Catalytic cycle, Isoforms, Induction and Inhibition of Cytochrome P450.

1. CYTOCHROME P450
ENZYMES
Presented By Deshmukh Md Faizan
M. Pharm (1st Sem)
DEPARTMENT OF PHARMACEUTICAL CHEMISTRY,
R.C.PATEL INSTITUTE OF PHARMACEUTICAL
EDUCATION AND RESEARCH, SHIRPUR
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2. CONTENTS…
 Introduction of Cytochrome P450.
 Location.
 Components.
 Classification And Nomenclature of Cytochrome
P450.
 Catalytic Cycle of Cytochrome P450.
 Isoforms of Cytochrome P450.
 Induction And Inhibition of P450.
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3. INTRODUCTION...
 Humans constantly exposed to xenobiotics(usually non polar)
that can cause harmful effect if not eliminated.
 Biotransformation convert non polar compounds to
polar and helps in their elimination.
 Biotransformation occurs in 2 phases(phase1 &phase
2).
 Cytochrome p-450 is a superfamily of enzymes that
catalyse most of the oxidation reactions of phase 1.
 Require both molecular oxygen and NADPH to effect
reaction.
 Also called mixed function oxidases and mono-oxygenases. 3

4. LOCATION…
 Located in endoplasmic reticulum of hepatic cells and
throughout of the body.
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5. COMPONENTS…
 Composed of an electron transfer chain consisting of 3
components.
1. heme protein.
2. flavoprotein.
3. phosphatidylcholine.
 Reaction involving the substrate RH which yields the
product ROH, is given by the following equation:
R-H + O2 + NADPH + H+ → R-OH + H2O + NADP+
Where NADPH=Nicotinamide adenosine dinucleotide
phosphate. 5

6. NOMENCLATURE AND CLASSIFICATION…
 Membrane-bound within a cell (cyto) + heme pigment
(chrome and P) + absorbs light at 450 nm (450) when
exposed to CO.
 Classified according to their amino acid sequence:
1. Families- proteins with 40% amino acid seq
e.g: CYP2, CYP3
2. Subfamilies-member of family have 55% AAS
e.g:CYP2D, CYP3A
3. Individual genes-denoted by arabic numerical
e.g:CYP2D6, CYP3A4
CYP1A2
family subfamily individual member of that
subfamily
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7. CATALYTIC CYCLE OF CYTOCHROME P450…
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8. CYTOCHROME P450 ISOFORMS…
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 Isoforms mean same enzyme that belong to cyp450
enzyme and which having ability to metabolized
drug.
 There are several isoforms of enzyme which is
given below:-
1. CYP1A2
2. CYP2C9
3. CYP2C19
4. CYP2D6
5. CYP2E1 etc.

9. 1) CYP1A2…
 Metabolize the chemical and environmental chemicals.
 13% total hepatic content of isoenzyme.
 Drug substrate
E.g- caffein, theophylline, propanolol.
 Drug inhibitor
E.g- ciprofloxacin, cimetidine, erythromycin.
 Drug inducer
E.g- hydrocarbon, phenobarbital, rifampicin.
2) CYP2C9…
 Responsible for metabolism of ibuprofen, tolbutamide,
torsemide.
 Warfarin is mainly metabolized by this isoforms. 9

10.  Inhibitor of warfarin
E.g- fluconazole, metronidazole, amiodarone.
3) CYP2C19...
 Responsible for metabolism of diazepam, omeprazole,
lansoprazole.
 Poor metabolizers.
4) CYP2D6...
 Enzyme not inducible by drug.
 Most drug use in clinical evaluation metabolized by
this isoforms.
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11. 5) CYP2E1...
 7% total CYP content in liver.
 Sevoflurane, isoflurane metabolized by this enzyme.
 Enzyme responsible for metabolism of paracetamol and
ethanol.
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12. Isoenzyme Substrate Inhibitor Inducers
CYP1A2 Theophylline
Caffein
Propanolol etc.
Cimetidine
Ciprofloxacin
Erythromycin etc.
Hydrocarbon
Omeprazole
Phenobarbital
etc.
CYP2C9 Ibuprofen
Tolbutamide
Torsemide etc.
Fluconazole
Amiodarone etc.
Phenobarbital
Phenytoin
Rifampicin etc.
CYP2C19 Omeprazole
Daizepam
Lansoprazole
etc.
Omeprazole
Cimetidine
Fluconazole etc.
Carbamazepine
Rifampicin
Phenobarbitone
etc.
CYP2D6 Amitriptyline
Codeine
Oxycodone etc.
Haloperidol
Indinavir
Quinidine etc.
Rifampicin
Dexamethason.
CYP2E1 Paracetamole
Ethanol
Halothane etc.
Disulfiram Ethanol and
isoniazid 12

13. INDUCTION AND INHIBITION OF CYP450...
 Induction of drug metabolism can lead to unexpected
drops in drug concentration or the build-up of
metabolites. The reverse can occur when there is
inhibition of drug metabolism.
 The major organ involved in metabolism is liver and the
major enzyme system involved in drug metabolism is
CYP 450, the well-known family of oxidative hemo-
proteins. Induction CYP 450 enzymes at the liver is
responsible for induction of metabolism of many drugs.
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14. INDUCTION OF CYP450...
 Induction means ability of drug to increase the synthesis of
CYP450 enzymes is called as induction.
 Most enzyme inducers have following properties
1. They are lipophilic compound.
2. They have long elimination half life.
3. They are substrate for inducted enzyme system.
 Mechanisms involved in enzyme induction are
1.Increase in both liver size and liver blood flow.
2.Increase stability of enzymes, synthesis of CYP450.
3.Decrease degradation of CYP450.
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15. TABLE:- INDUCERS OF DRUG METABOLISING
ENZYME SYSTEM AND DRUGS COMMONLY
AFFECTED BY THEM
Inducers Drugs with enchanced
Metabolism
Barbiturates Coumarins, phenytoin,
testosterones etc.
Alcohol Pentobarbital, Coumarins,
phenytoin.
Phenytoin Tolbutamide, cortisol etc.
Rifampicin Rifampicin, tolbutamide etc.
Cigarette Smoke Nicotine and amino azo dyes.
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16. INHIBITION OF CYP450...
 Decrease in the drug metabolism ability of enzyme by
drug is called enzyme inhibition.
 inhibition of CYP enzymes can be classified
1.Reversible inhibition.
2.Irreversible inhibition.
1. Reversible inhibition...
 Direct competition for binding site between substrate
and inhibitor.
 determination of potency of inhibitor.
 for example:- cimetidine H2 receptor antagonist.
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17. 2. Irreversible inhibition...
 occur by reactive metabolites generation.
 first type of irreversible inhibition involves the
formation of metabolic intermediate complexes.
 inhibition of CYP3A by erythromycin.
 Transformation reactions such as N-hydroxylation, N-
demethylation, N-oxidation catalyzed by CYP3A.
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18. TABLE:-ENZYME INHIBITOR AND DRUGS AFFECTED
BY THEM
Inhibitors Drugs with Decreased
metabolism
MAO inhibitors Barbiturates, Tyramine
Coumarins Phenytoin
Allopurinol 6-Mercatopurine
PAS Phenytoin, Hexobarbital
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19. REFERENCES...
1) Wilson and Gisvolds Textbook of Organic medicinal
and pharmaceutical chemistry, Ninth edition,Page
no.48 - 49 &117.
2) William O. Foye Principle of medicinal chemistry,3rd
edition, varghese publishing house, Page no.84-85.
3) Bramhankar, D.M. and Sunil B Jaiswal. 2001
Biopharmaceutics and Pharmacokinetics A Treatise, 2nd
ed.,Vallabh Prakashan. New Delhi, CT ISBN 978-81-
85731=47-6: Page no.181-184.
4) Bhupinder singh kalra, Cytochrome P450 enzyme
isoforms and their therapeutic implication: an update,
Indian J Med Sci, vol. 61, no. 1, Feb 2007. 19

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