Biotransformation

IntroductionForeign and potentially toxic compounds absorbed into biological systems are generally lipophilic substances. They are therefore not ideally suited for excretion.In the biotransformation of foreign compounds, the body attempts to convert such lipophilic substances into more polar, and consequently, more readily excreted metabolites.The conversion to water-soluble chemicals through biotransformation is catalyzed by multiple enzymes primarily in the liver with contributions from other tissues.Drug to inactive metabolite or drug to active metabolite such as benzodiazepines. 1Highly lipophilic substances such as polybrominated biphenyls and DDT are poorly excreted and therefore may remain in the animals body for years.The enzymes involved in biotransformation also have a particular subcellular localization: many are found in the smooth endoplasmic reticulum (SER).Metabolism can be simply and conveniently divided into two phases: phase 1 and phase 2.

The half life of DDT is 75 years. The smooth endoplasmic reticulum (abbreviated SER) has functions in several metabolic processes. It synthesizeslipids, phospholipids and steroids. The smooth endoplasmic reticulum also contains the enzymeglucose-6-phosphatase, which convertsglucose-6-phosphateto glucose, a step ingluconeogenesis2

Example of Phase 1 and Phase 2 Biotransformation The foreign molecule benzene is a highly lipophilic molecule, which is not readily excreted from the animal except in the expired air, as it is volatile.The Phase 1 metabolism converts benzene into a variety of metabolites, but the major one is phenol. The insertion of a hydroxyl group allows a phase 2 conjugation reaction to take place with the polar sulfate group being added. Phenyl sulfate, the final metabolite is very water soluble and is readily excreted in the urine.(Principles of Biochemical Toxicology, John A. Timbrell)

(Principles of Biochemical Toxicology, John A. Timbrell)

http://goo.gl/MQPXUz phase 1 and phase 2 metabolism

Phase 1 metabolismThe major phase 1 reactions are oxidation, reduction, and hydrolysis, exposing or introducing a functional group (-OH) to increase reactivity and slightly increase hydrophilicity. Cytochrome P450:Cytochrome P450 (CYP) enzymes are the most important in biotransformation in terms of the catalytic versatility and number of xenobiotics that it metabolizes: 400 isozymes and 36 families.

Isozymes are enzymes that differ in amino acid sequence but catalyze the same chemical reaction.7Cytochrome P450 isozymes are localized in the smooth endoplasmic reticulum of the cell.P450s have an absolute requirement for molecular oxygen.Oxidations include hydroxylations and dealkylations.Has multiple CYP Families.Example of CYP 450 families are 1A2, 2C9, 2D6, 3A4. CYPs are heme-containing proteins.

Dealkylationis the removal of alkyl groups from a compound. Analkyl groupis a piece of a molecule with the general formula CnH2n+11A2, Theophylline Acetaminophen, smoke aromatic hydrocarbons.8

OxidationThe majority of these reactions are catalyzed by one enzyme system, the cytochrome P450 mono-oxygenase.The liver has the highest concentration of this enzyme although it can be found in most, if not all tissues.The reactions catalyzed also require NADPH, molecular oxygen and magnesium,

(Introduction to toxicology, John Timbrell)

Nicotinamide adenine dinucleotide phosphate cofactor, Alcohol dehydrogenase is another enzyme helping in the process of oxidation.

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ReductionReductases in the microflora present in the gastrointestinal tract has an important role in the reduction of xenobiotics. There are a number of different reductases, which can catalyze the reduction of azo and nitro compounds. Reduction of the azo dye prontosil to produce the antibacterial drug sulfanilamide is a well-known example of azo reduction.-During the 1930's medicine was revolutionized by the discovery that Prontosil, an industrial red dye, could cure bacterial infections. A small particle in the cytoplasm of a cell, typically consisting of fragmented endoplasmic reticulum to which ribosomes are attached. 5-alpha reductaseDihydrofolate reductaseHMG-CoA reductaseMethemoglobin reductaseRibonucleotide reductaseThioredoxin reductaseE. colinitroreductaseMethylenetetrahydrofolate reductase

10(Principles of Biochemical Toxicology, John A. Timbrell)

Phase 2 MetabolismPhase 2 reactions include glucuronidation, sulfation, acetylation, methylation, conjugation with glutathione, and conjugation with amino acids (glycine, taurine and glutamic acid) that strongly increase hydrophilicity.

SulfationThe addition of the sulphate moiety to a hydroxyl group is a major route of conjugation for foreign compounds. It is catalyzed by a cytosolic sulphotransferase enzyme and utilizes the coenzyme phosphoadenosine phosphosulphate.The product is an ester which is very polar and water soluble.

(Introduction to toxicology, John Timbrell)

Thecytosolor intracellular fluid (ICF) or cytoplasmic matrix is the liquid found inside cells. Coenzymesare small molecules. They cannot by themselves catalyze a reaction but they can help enzymes to do so. In technical terms,coenzymesare organic nonprotein molecules that bind with the protein molecule (apoenzyme) to form the active enzyme (holoenzyme)13

GlucuronidationGlucuronic acid is a polar and water soluble carbohydrate molecule which may be added to hydroxyl groups, carboxylic acid groups, amino groups and thiols.This process is a major route of phase 2 metabolism and utilizes glucuronosyl transferases, which are microsomal enzymes, with uridine diphosphate glucuronic acid as the cofactor.

(Introduction to toxicology, John Timbrell)

Epoxides, which can be stable metabolic intermediates,may undergo hydration catalyzed bythe enzyme epoxide hydrolase located in themicrosomal fraction.14

ReferencesTimbrell, J. A. 2009. Principles of Biochemical Toxicology. Informa Healthcare(USA).Timbrell, J. 2002. Introduction to Toxicology. Taylor & Francis Inc, London.www.xenotechllc.com/chapter-6nature.berkeley.edu/~dnomura/pdf/Lecture4MetabolismPhaseIv2.pdfnature.berkeley.edu/~dnomura/pdf/Lecture5MetabolismPhaseII.pdf