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

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Abstract
Rat small intestinal microsomes have been compared with liver preparations for the ability to activate promutagens using the Salmonella mutagenicity assay. Induced levels of arylhydrocarbon hydroxylase and cytochrome P-450 in intestinal microsomes are significantly lower than the corresponding amounts in liver microsomes. Greater activation of benzo[a]pyrene (BP) by liver extracts would thus be expected. Although this was observed at > 1 .mu.g BP/plate, at lower doses comparatively high levels of activation were obtained with intestinal microsomes. This could be due to preferential formation of the mutagenic 4,5-oxide with intestinal microsomes, as opposed to the putative major active metabolite, the 7,8-diol-9,10-epoxide. Microsomal epoxide hydrolase inactivates the K-region epoxide by forming the corresponding dihydro-diol. Differences in the levels of these metabolites may thus be a result of higher activity of the enzyme in liver extracts. This hypothesis has been studied using the epoxide hydrolase inhibitor, 1,2-epoxy-3,3,3,-trichloropropylene oxide (TCPO). Enzyme activity has been measured using [3H]-BP-4,5-oxide as substrate. Since aflatoxin B1 (AFB) may also be activated via analogous epoxide intermediates, the effects of TCPO on activation of AFB were also investigated. Intestinal microsomal expoxide hydrolase activities were significantly lower than those in liver preparations obtained from animals pre-treated with enzyme inducers. Enzyme activity and promutagen activation ability of intestinal microsomes, respectively, were less susceptible to and not inhibited by TCPO. However, TCPO strongly inhibited microsomal epoxide hydrolase activity and activation of BP and AFB due to liver microsomes. The differences in dose-responses for mutagenicity of BP and AFB obtained are discussed with respect to the relative involvement of epoxide hydrolase in the activation of the two promutagens by the different microsomal preparations used.