Abstract

Trans-stilbene oxide is a potent inducer of epoxide hydratase in rat liver which does not significantly alter cytochrome P-450 content or the specific activities of aminopyrine N-demethylase and benzo(a)pyrene monooxygenase measured by tritium release or fluorescence of phenols. Although three of these monooxygenase parameters are directly related to the metabolism of benzo(a)pyrene, we now report the surprising observation that the pattern of benzo(a)pyrene metabolites is substantially altered by trans-stilbene oxide treatment. The formation of metabolites oxidized at the 7,8,9,10-benzo ring was decreased by a factor of 3 whereas the formation of 4,5-dihydroxy-4,5-dihydrobenzo(a)pyrene was increased 6-fold. The ratio between 7,8-dihydroxy-7,8-dihydrobenzo(a)pyrene (dihydrodiol bay region epoxide precursor) and 4,5-dihydroxy-4,5-dihydrobenzo(a)pyrene was decreased more than 20-fold. We also found that ethoxycoumarin O-deethylase activity was increased 2.5-fold and preferentially inhibited by metyrapone. These marked effects on monooxygenase activities severely limit the selectivity of trans-stilbene oxide as epoxide hydratase inducer. The mutagenicity with his^- Salmonella typhimurium was up to 10 times lower when benzo(a)pyrene was activated by microsomes or postmitochondrial fraction from trans-stilbene oxide-treated as compared to control animals. Two factors contribute to this protection: (1) A decreased formation of mutagenic dihydrodiol epoxides at the 7,8,9,10-benzo ring due to the shift of the metabolism to the 4,5-position, (2) a more efficient inactivation of the mutagenic benzo(a)pyrene 4,5-oxide due to the induction of epoxide hydratase. Thus trans-stilbene oxide causes two effects in the rat liver, which can synergistically reduce the harmful effects of benzo(a)pyrene. This study shows that benzo(a)pyrene metabolite patterns and consequent biological effects can be substantially altered without any significant change in the fluorescence intensity of the alkali extractable metabolites, the so called AHH (aryl hydrocarbon hydroxylase) activity.