Modification of microsomal metabolism of benzo(a)pyrene (BaP) by superoxide generated in vitro

Reactive oxygen species may play a role in bioactivation of BaP and its procarcinogenic metabolites. The effect of superoxide (O2) and/or other active oxygen species, generated in vitro by xanthine/xanthine oxidase (X/XO) +/- Fe3+, on BaP metabolism by mouse liver microsomes fortified with NADPH and UDP-glucuronic acid was examined. Analysis of the reaction mixtures by reversed-phase high pressure liquid chromatography in methanol-water gradient showed that the peroxidative conditions evoked by X/XO + Fe3+ decreased the overall metabolism of 14C-BaP by some 40% and decreased glucuronidation of BaP metabolites by about 85%, while protein bound metabolites increased by 80%. Even without added Fe3+, O2 increased the production of epoxides by 174%. Addition of superoxide dismutase reversed the decreases in both glucuronidation and overall metabolism of BaP. This indicates that O2 switches the BaP metabolism to direct epoxidation and increases the amount of intermediates able to bind protein, while damage to membrane bound enzymes curtails further metabolism of BaP. Thus, it appears that during oxidative burst, such as in inflammatory reactions, there would be a limited mixed function oxidase mediated bioactivation of BaP