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Phase I reactions-oxidative occur in the endoplasmic reticulum of liver (microsomal fractions)

- catalyzed by the microsomal P450 monooxygenase system

59-291 Section 1, Lecture 4

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Phase I Biotransformation

• Oxidative reactions– Catalyzed by enzymes of microsomal fraction

(ER) of liver• Microsomal cytochrome P450 monooxygenase

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Microsomal cytochrome P450 monooxygenase system is a family of enzymes that biotransforms many drugs

Fe3+

Fe2+-O=O

Fe2+

Fe3+Only Heme-Fe2+ can bind O=O

P450 Enzyme Complex

Hemoprotein

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-many drugs affect the activity of p450 enzymes:

induction or inhibition

-phenobarbital and rifampin

2 drugs that induce increased transcription and translation of the P450 genes

-net result all drugs metabolized by the P450 system are destroyed and excreted faster

What are the consequences?

Drugs combine with P450 enzymes and block their activity

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Cytoplasmic Enzymes Catalyze the hydrolysis (add H2O) or reduction of drugs

Reductive Drugs

Hydrolytic reactions:

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Phase II Biotransformation• Conjugation Reactions with endogenous

substances– Acetylation

• N- acetyltransferase uses acetyl CoA as a source of acetate

– Glucuronide formation• Glucuronosyltransferase conjugates glucuronate

molecule with a drug

– Sulfation• Sulfotransferase sulfonates several drugs. The

sulfate metabolites are active

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Phase II Biotransformation-here drugs are conjugated (added) to other moleculesAcetylation- by N-acetyltransferase enzymes

Glucuronide formation- adds carbohydrate making drugs more soluble- via glucosyltransferases

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Sulfation- catalyzed by sulfotransferases- sulfated drugs retain bioactivity

R O S

O

O

OH

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Other genetic variations- 1:3000 occurrence atypical cholinesterase- will not metabolize topical anasthetic succinycholine these individuals may stay “frozen” for days or weeks- need to be screened for this enzyme

Genetic variations in drug biotransformation

-Acetylation slow r/r(Middle Eastern) ; fast r/R or R/R alleles (Japanese)

-P450- genetic variations in the CYP2D6 4-hydroxylase-Extensive metabolizers; or -poor metabolizers no pain relief from codeine prodrug for morphine catalyzed by CYP2D6 4-hydroxylase

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Practice questions(to be discussed in the class)

• 1000 g of a drug is injected to patient and Cp0 was measured, which is 10 mg/L.

• Where is this drug concentrated mainly? Why?

• Vd=Dose/Cp0 >> Vd= (1g)1000/10= 100

• Since Vd is very large for this drug, more than total volume of body compartment, it is concentrated mostly in the cells due to ion trapping.

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• Identify the phase and type of the following drug transformation– Drug-OCH3 Drug-OH + CH2O

• Phase I, O-Dealkylation

– Drug- NH2 + AcetylcoA-CO-CH3 Acetyl coA-SH + Drug-NH-CO-CH3

• Phase II, Acetylation

– Drug-CH2CH3 Drug-CHOH- CH3

• Phase I, Aliphatic hydroxylation