripeptide consisting of glutamic acid, cysteine, and glycine. Glutathione is commonly abbreviated to GSH; the SH indicates the sulfhydryl group of its cysteine and is the business part of the molecule. A number of potentially toxic electrophilic xenobiotics (such as certain carcinogens) are conjugated to the nucleophilic GSH. The enzymes catalyzing these reactions are called glutathione S-transferases and are present in high amounts in liver cytosol and in lower amounts in other tissues. glutathione conjugates are subjected to further metabolism before excretion. The glutamyl and glycinyl groups belonging to glutathione are removed by specific enzymes, and an acetyl group (donated by acetyl-CoA) is added to the amino group of the remaining cystenyl moiety. The resulting compound is a mercapturic acid, a conjugate of L-acetylcysteine, which is then excreted in the urine.

 Glutathione is also an important antioxidant. This combination of detoxification and free radical protection, results in glutathione being one of the most important anticarcinogens and antioxidants in our cells, which means that a deficiency is cause of serious liver dysfunction and damage. Exposure to high levels of toxins depletes glutathione faster than it can be produced or absorbed from the diet. This results in increased susceptibility to toxin-induced diseases, such as cancer, especially if phase I detoxification system is highly active. Disease states due to glutathione deficiency are not uncommon.

A deficiency can be induced either by diseases that increase the need for glutathione, deficiencies of the nutrients needed for synthesis, or diseases that inhibit its formation. Smoking increases the rate of utilization of glutathione, both in the detoxification of nicotine and in the neutralization of free radicals produced by the toxins in the smoke. Glutathione is available through two routes: diet and synthesis. Dietary glutathione (found in fresh fruits and vegetables, cooked fish, and meat) is absorbed well by the intestines and does not appear to be affected by the digestive processes. Dietary glutathione in foods appears to be efficiently absorbed into the blood. However, the same may not be true for glutathione supplements.

In healthy individuals, a daily dosage of 500 mg of vitamin C may be sufficient to elevate and maintain good tissue glutathione levels. In one double-blind study, the average red blood cell glutathione concentration rose nearly 50% with 500 mg/day of vitamin C. Increasing the dosage to 2,000 mg only raised red blood cell (RBC) glutathione levels by another 5%. Vitamin C raises glutathione by increasing its rate of synthesis. In addition, to vitamin C, other compounds which can help increase glutathione synthesis include N-acetylcysteine (NAC), glycine, and methionine. In an effort to increase antioxidant status in individuals with impaired glutathione synthesis, a variety of antioxidants have been used. Of these agents, only microhydrin, vitamin C and NAC have been able to offer some possible benefit.

Over the past 5-10 years, the use of NAC and glutathione products as antioxidants has become increasingly popular among nutritionally oriented physicians and the public. While supplementing the diet with high doses of NAC may be beneficial in cases of extreme oxidative stress (e.g. AIDS, cancer patients going through chemotherapy, or drug overdose), it may be an unwise practice in healthy individuals.

4. Acetylation. These reactions is represented by X + Acetyl-CoA → Acetyl-X + CoA, where X represents a xenobiotic. These reactions are catalyzed by acetyltransferases present in the cytosol of various tissues, particularly liver. The different aromatic amines, aromatic amino acids, such drug as isoniazid, used in the treatment of tuberculosis, and sulfanylamides are subjects to acetylation. Polymorphic types of acetyltransferases exist, resulting in individuals who are classified as slow or fast acetylators, and influence the rate of clearance of drugs such as isoniazid from blood. Slow acetylators are more subject to certain toxic effects of isoniazid because the drug persists longer in these individuals.

Conjugation of toxins with acetyl-CoA is the primary method by which the body eliminates sulfa drugs. This system appears to be especially sensitive to genetic variation, with those having a poor acetylation system being far more susceptible to sulfa drugs and other antibiotics. While not much is known about how to directly improve the activity of this system, it is known that acetylation is dependent on thiamine, pantothenic acid, and vitamin C.

5. Methylation. A few xenobiotics (amines, phenol, tio-substances, inorganic compounds of sulphur, selen, mercury, arsenic) are subject to methylation by methyltransferases, employing S-adenosylmethionine as methyl donor. Also catecholamines and nicotinic acid amid (active form of vitamin PP) are inactivated due to methylation.