Cytochrome P450TB (CYP2C): A major monooxygenase catalyzing diclofenac 4′-hydroxylation in human liver

doi:10.1016/0024-3205(93)90285-B

Life Sciences

Volume 52, Issue 1, 1993, Pages 29-34

https://doi.org/10.1016/0024-3205(93)90285-B Get rights and content

Abstract

The nature of the enzyme(s) catalyzing the major metabolic pathway of diclofenac, 4′-hydroxylation, was investigated in human licer microsomes. Inhibition studies were performed with tolbutamide and sulfaphenazole (respectively the prototype substrate and a selective inhibitor of cytochrome P450TB - CYP2C subfamily), and with phenytoin and (±)-warfarin, other proposed substrates of P450TB. Diclofenac 4′-hydroxylation displayed single enzyme Michaelis-Menten kinetics and was similar in microsomes from one poor and five extensive metabolizers of debrisoquin (CYP2D6), with a K_m of 5.6 ± 1.5 μM (mean ± sd) and a V_max of 60.6 ± 23.5 nmol/mgP/h. Inhibition by tolbutamide, sulfaphenazole, phenytoin and (±)-warfarin was comparable in all livers, with values predicted from their K_m or K_i for cytochrome P450TB determined in separate studies and a competitive inhibition model. Sulfaphenazole competitively inhibited diclofenac 4′-hydroxylation (K_i = 0.11 ± 0.08 μM, n = 3). Diclofenac 4′-hydroxylation is predominantly catalyzed by a cytochrome P450 isozyme of the CYP2C subfamily, most likely CYP2C9. This particular isozyme therefore appears to be responsible for the oxidation of polar acidic substances such as non-steroidal anti-inflammatory drugs from different chemical classes. It also constitutes a common site for drug interactions involving these compounds, as well as tolbutamide, phenytoin and warfarin.

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^☆: Presented in part at the 24th Annual Meeting of the Union of Swiss Societies of Experimental Biology, Basel, March 1992 and at the 5th World Conference on Clinical Pharmacology and Therapeutics, Yokohama, July 1992.

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Cytochrome P450TB (CYP2C): A major monooxygenase catalyzing diclofenac 4′-hydroxylation in human liver☆

Abstract

Life Sci.

Biochem. Biophys. Res. Commun.

Biochem. Pharmacol.

Biochem. Pharmacol.

Experientia

Dna

J. Pharmacol. Exp. Ther.

Mol. Pharmacol.

J. Biochem.

Nucleic Acids Res.

Biochemistry