Xenobiotic Metabolism Lecture Notes PDF

Metabolism of Xenobiotics Xenos = foreign, and xenobiotic is a compound that is foreign to the body. (drugs, chemical carcinogens, insecticides and pollutants such as polychlorinated biphenyls. Site of xenobiotics metabolism: Most of these compounds are subjected to metabolism (chemical alteration...

Metabolism of Xenobiotics Xenos = foreign, and xenobiotic is a compound that is foreign to the body. (drugs, chemical carcinogens, insecticides and pollutants such as polychlorinated biphenyls. Site of xenobiotics metabolism: Most of these compounds are subjected to metabolism (chemical alteration) in the human body, with the liver being the main organ involved. Occasionally, a xenobiotic may be excreted unchanged or changed into more dangerous form. Phases of xenobiotics metabolism The metabolism of xenobiotics has three phases (Phase I and II and phase III). In reactions of phase I, xenobiotics are generally converted to more polar, hydroxylated derivatives. In phase II reactions, these derivatives are conjugated with molecules such as glucuronic acid, sulfate, or glutathione. This renders them even more water-soluble and facilitates their excretion from the body into the urine or bile (phase III).  Phase I Hydroxylation: - It is the major phase I reaction involved that is catalyzed by members of a class of enzymes referred to as mono-oxygenases particularly cytochrome p450 species. -Monooxygenases (e.g., cytochrome p450 species) split oxygen (O2) molecule into two ions and adding one atom into the substrate or xenobiotic (RH) to give its hydroxylated form (R-OH) and add the other ion into two protons to give water. -RH + O2 + reduced cytp450 (NADPH.H+)  R-OH + H2O + oxidized cytp450 (NADP)  cytp450 - RH Substrate, Fe3+ RH NADP + FADH2 e- Reductase1 Reductase2 NADPH.H+ FAD cytp450 - RH cytp450 e- Fe2+ + 3+ H2O 2H Fe O2 cytp450 - RH cytp450 - RH ROH.- Reductase3 Fe2+ - O2 Fe2+ - O2 Hydroxylated Substrate Cytochrome P450 species (cytp450s): 1- Cytochrome P450 (cytp450) is the term used to describe members of the enzyme hemoprotein (like hemoglobin) superfamily that catalyses the oxidative biotransformation of lipophilic substrates to more polar metabolites. 2- Xenobiotics include, a very wide variety of drugs, food additives, insecticides,carcinogens, and pollutants (such as a mixture of polychlorinated biphenols).Certain endogenous compounds such assteroids,fatty acids, cholesterol, retinoids, eicosanoids and bile acids are also metabolized by these enzymes. 3- Cytochrome P450 enzymes metabolize approximately 50% of the ingested drugs. 4- There are about 14 families of the enzyme consist of up to 60 distinct isoforms of cytp450 in human. 5- Cytochrome p450 is the most diverse biocatalyst known 6- Most of isoforms of cytochrome P450 are inducible. For instance, the administration of Phenobarbital or other drugs that causes hypertrophy of endoplasmic reticulum lead to increase in the amount of cytp450 within 4-5 days three to four folds. -In the endoplasmic reticulum (i.e., microsomes) cytp450 activity is dependent upon a flavoprotein, NADPH-cytp450 reductase that functions in the transfer of electrons from NADPH to cytp450 substrate complex and requires binding to lecithin. 7- There are two types of cytp450: a- Mitochonderial which inactivates O2 molecules b-Microsomal which hydroxylates xenobiotics and mostly present in the endoplasmic reticulum of the liver 9- Certain isoforms of cytp50 are particularly involved in the metabolism of polycyclic aromatic hydrocarbons (PAHs). 10- In the lung, these enzymes may be involved in the conversion of inactive PAHs (procarcinogens) inhaled by smoking to active carcinogens by hydroxylation reaction 11- Although cytp450 reaction aims at detoxification of xenobiotic, its reaction under some circumstances may activate xenobiotics (e.g., procarcinogens) to more reactive carcinogen, mutagen and/or cytotoxic products (e.g., ultimate carcinogen).  Phase II Five types of phase II reactions: 1. Glucuronidation: - A major mechanism is through conjugation of xenobiotics and endogenous compounds with glucuronic acid catalyzed by UDP-glucuronyl transferase in cytosol and endoplasmic reticulum. Glucuronidation of xenobiotics includes acetylaminofluorene (a carcinogen), chloramphinicol (an antibiotic) aniline, benzoic acid, meprobamate (a tranquilizer) and phenol. It also helps excreting endobiotics such as many steroid and thyroid hormones and bilirubin. Glucuronidation generally results in the formation of products that are more water-soluble and that are more readily excreted by renal or hepatic processes. 1. Sulfation: - Some alcohols, arylamines, cyanide and phenols are sulfated. - The sulfate donor in these and other biologic sulfation reactions (e.g., sulfation of steroids glycosaminoglycans, glycolipids, and glycoproteins) is 3-phosphate-adenosine-5-phospho-sulfate (PAPS), this compound is called “active sulfate”. It is catalyzed by sulfotransferases (sulfatases). 2. Conjugation with glutathione: - Glutathione (-Glutamyl-cysteinyl-glycine) is a tripeptide consisting of glutamic acid, cysteine and glycine. - A number of potentially toxic electrophilic xenobiotics (such as some drugs, carcinogens, leukotrienes and lipid peroxides) are conjugated to the reducing glutathione (GSH) in reactions that can be represented as follows: R + GSH  R – S – G, - 4. Acetylation: - Acetylation is represented by, X + Acetyl-CoA  Acetyl-X + CoA, where X represents a xenobiotic. -As for other acetylation reaction, acetyl-CoA (active acetate) is the acetyl donor. - The reaction is catalyzed by the enzyme acetyl transferase present in the cytosol of various tissues, particularly liver. -Acetyl transferases are involved in the bioactivation and inactivation of a wide variety of arylamines, hydrazine, carcinogens, and drugs such as isoniazid (antitubeculosis). - Individuals are of two types, rapid and slow acetylators that should be considered on drug prescription (due to polymorphism in acetyl transferases). 5. Methylation: - A few endo- or xeno-biotics are subjected to methylation by methyl-transferases, employing s-adenosyl-methionine as the methyl donor, such as estrogens and catecholamines.  Phase III It is the active transport of the conjugated products of phase II into bile and/or urine by specific membrane transporters. Defective transport leads to accumulation of these compounds in the body. An example is the chronic idiopathic jaundice (Dubin- Johnson Syndrome), an autosomal recessive disorder due to inability of liver cells to excrete conjugated bilirubin (leads to conjugated hyperbilirubinemia) and estrogens into bile. N.B.: There are other pathways involved in drug metabolism such as deamination. The enzymes that catalyze the deamination of amines are monoamine oxidase, plasma amine oxidase and diamine oxidase.  Factors affecting activity of xenobiotic metabolizing enzymes i. Species differ in activities of these enzymes. ii. Individuals due to genetic factors vary in some enzyme activities. iii. Age and sex affect activities of some of these enzymes. iv. Certain xenobiotics cause enzyme induction. v. Metabolites of certain xenobiotics cause enzyme inhibition. vi. They are under hormonal and cytokine control.  Safe Cyt. P450 Reactive Conjugation Xenobiotic Non-reactive Metabolite Metabolite Excretion Covalent Urine Binding to Sweat Macromolecules Milk Cancer Mutation Damage Bile, DNA Feces Cell Lipids Death Cell Injury Protein Antigenic Change Allergy, Autoimmune Diseases