Activation of benzo[a]pyrene and aflatoxin B1 to mutagenic chemical species by microsomal preparations from rat liver and small intestine in relation to microsomal epoxide hydrolase

Rat small intestinal microsomes have been compared with liverpreparations for their ability to activate promutagens usingthe Salmonella mutagenicity assay. Induced levels of arylhydrocarbonhydroxylase and cytochrome P-450 in intestinal microsomes aresignificantly lower than the corresponding amounts in livermicrosomes. Greater activation of benzo[a]pyrene (BP) by liverextracts would thus be expected. Although this was observedat >1 µg BP/plate, at lower doses comparatively highlevels of activation were obtained with intestinal microsomes.This could be due to preferential formation of the mutagenic4,5-oxide with intestinal microsomes, as opposed to the putativemajor active metabolite, the 7,8-diol-9,10-epoxide. Microsomalepoxide hydrolase inactivates the K-region epoxide by formingthe corresponding dihydro-diol. Differences in the levels ofthese metabolites may thus be a result of higher activity ofthe enzyme in liver extracts. This hypothesis has been studiedusing the epoxide hydrolase inhibitor, 1,2-epoxy-3,3,3-trichloropropyleneoxide (TCPO). Enzyme activity has been measured using [³H]-BP-4,5-oxideas substrate. Since aflatoxin B¹ (AFB) may also be activatedvia analogous epoxide intermediates, the effects of TCPO onactivation of AFB were also investigated. In testinal microsomalexpoxide hydrolase activities were significantly lower thanthose in liver preparations obtained from animals pre-treatedwith enzyme inducers. Enzyme activity and promutagen activationability of intestinal microsomes, respectively, were less susceptibleto and not inhibited by TCPO. However, TCPO strongly inhibitedmicrosomal epoxide hydrolase activity and activation of BP andAFB due to liver microsomes. The differences in dose-responsesfor mutagenicity of BP and AFB obtained are discussed with respectto the relative involvement of epoxide hydrolase in the activationof the two promutagens by the different microsomal preparationsused. ^*To whom reprint requests should be addressed