Lecture 5: Cytochromes P450 Flashcards
Cytochromes P450
-powerful detox system
-steroid hormone synthesis
-polyunsaturated fatty acid metabolism
-certain proteins in the ER
-proteins in liver metabolize potential toxins
P450 Detox system
-first line of defense against toxins
-susbstrates are unusual chemicals (drugs, poisons)
-convert compounds to be more easily excreted
Super family of P450
-dif ones act on molecules with dif structure
-1000s identified in the organism domains
Plant P450
-need for biosynthesis of pigments and toxins for protection
-used in metabolic efforts for large-scale medicine production
Nomenclature
Superfamily > family (CYP#)> subfamily (letter)> members (#)
P450 families
-sequence identity > 40%
-CYP1, CYP2, CYP3, etc
P450 subfamilies
-sequence identity > 55%
-CYP1A, CYP1B, CYP1C, etc
P450 individual members
-numbered
-CYP1A1, CYP1A2, CYP1A3
Human P450
-18 families
-41 subfamilies
-majority CYP1-4
-other families rarely induced
CYP1-4
-4 inducible (diet drugs etc) human P450 families
-have several members
-metabolize eicosanoid
Cytochromes P450 enzymatic function
-monooxygenases
-catalyze O from radical, one O to product, one to water
-product more hydrophilic=better substrate for detoxifying
-requires heme iron
Endogenous P450 substrates
-cholesterol
-steroid hormones
-fatty acids
-higher substrate specificity than exogenous
P450 substrates
endogenous vs exogenous
-vary in how many substrates each can metabolize
Exogenous P450 substrates
-drugs
-food additives
-environmental contaminants
P450 biological functions
-synthesis: of steroid hormones, vit A and D, lipid-like eicosanoid molecules involved in signaling
-metabolism: of fatty acids and eicosannoids
-Detoxification
-Hydroxylation
Hydroxylation
-increases aqueous solubility of lipid-soluble substrates
Substrate variety
= reaction variety
Drug interactions of cytochrome P450
-responsible for >70% of clearance of drugs
-CYP3A4 acts on 30-50% of most drugs
-some reactions harmful: acetaminophen
CYP3A4A
-liver
-act on 30-50% of drugs (ex erythromycin (antibiotic))
-catalysis of acetaminophen generates highly reactive compound that causes toxicity at high doses
P450 structure
-integral membrane protein with single heme group
-anchored to membrane by N-terminal helix
-well conserved
-conformational changes can occur in binding
P450 heme group
-site of oxidation
-iron coordinated with porphyrin ring and protein
-6th coord site is open or occupied by O2 or other ligands
-on top of heme just above orange sphere
P450 Catalytic cycle
- water and iron3
- substrate removes water
- iron3 to iron 2 via e-
- O2 added = oxy-ferrous (one O has -1 charge)
- e- transfer (O-1 to O-2) = peroxanion
- Compound I
DOG just look at the slide 7
Reactions catalyzed by P450
-aliphatic and aromatic hydroxylation
-epoxidation
-dealkylation
-N-oxidation
-all add oxygen
aliphatic hydroxylation
R-CH2-CH3 –> R-CH2-CH2OH
Aromatic hydroxylation
add OH to ring
Epoxidation
add oxygen to alkene and makes triangle
Dealkylation
R-CH2-NH-CH3 –> R-CH2-NH-CH2-OH –> R-CH2-NH2 + HCHO
-get rid of carbon
-N or O or S dealkylation
N-oxidation
add OH to NH2
Lipophilic xenobiotics
-drugs, food additive, environment
-oxidized by P450
-exogenous compounds
P450 oxidation of xenobiotics
-exogenous compounds
-promotes elimination
-phase 1 metabolism
-phase 2 biosynthesis rxs like linking to glutatione, sulfate, etc
-CYP3A4
CYP3A4
-GI tract, liver
-responsible for poor bioavailability of some drugs
P450 isoforms of CYP3A4
-less discriminating
-variety of lipophlic substrates
-multiple sites of oxidation
-lower regioselectivity
Metabolism of xenobiotic outcomes
-inactivation (drug metabolism)
-activation (prodrug conversion)
-formation of highly toxic metabolite (like benzoapyrene from cig burning)
Benzo[a]pyrene
-weak carcinogen
-metabolized multiple times to make stronger carcinogen
Steroid hormone pathways
-endogenous substrates
-higher P450 specificity
-can involve several P450s
-one P450 can catalyze multiple steps
-SLIDE 15 mech
Induction of P450
decreased drug plasma levels
inhibition of P450
increased drug plasma levels
Drug-drug interactions
-unintended effects can occur due to altering P450 activity
-problematic for drugs requiring tight control of plasma levels
-vital importance to patient, critical side effects, small therapuetic index
CYP3A4 induction as cause of drug interactions
-upregulation increases rate of metabolism of drugs in liver
-Rifampicin
-St. John’s wort
Rifampicin (induction)
-antituberculosis drug
-inducer of CYP3A4
-makes many drugs less effective
-increases elimination of warfarin which might increase risk of undertreating anticoagulation
St. John’s-wort
-herbal med for depression
-induces CYP3A4 and causes multiple adverse drug reactions
CYP3A4 inhibition as a cause of drug interaction
-certain drug inhibits activity (erythromycin/ketoconazole)
-patient takes terfenadine for allergies
-increased plasma levels that can cause serious cardiac problems
Terfenadine (seldane)
-H1 antihistamine for seasonal allergies
-pro drug rapidly metabolized by CYP3A4 into fexofenadine (the active compound)
-CYP3A4 inhibition can lead to cardiac problems
-avoid risk by using drug replaced by fexofenadine (allegra)
NAPQI
-generated in a small amount from metabolization of acetaminophen by CYP2E1
-normally conjugated with glutathione (GSH) to generate non toxic form
Acetaminophen-induced liver toxicity
-high doses increase NAPQI and can deplete glutathione pool
-NAPQI not conjugated = toxic
-35% of liver failure cases
CYP2E1
-metabolizes acetaminophen
-induced by alcohol = more NAPQI
-BUT alcohol is one of its substrates so it can inhibit the metabolism of other drugs
-effect depends on time of drinking and taking acetaminophen
Genetic Polymorphisms of P450: CYP2A6
-nicotine metabolism
-SLIDE 21
