Metabolism and DNA binding of benzo[a]pyrene in cultured human bladder and bronchus

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Abstract
The effects of modulating epoxide hydrolase activity on the binding of individual metabolites of benzo[a]pyrene to DNA in isolated rat liver nuclei were studied. trans-Stilbene oxide is shown to induce nuclear epoxide hydrolase (3.7-fold). This induction leads to a decrease in the binding of 9-hydroxy-benzo[a]pyrene-4,5-oxide to nuclear DNA, but to no appreciable alteration in the amount of benzo[a]pyrene-7,8-di-hydrodio)-9,10-oxide bound. The addition of a large excess of purified epoxide hydrolase, which had been solubilized from microsomal membranes and purified to apparent homo-geneity, to the incubation containing nuclei caused a marked decrease in the amount of 9-hydroxybenzo[a]pyrene-4,5-oxide bound to nuclear DNA, but had no appreciable effect on the binding of benzo[a]pyrene-7,8-dihydrodiol-9,10-oxide. This is in contrast to the previously observed marked increase in binding of benzo[a]pyrene-7,8-dihydrodiol-9,10-oxide to exogenous DNA when benzo[a]pyrene was incubated with microsomes in the presence of increased levels of microsomal epoxide hydrolase. The addition of microsomes to the nuclei caused an increase of binding of both metabolites, especially of benzo[a]pyrene-7,8-dihydrodiol-9,10-oxide. However, the influence of modulations of epoxide hydrolase activities on the binding pattern were similar to those observed with nuclei alone and not to those with microsomes and exogenous DNA. The addition of 125 μM 1,1,1-trichloropropene 2,3-oxide in vitro, whereby epoxide hydrolase was strongly inhibited, resulted in a decrease in the binding of benzo[a]-pyrene-7,8-dihydrodioi-9,10-oxide, and an increase in the binding of 9-hydroxybenzo[a]pyrene-4,5-oxide in all situations studied. We conclude that epoxide hydrolase is highly inhibitory to the binding of 9-hydroxybenzo[a]pyrene-4,5-oxide to nuclear DNA, in contrast to the binding of the potent ultimate carcinogen benzo[a]pyrene-7,8-dihydrodiol-9,10-oxide. Moreover, epoxide hydrolase is not limiting for the formation of benzo[a]pyrene-7,8-dihydrodiol-9,10-oxide from benzo[a]pyrene in systems where the ratio of the required epoxide hydrolase to the required monooxygenase activities is high as in rat liver nuclei or in systems containing rat liver nuclei and microsomes.