In vitro metabolism of 4-vinylcyclohexene in rat and mouse liver, lung, and ovary.

ABSTRACT

4-Vinylcyclohexene (4-VCH), the dimer of 1,3-butadiene, is an ovarian toxicant in mice due to the formation of a diepoxide metabolite, but the tissue-specific site of formation of the metabolites is unknown. Microsomal preparations from liver, lung, and ovaries obtained from female CrlCD BR rats and female B6C3F1 mice were tested for their ability to metabolize the following reactions 4-VCH to 4-VCH-1,2-epoxide and 4-VCH-7,8-epoxide; 4-VCH-1,2-epoxide to 4-VCH diepoxide and 4-VCH-1,2-diol; 4-VCH-7,8-epoxide to 4-VCH diepoxide and 4-VCH-7,8-diol; and hydrolysis of 4-VCH diepoxide. Microsomes were incubated with the test chemical and the reaction products were analyzed by gas chromatography. Rat liver and lung microsomes and mouse liver and lung microsomes metabolized 4-VCH to 4-VCH-1,2-epoxide at detectable rates. Mouse liver had a Vmax for the reaction that was 56-fold higher than that for rat liver (11.1 and 0.20 nmol/min/mg protein, respectively). The Vmax for mouse lung was 2-fold higher than that for rat lung. 4-VCH-1,2-epoxide formation was not detected in ovarian microsomes from rats or mice. Metabolism of 4-VCH to 4-VCH-7,8-epoxide was detected in microsomes from rat liver and mouse liver and lung, at rates very low compared to those for metabolism to the 1,2-epoxide. Rat and mouse liver had very similar K(m) and Vmax values for metabolism of 4-VCH-1,2-epoxide to 4-VCH diepoxide. The Vmax for rat liver was 3.69 and for mouse liver was 5.35 nmol/min/mg protein. Rat and mouse ovaries did not have detectable capacity to metabolize 4-VCH-1,2-epoxide to the diepoxide. Rat and mouse liver and lung have very similar K(m) and Vmax values for metabolism of 4-VCH-7,8-epoxide to the diepoxide, while ovaries did not have detectable rates for this reaction. Hydrolysis of 4-VCH-1,2-epoxide to 4-VCH-1,2-diol was at similar rates in rat and mouse liver microsomes. Hydrolysis of 4-VCH-7,8-epoxide to 4-VCH-7,8-diol was detected only in rat liver microsomes. Hydrolysis of 4-VCH diepoxide was detected in rat and mouse liver and lung, and in rat ovary microsomes. The Vmax for rat liver was 9-fold greater than that for mouse liver (5.51 and 0.63 nmol/min/mg protein, respectively), and lung and ovary tissues were not as active as rat liver. The balance of activation versus detoxication reactions in rats and mice suggests that the mouse may be more susceptible to 4-VCH toxicity because of generation of high levels of epoxide metabolites. In general, the mouse is more efficient at metabolism of 4-VCH to epoxides than is the rat. In contrast, the rat may be more efficient at hydrolysis of epoxides. Thus, the rat would tend to produce a lower concentration of epoxide metabolites than the mouse, at equal doses of 4-VCH.