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Cytochrome P450 Oxidase

Created by Joseph AmanEdited by Margaret Hilton

Honors Organic ChemistryChem 2321 (Sigman), 2013

Cytochrome P450 Oxidase

Overview: Cytochrome P450 Oxidases (CYPs) are the principal enzymes implicated in drug metabolism, catalyzing approximately 75% of reactions (1). The heme iron reacts with molecular oxygen and NADPH to produce a highly reactive intermediate called compound I.

CYP Wiki Page: http://en.wikipedia.org/wiki/Cytochrome_P450 Drug Metabolism: http://en.wikipedia.org/wiki/Drug_metabolismOther References: 1. Guengerich, et. al. Chem. Res. Toxicol, 2008, 21 (1), pp 70–83 2. Axelrod, et. al, Journal of Pharmacology, 1955, vol. 114 no. 4, pp. 430-438 3. Ryan et. al, The Journal of Biological Chemistry, 1957, 225, pp. 103-114

Early Examples

OH

HN

OH

NH2

rabbit liver microsomes

O2, NADPH

Axelrod et. al, 1955 (Nobel Prize 1970)

Ephedrine Norephedrine O

OOHHO

H

H

H

17,21-DihydroxyprogesteroneO

OHO

H

H

H

17a-hydroxyprogesterone

beef adrenal microsomes

Ryan and Engel, 1957

O2, NADPH

Cytochrome P450 Oxidase

Overview: Cytochrome P450 Oxidases (CYPs) are the principal enzymes implicated in drug metabolism, catalyzing approximately 75% of reactions (1). The heme iron reacts with molecular oxygen and NADPH to produce a highly reactive intermediate called compound I.

CYP Wiki Page: http://en.wikipedia.org/wiki/Cytochrome_P450 Drug Metabolism: http://en.wikipedia.org/wiki/Drug_metabolismOther References: 1. Guengerich, et. al. Chem. Res. Toxicol, 2008, 21 (1), pp 70–83 2. Axelrod, et. al, Journal of Pharmacology, 1955, vol. 114 no. 4, pp. 430-438 3. Ryan et. al, The Journal of Biological Chemistry, 1957, 225, pp. 103-114

Mechanism

abstraction reboundR H R R

OH

O

FeIV

SCys

O

FeIV

SCys

FeIII

SCys

H

Compound I Caged Radical

O

FeIII

SCys

R H

FeIII

SCys

R HFeII

SCys

FeIII

SCys

e-R H R H

H+e-O2

OOR H

FeIII

SCys

OOR H

H

H+

FeIV

SCys

OR H

HO

H

Compound I

CYP and Tamoxifen Metabolism

Dehal et. al, Cancer Research, 1997, 57, pp. 3402-3406 Desta et. al, Journal of Pharmacology and Experimental Therapeutics, 2004, vol. 310 no. 3, pp. 1062-1075Shiau et. al, Cell, 1998, vol. 95 no. 7, pp. 927-937

Tamoxifen, an antagonist of the estrogen receptor, is a prodrug used to treat breast cancer. Cytochrome P450s convert tamoxifen to active metabolites.

N-Dealkylation converts tamoxifen to N-desmethyl tamoxifen, which is subsequently hydroxylated to form the active metabolite endoxifen.

NO

Tamoxifen

HN

ON-desmethyl-tamoxifen

HN

O4-hydoxy N-desmethyl-tamoxifen

(endoxifen)

HO

CYP2D6CYP3A4 & CYP3A5

4-hydoxy-tamoxifen complexed with ligand-binding domain of estrogen

receptor alpha (Shiau et. al.)

Problems

1. Caraco et. al, Drug Metabolism and Disposition, 1996,  vol. 24 no. 7, pp. 761-764 4. Dayton et. al, Drug Metabolism and Disposition, 1973, vol. 1 no. 6, pp. 742-751

2. Claesen et. al, Drug Metabolism and Disposition, 1982, vol. 10 no. 6, pp. 667-671 3. Yoshii et. al, Life Sciences, 2000, vol. 67, no.2, pp. 175-184

1. Hydroxylation of codeine by CYP2D6 yields a hemiacetal that spontaneously cleaves to an alcohol and formaldehyde. Propose an arrow-pushing mechanism for this decomposition under acidic and basic conditions.

2. CYP2D6 oxidizes an aromatic pi bond in phenytoin to form an arene oxide intermediate that rapidly rearranges to a phenol via a mechanism called the NIH shift. Propose an arrow-pushing mechanism for this rearrangement.

3. Predict the site of hydroxylation for the antipsychotic chlorpromazine.

HO

HO

O H

N

Morphine

O

O

HO

H

N

Codeine

Spontaneous

O

O

HO

H

N

CYP2D6

HOO

H H

O

NH

HN

O

Phenytoin

O

NH

HN

OHPPH (major metabolite)

O

NH

HN

O

OHO

CYP2D6 NIH shift

O

HOS

O

O

N(CH2CH2CH3)2

Probenecid

4. The uricosuric drug probenecid undergoes no observable aromatic hydroxylation by CYPs. Explain this observation.

N

N

S

Cl

chlorpromazine

CYP2D6 ?

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Contributed by: Created by Joseph Aman (Undergraduate)

Edited by Margaret HiltonHonors Organic ChemistryChem 2321 (Sigman), 2013

University of Utah