Differential inhibition of aflatoxin B1 oxidation by gestodene action on human liver microsomes

Human cytochrome P450 (P450) 3A is known to be involved in the formation of both aflatoxin B-1-exo-8,9-epoxide (exo-epoxidation) and aflatoxin Q(1)(3 alpha-hydroxylation). Gestodene, a known inactivator of P450 3A4, inhibited the formation of AFB(1) metabolites in a variety of ways depending on the incubation condition. Preincubation of gestodene with human liver microsomes prior to the addition of AFB(1) inhibited both exo-epoxidation and 3 alpha-hydroxylation whereas simultaneous incubation of gestodene with AFB(1) only inhibited 3 alpha-hydroxylation. These results suggest that two independent substrate binding sites exist in P450 3A4, and AFB(1) binds to both of the binding sites. Gestodene selectively binds to one of the binding sites leading to the formation of AFQ(1), whereas it does not affect the formation of exo-epoxide via the other binding site.