Metabolism - Biotransformation Supplementary readings: Casarett and Doull,Chapter 6 Timbrell, Chapters 4 and 5

Non-polar (lipophilic) Hydrophobic XENOBIOTIC Phase I Metabolism Oxidation Can accumulate in tissues Solubility in lipids INTERMEDIATE METABOLITE Phase II Metabolism Conjugation Solubility in water May be reactive/toxic WATER-SOLUBLE METABOLITE Lipophobic Hydrophilic (Polar) ELIMINATION

What is a xenobiotic ?

Phase I reactions Chemical modification of xenobiotics Introduces or uncovers polar functional groups that provide sites for Phase II metabolism Major classes of reaction: Oxidation Reduction Hydrolysis

Overview of oxidations, reductions, hydrolyses Loss of electrons M M+ + e- Gain of oxygen R + O RO

Oxidation reactions Hydroxylation

Epoxidation

Overview of oxidations, reductions, hydrolyses Gain of electrons M+ + e- M Loss of oxygen RO R + O Gain of hydrogen R + H RH

Reduction Cr6+ + 3 e- Cr3+ Nitro to amino group Chromium VI to Chromium III Cr6+ + 3 e- Cr3+

Hydrolysis Addition of water Cleavage of R-O or R-N bond accompanied by addition of H2O R’-O-R + H2O R’-O-H + R-OH R’-N-R + H2O R’-N-H + R-OH H H

Principal Phase I enzymes Cytochrome P450 Flavin monooxygenase Monoamine oxidase Esterases Amidases Hydrolases Reductases, dehydrogenases, oxidases

Flavin monooxygenase Flavoprotein Mixed-function amine oxidase Located in smooth endoplasmic reticulum, in human, pig, rabbit liver, guinea-pig lung, human kidney Uses NADPH as a source of reducing equivalents Not inducible

Overall reaction R-H + O2 + NADPH + H+ R-OH + H2O + NADP+

Monoamine oxidase Metabolizes endogenous monoamine neurotransmitters Uses NADPH as a source of reducing equivalents Found in the endoplasmic reticulum and in mitochondria, of nerve endings and liver

Esterases Hydrolyse esters to carboxylic acid and alcohol functional groups Non-specific esterases in plasma, more substrate-specific forms in liver cytosol

Amidases Hydrolyse amides to carboxylic acids and amines (or ammonia) Found in plasma and in liver cytosol

Hydrolases Hydrolyse ethers

Reductases, dehydrogenases, oxidases In cytosol, endoplasmic reticulum, mitochondria

Cytochrome P450 Heme protein Terminal oxidase of the mixed-function oxidase (MFO) electron-transfer system Located in the smooth endoplasmic reticulum of all major organs and tissues Uses NADPH as a source of reducing equivalents Inducible