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

59-291 Section 1, Lecture 4. Phase I reactions-oxidative occur in the endoplasmic reticulum of liver (microsomal fractions) - catalyzed by the microsomal P450 monooxygenase system. Phase I Biotransformation. Oxidative reactions Catalyzed by enzymes of microsomal fraction (ER) of liver

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

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  1. 59-291 Section 1, Lecture 4 Phase I reactions-oxidative occur in the endoplasmic reticulum of liver (microsomal fractions) - catalyzed by the microsomal P450 monooxygenase system

  2. Phase I Biotransformation • Oxidative reactions • Catalyzed by enzymes of microsomal fraction (ER) of liver • Microsomal cytochrome P450 monooxygenase

  3. P450 Enzyme Complex Microsomal cytochrome P450 monooxygenase system is a family of enzymes that biotransforms many drugs Fe3+ Fe2+ Hemoprotein Fe3+ Only Heme-Fe2+ can bind O=O Fe2+-O=O

  4. -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

  5. Hydrolytic reactions: Cytoplasmic Enzymes Catalyze the hydrolysis (add H2O) or reduction of drugs Reductive Drugs

  6. 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

  7. Phase II Biotransformation -here drugs are conjugated (added) to other molecules Acetylation- by N-acetyltransferase enzymes Glucuronide formation- adds carbohydrate making drugs more soluble- via glucosyltransferases

  8. Sulfation- catalyzed by sulfotransferases- sulfated drugs retain bioactivity

  9. 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 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

  10. 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.

  11. 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

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