Cytochrome p450

Substrates of CYP450

Cholesterol steroid hormones fatty acids drugs food additives pesticides chemicals inhaled ingested or absorbed

Main functions of CYP450

Produce steroid hormones metabolize fatty acids PG leukotrienes retinoids activate or inactivate drugs form reactive metabolites cause drug interactions

CYP450 structure

Integral membrane protein with single heme group

Heme iron bonds

4 bonds to pyrrole nitrogens 2 axial ligands

Reason for P450 name

Absorbance peak at 450 nm when CO binds to reduced heme

General CYP450 reaction

Substrate reacts with oxygen NADPH and H plus forming hydroxylated product SOH

Purpose of hydroxylation

Increase water solubility for excretion

Location of most CYP metabolic activity

Liver

Second most important organ for drug metabolism

Small intestine

Other significant CYP locations

Nasal mucosa lungs kidneys CNS

First pass organs for orally administered drugs

Gut and liver

Cellular location of xenobiotic metabolizing enzymes

Endoplasmic reticulum and cytosol

Enzymes located in ER

CYPs FMOs epoxide hydrolases some phase II enzymes

Definition of xenobiotic

Foreign compound not naturally produced by body

Most abundant drug metabolizing CYP

CYP3A4

Percentage of drugs metabolized by CYP3A4

50 percent

Drugs metabolized by CYP2D6

20 percent of therapeutic drugs

Drugs metabolized by CYP2C9 and CYP2C19

15 percent

CYPs mainly involved in drug metabolism

CYP2C CYP2D CYP3A

CYPs mainly involved in activation of protoxins

CYP1A CYP1B CYP2A CYP2B CYP2E

Diazepam metabolism pathway

CYP2C19 to nordazepam then CYP3A4 to oxazepam

Meaning of multiple CYP metabolism

One drug can be metabolized by several CYPs at different sites

Inactive metabolite of diazepam

Oxazepam

Goal of Phase I reactions

Introduce or expose functional groups

Functional groups introduced in Phase I

Hydroxyl carboxyl SH oxygen or amine

Major enzymes in Phase I

CYPs FMOs epoxide hydrolases

Types of Phase I reactions

N dealkylation O dealkylation aromatic hydroxylation N oxidation S oxidation deamination dehalogenation

Flavin monooxygenases role

Oxidize soft nucleophiles such as nitrogen sulfur and phosphorous

FMO cofactor

FAD

FMO location

Endoplasmic reticulum

FMO function

Detoxify drugs by N oxidation S oxidation

FMO difference from CYP

Not inducible by smoking or alcohol and generates fewer reactive species

Example substrates of FMO

Cimetidine ranitidine nicotine

Epoxide hydrolase function

Converts reactive epoxides into safer diols

Location of microsomal epoxide hydrolase

ER membrane

Location of soluble epoxide hydrolase

Cytosol

Role of epoxide hydrolases

Protect against DNA protein and lipid damage from epoxides

Examples of epoxide forming toxins

Benzo a pyrene aflatoxin vinyl chloride

Clinical significance of epoxide hydrolase

Prevents carcinogenesis and organ toxicity

Purpose of Phase II reactions

Increase solubility and prepare for renal or biliary excretion

Main Phase II reactions

Glucuronidation sulfation acetylation methylation glutathione conjugation amino acid conjugation

Most common Phase II reaction

Glucuronidation

Glucuronidation enzyme family

UGT UDP glucuronosyltransferase

Cofactor for glucuronidation

UDP glucuronic acid

Common glucuronidated drugs

Morphine bilirubin chloramphenicol

Clinical relevance of UGT1A1 deficiency

Gilbert syndrome and Crigler Najjar syndrome

Sulfation enzyme family

SULT

Cofactor for sulfation

PAPS

Sulfation substrate preference

Low concentration high affinity

Glucuronidation vs sulfation

High concentration favors glucuronidation

Acetylation enzyme

NAT1 and NAT2

Slow acetylator clinical risks

Isoniazid toxicity peripheral neuropathy lupus hepatotoxicity

Ethnicity linked to slow acetylation

African and European descent

Rapid acetylators clinical issues

Poor drug response more rapid clearance

Methylation enzymes

COMT TPMT PNMT

Glutathione conjugation enzyme

GST glutathione S transferase

Glutathione conjugation function

Detoxify reactive electrophiles prevent oxidative damage

N acetylcysteine therapeutic role

Replenishes glutathione in acetaminophen overdose

Amino acid conjugation purpose

Excrete aromatic acids such as benzoic acid via glycine conjugation

Definition of CYP induction

Increased enzyme synthesis leading to faster drug metabolism

Common CYP inducers

Rifampin carbamazepine phenytoin phenobarbital St Johns wort smoking

Effect of CYP induction

Lower drug levels risk of treatment failure

Definition of CYP inhibition

Reduced metabolism leading to higher drug levels

Common CYP inhibitors

Macrolides azole antifungals cimetidine omeprazole isoniazid grapefruit juice

Effect of CYP inhibition

Toxicity due to drug accumulation

Grapefruit juice inhibits

CYP3A4 in the intestinal wall

Smoking induces

CYP1A2

CYP1A2 metabolizes

Theophylline caffeine clozapine

Smoking effect on theophylline

Increases clearance requires higher dose

Alcohol chronic use induces

CYP2E1

CYP2E1 substrates

Acetaminophen ethanol anesthetics

Binge drinking effect on CYP2E1

Acute inhibition increasing drug toxicity

Chronic alcohol plus acetaminophen risk

Increased NAPQI formation causing hepatotoxicity

Rifampin induction effect

Reduces efficacy of oral contraceptives antiretrovirals and warfarin

Macrolide inhibition effect

Raises levels of calcium channel blockers statins and warfarin

CYP2D6 polymorphism

Poor intermediate extensive and ultra rapid metabolizers

Ultra rapid metabolizer risk

Codeine converted too quickly to morphine causing respiratory depression

Poor metabolizer risk

No analgesic effect of codeine

CYP2C19 poor metabolizer effect

Reduced activation of clopidogrel treatment failure

CYP2C19 ultra rapid metabolizer effect

Excessive drug activation higher bleed risk

CYP2C9 polymorphism effect

Poor warfarin metabolism increases INR bleeding risk

CYP3A5 polymorphism

Expression varies alters tacrolimus dosing in transplant patients

NAT2 polymorphism

Determines slow or fast acetylator status

TPMT polymorphism

Low TPMT increases risk of bone marrow toxicity with azathioprine

Most common reactive intermediates

Epoxides quinones free radicals

Role of CYP2E1 in toxicity

Generates reactive oxygen species and toxic metabolites

Acetaminophen toxic metabolite

NAPQI

NAPQI detoxification

Conjugated by glutathione

NAPQI accumulation cause

Glutathione depletion in overdose or alcohol use

Aflatoxin B1 metabolism

CYP converts to epoxide causing liver cancer

Benzopyrene activation

CYP1A1 forms carcinogenic epoxide

Vinyl chloride metabolism

Forms chloroethylene oxide causing angiosarcoma of liver

Carbon tetrachloride toxicity

Converted to free radicals causing lipid peroxidation

Halothane toxicity

CYP metabolism forms reactive intermediates causing hepatitis

First pass metabolism importance

Reduces oral bioavailability of many drugs

Drugs with high first pass metabolism

Morphine propranolol nitroglycerin

Drug bioavailability increased by

CYP inhibition liver disease low first pass

Drug clearance increased by

CYP induction and enhanced Phase II

Disease states reducing drug metabolism

Liver cirrhosis fatty liver heart failure malnutrition

Age and metabolism effect

Newborns have low Phase I and Phase II activity