A number of drugs inhibit the metabolism of carbamazepine catalyzed by cytochrome P450, sometimes resulting in carbamazepine intoxication. However, there is little information available concerning the identity of the specific isoforms of P450 responsible for the metabolism of this drug. This study addressed the role of CYP3A4 in the formation of carbamazepine-10,11-epoxide, the major metabolite of carbamazepine. Results of the study showed that: (1) purified CYP3A4 catalyzed 10,11-epoxidation; (2) cDNA-expressed CYP3A4 catalyzed 10,11-epoxidation (Vmax = 1730 pmol/min/nmol P450, Km = 442 microM); (3) the rate of 10,11-epoxidation correlated with CYP3A4 content in microsomes from sixteen human livers (r2 = 0.57, P < 0.001); (4) triacetyloleandomycin and anti-CYP3A4 IgG reduced 10,11-epoxidation to 31 +/- 6% (sixteen livers) and 43 +/- 2% (four livers) of control rates, respectively; and (5) microsomal 10,11-epoxidation but not phenol formation was activated 2- to 3-fold by alpha-naphthoflavone and progesterone and by carbamazepine itself (substrate activation). These findings indicate that CYP3A4 is the principal catalyst of 10,11-epoxide formation in human liver. Experiments utilizing a panel of P450 isoform selective inhibitors also suggested a minor involvement of CYP2C8 in liver microsomal 10,11-epoxidation. Epoxidation by CYP2C8 was confirmed in incubations of carbamazepine with cDNA-expressed CYP2C8. The role of CYP3A4 in the major pathway of carbamazepine elimination is consistent with the number of inhibitory drug interactions associated with its clinical use, interactions that result from a perturbation of CYP3A4 catalytic activity.