Vol. 29, Issue 9, 1236-1242, September 2001

Role of Induction of Specific Hepatic Cytochrome P450 Isoforms in Epoxidation of 4-Vinylcyclohexene

S. M. Fontaine, P. B. Hoyer, J. R. Halpert, and I. G. Sipes

Department of Pharmacology and Toxicology, Center for Toxicology (S.M.F., I.G.S.), and Department of Physiology (P.B.H.), University of Arizona, Tucson, Arizona; and Department of Pharmacology and Toxicology (J.R.H.), University of Texas Medical Branch, Galveston, Texas

4-Vinyl-1-cyclohexene (VCH) is ovotoxic in B6C3F₁ mice but not in Fischer-344 rats, which can be partially attributed to greater formation of toxic epoxides from VCH in mice compared with rats. Since repeated exposure to VCH is necessary to cause ovotoxicity in mice, it is important to determine whether repeated exposure results in induction of cytochrome P450 (CYP) enzymes involved in its bioactivation. Hepatic microsomes prepared from mice or rats treated repeatedly with VCH demonstrated significantly increased VCH bioactivation in vitro, as assessed by VCH-1,2-epoxide, VCH-7,8-epoxide, or vinylcyclohexene diepoxide (VCD) formation. Mice and rats were then dosed with VCH, VCH-1,2-epoxide, or VCD for 10 days and measured for increases in hepatic microsomal CYP levels or activities. Total hepatic CYP levels were elevated only in microsomes from mice pretreated with VCH or VCH-1,2-epoxide. Immunoblotting analysis of microsomes from VCH-treated rodents revealed elevated levels of CYP2A and CYP2B in mice but not rats. VCH-1,2-epoxide pretreatment also increased CYP2B levels in the mouse. Activities toward specific substrates for CYP2A and CYP2B (coumarin and pentoxyresorufin, respectively) confirmed that VCH and VCH-1,2-epoxide pretreatments resulted in increased catalytic activities of CYP2A and CYP2B in the mouse but not the rat. Pretreatment with phenobarbital, a known inducer of CYP2A and CYP2B, increased VCH bioactivation in both species. Interestingly, metabolism studies with human CYP "Supersomes" reveal that, of eight isoforms tested, only human CYP2E1 and CYP2B6 were capable of significantly catalyzing VCH epoxidation, whereas CYP2B6, CYP2A6, CYP2E1, and CYP3A4 were capable of catalyzing the epoxidation of the monoepoxides.