The biochemical basis for the species difference in hepatic microsomal 4-vinylcyclohexene epoxidation between female mice and rats.

Mice but not rats are susceptible to 4-vinylcyclohexene (VCH)-induced ovarian toxicity and carcinogenicity. This is due in part to a 4- to 6-fold greater rate of hepatic microsomal bioactivation of VCH to the ovotoxicant VCH-1,2-epoxide. The biochemical basis for this difference was investigated in microsomes using enzyme induction, enzyme inhibition with chloramphenicol or specific inhibitory antibodies, and correlation with marker steroid hydroxylase activities to associate VCH epoxidation with particular cytochrome P450 forms. Testosterone 6 beta- and 15 alpha-hydroxylase activities and VCH epoxidation were decreased in microsomes from chloramphenicol-treated mice, initially suggesting the possible involvement of P450IIIA and P450IIA forms in VCH metabolism. Although both testosterone 6 beta-hydroxylase and VCH epoxidase activities were increased by dexamethasone treatment (P450IIIA inducer), anti-rat P450IIIA IgG inhibited testosterone 6 beta-hydroxylase (68%) but not VCH epoxidase activity. These latter results do not support the involvement of mouse P450IIIA forms in VCH epoxidation. However, results were obtained which indicated that mouse P450IIA forms are involved in VCH epoxidation. In microsomes from untreated female mice VCH epoxidase activity was inhibited 48% by antibodies to mouse P45015 alpha (P450IIA3) at a concentration that inhibited testosterone 15 alpha-hydroxylase activity by 86%. No protein immunochemically related to mouse P45015 alpha was detected in female rat hepatic microsomes. VCH epoxidation by hepatic microsomes was increased in female mice and rats by phenobarbital treatment and was inhibited by approximately one-third by anti-rat-P450IIB1 IgG in microsomes from untreated animals of both species. Furthermore, microsomal VCH epoxidase and testosterone 16 alpha-hydroxylase activities were lower (34%) in female 129/J mice (deficient in constitutive expression of P450IIB forms) than in B6C3F1 mice. These results suggested partial involvement of P450IIB forms in the microsomal epoxidation of VCH. Therefore, P450 forms IIA and IIB account for the majority of VCH bioactivation in female mouse liver, which explains in part the susceptibility of mice to VCH-induced ovarian toxicity and carcinogenicity.