Detoxification Mechanisms PDF

XENOBIOTIC METABOLISM “DETOXIFICATION MECHANISMS” Dr. Önder Şirikçi  OUTLINE Introduction Absorbtion Distribution Metabolism (Biotransformation) – Phase 1 reactions – Phase 2 reactions Excretion  TERMINOLOGY – X...

XENOBIOTIC METABOLISM “DETOXIFICATION MECHANISMS” Dr. Önder Şirikçi  OUTLINE Introduction Absorbtion Distribution Metabolism (Biotransformation) – Phase 1 reactions – Phase 2 reactions Excretion  TERMINOLOGY – Xenobiotic (Biology of the foreign) – Detoxification term is not always correct…  sometimes the metabolites of xeno compounds that are themselves inert or harmless become reactive or biologically active. This may be desirable, as in the activation of a prodrug to an active compound, or it may be undesirable, as in the formation of a carcinogen or mutagen from an inert precursor.  Xenobiotics chemicals that enters the body via GIS, lungs, skin or mucosa compounds in plant foods, compounds in medicines food additives, environmental pollutants, household chemicals, cosmetics, agricultural chemicals, bacterial compounds BIOTRANSFORMATION & EXCRETION  Aim of biotransformation More than 200,000 manufactured environmental chemicals exist Foreign, toxic compound metabolite that are less toxic that are more polar which can be more readily excreted from the body  How are the drugs introduced into the body? Enteral (Oral; from the intestinal system; Major) Parenteral (Other than the intestinal system) – Subcutaneous injection – Intravenous injection – Intramuscular injection – Inhalation – Skin/mucosa contact Four stages of drugs in the body Absorbtion Distribution Metabolism Excretion  Absorbtion Drugs and other chemicals are absorbed from the digestive tract by: a) Passive diffusion (major) b) facilated diffusion c) active transport Hydrophobic compounds can easily penetrate cell membrane, neutral drugs are absorbed more easily than charged and/or hydrophilic compounds Acidic drugs are absorbed from the stomach, whereas basic drugs are absorbed from the intestine.  Distribution Drugs and chemicals that have entered the blood (Tetracyclines, chloramphenicol, penicillin, barbiturates, sulfonamides) are bound to the albumin and transported (drug-protein complex) Drug-protein complex dissociate Drug enters the cell  Distribution Tetracyclin: stored in bones and teeth Thiopental: stored in adipose tissue Iodine: stored in thyroid gland Fat soluble drugs: pass blood brain barrier Liver: Main site for the biotransformation  Metabolism (Biotransformation) 1) Forms an inactive metabolite from an active drug 2) Forms an active drug from an inactive drug (Pro- drug, procarcinogen) 3) Forms an active drug from another active drug Codeine Morphine Diazepam Oxazepam 4) Forms a toxic metabolite from a less toxic drug Isoniazid Acetylation + pyridoxine deficiency  Xenobiotic Metabolism (2 phases) How many phases dose xenobiotic metablosim Phase 1: Chemical modification hve Phase 2: Conjugation after chemical modification Results in elimination (excretion)  Phase 1 Phase 1 metb. renders compounds more reactive, introducing groups (-OH) that can be conjugated with glucuronic acid, sulfate, acetate, glutathione, or amino acids in phase 2 metabolism; producing water soluble polar compounds that can readily be excreted in urine or bile. Very hydrophobic xenobiotics would persist in adipose tissue indefinitely if not converted to more polar forms.  Phase 1 the major reaction involved is hydroxylation by members of monooxygenases or cytochromes P450. Which one of below is not a phase 1 Also reactions of deamination, reaction ? dehalogenation, desulfuration, epoxidation, peroxygenation, and reduction. Reactions involving hydrolysis (eg, catalyzed by esterases) and certain other non-P450- catalyzed reactions also occur in phase I. Metabolism (Biotransformation) Xenobiotic a) Oxidation Hydroxylation Dealkylation Deamination b) Reduction c) Hydrolysis Primary metabolites Glucuronidation Sulfation Methylation Acetylation Glutathione conjugation Glycine conjugation Glutamine conjugation Secondary metabolites Altern Med Rev 1998;3(3):187-198  Phase 1 Reactions Oxidation Addition of polar and reactive group to the molecule can subsequently undergo conjugation OXIDATION NON-MICROSOMAL OXIDATION MICROSOMAL OXIDATION  Non-microsomal oxidation Cytoplasmic and mitochondrial enzymes Monoamine oxidases (monoamine oxidation) NAD-linked alcohol dehydrogenase (alcohol oxidation) NAD-linked aldehyde dehydrogenase (aldehyde oxidation)  Non-microsomal oxidation Amine oxidation The most important enzyme: monoamine oxidase; remove amino groups using oxygen Outer mebrane of the mitochondria, rich in liver, heart, CNS, vascular tissue Flavoproteins (contain FAD or FMN as co- factors Important in the metabolism of endogenous compounds: 5-hydroxytryptamine (serotonin)  Non-microsomal oxidation Alcohol and aldehyde oxidation Alcohol and aldehyde dehydrogenase Nicotinamide; Niacin (vit B3)  Microsomal Oxidation Aromatic hydroxylation Aliphatic hydroxylation Alicyclic hydroxylation Heterocyclic hydroxylation N-, S-, O-dealkylation N-oxidation N-hydroxylation S-oxidation Desulphuration Deamination Dehalogenation  Microsomal Oxidation Includes hydroxylation reactions cytochrome P450 containing microsomal enzyme system mixed function oxidase/monooxygenase One atom of oxygen is introduced into xenobiotics, the other atom is reduced to water System is located in smooth endoplasmic reticulum membranes (liver, kidney, lung, intestine) highest  Cytochrome P450 monooxygenase system Hemoproteins (Iron protoporphyrin IX as prosthetic group) Possess broad substrate specificity Cyt-P450 reductase is required for regeneration Phospholipids are required in the enzyme complex:for the interrelationship between cyt P-450 and the reductase Cyt-P448 is Aromatic hydrocarbon hydroxylase (PAHH) Adrenal glands: CYP450 is located in mitochondria (adrenodoxin/adrenodoxin reductase)  Cytochrome P450 monooxygenase system 50% of the common drugs are metabolized by isoforms of cyt P450. Also important in the metabolism of steroid hormones, hydroxylation of vit D to 25-0H, carcinogens, and pollutants. The major cytochromes P450 in drug metabolism are members of the CYP1, CYP2, and CYP3 families.  Microsomal Oxidation RH (Substrate) + O2 + NADPH + H+ ROH (product) + H2O + NADP+ Aromatic Hydroxylation Aromatic and Aliphatic Hydroxylation Epoxidation following aromatic hydroxylation Epoxides are lipophilic and electrophilic and therefore important metabolites Reaction with cellular constituents N-, S-, O-dealkylation N-, S-, O-dealkylation Deamination Amphetamine Phenylacetone N-Oxidation Acetylaminofluorene  Clinical implications in the induction of CYP450 system Drug interaction: when the effect of one drug is altered by prior Warfarin (anticoagulant): metabolized by CYP2C9 Phenobarbital (epilepsy treatment): inducer of CYP2C9 If the dose of warfarin is not changed after 5 days or so, The levels of CYP2C9 will be elevated Warfarin will be metabolized more quickly The dose will not be adequate and warfarin will not be effective  Reduction Enzymes located in microsomal fraction (Cytochromes P-450 and flavoproteins) and soluble cell fraction NADPH/NADH is the main coenzyme for the reaction 1. Prontosil Sulphanilamide 2. One-electron reduction of adriamycin (quinone) by NADPH cytochrome P-450 reductase  Reduction of ketones Anticoagulant  Hydrolysis Aspirin Salicylic acid Procainamide antiarrhythmic  Phase 2 Reactions CONJUGATION  Addition of endogenous groups (polar and readily available) to xenobiotics  These endogenous groups are added to a suitable functional group present on the foreign molecule or introduced by phase 1 metabolism Product is more polar, less lipid soluble Increases excretion Reduces toxicity  Glucuronidation Transfer of glucuronic acid in an activated form as uridine diphosphate glucuronic acid (UDPGA) to hydroxyl, carboxyl, nitrogen, sulphur and carbon atoms. UDPGA formation in the cytosol  Glucuronidation UDP-glucuronosyl transferase (glucuronyl transferase) catalyzes the conjugation Located in ER and in many tissues including liver Exists in four or more forms Different forms of glucuronyl transferase and their substrates Form A 1-napthol Form B Bilirubin Form C Estrone Form D Morphine Estrone, E1 Bilirubin glucuronidation Bilirubin metabolism  Sulphate Conjugation Substrates include aliphatic alcohols, phenols, aromatic amines, steroids and carbohydrates Sulphate donor: 3’-phosphoadenosyl-5’-phosphosulphate (PAPS; formed from inorganic sulphate and ATP) Sulphate conjugation is catalyzed by sulphotransferase enzyme located in the cytosol and found in the liver, GI mucosa and kidney Sulphate esters are much more soluble Usually pharmacologically inactive  Sulphate Conjugation Sulphotransferase + PAPS Phenyl sulphamate Aniline  Glutathione Conjugation Glutathione is a tripeptide composed of glutamic acid, cysteine and glycine Binds to the substrate through nucleophilic cysteine group Glutathione conjugate may be excreted or undergo further metabolization Glutathione Conjugation  Acetylation Includes the substrates aromatic amines, sulphonamides, hydrazines, hydrazides Product is less water soluble than the parent compound. Problems with sulphanilamide when given in high doses (metabolites are crystallized out in kidney tubules, causing tubular necrosis)  Methylation Methyl donor: S-adenosyl methionine which is formed from methionine and ATP Methylation is catalyzed by methyltransferases (cytosol and ER) Tends to reduce water solubility Mercury methylation: more lipophilic and neurotoxic Paracetamol metabolism  EXCRETION Biliary excretion: Glucuronides, Sulfates, GSH-derivatives and other derivatives are secreted into bile by the liver. Some of them are reabsorbed in the intestines and they form an enterohepatic cycle. Renal excretion Respiration (Exhalation) Sweat References: Principals of Biochemical Toxicology, John Timbrell, Taylor & Francis, 3rd Edition Harper's Biochemistry, Murray, Granner, 23rd Ed., Lange