Benzo[a]pyrene oxidation and microsomal enzyme activity in the mussel (Mytilus edulis) and other bivalve mollusc species from the Western North Atlantic

Analysis of subcellular fractions revealed a complement of microsomal electron transport components including reductases and heme proteins in several organs of the three bivalve species Mytilus edulis, Macrocallista maculata and Area zebra. Dithionite difference spectroscopy of CO-treated microsomes yielded spectra typical of cytochrome P-450 in digestive gland and gill, with absorption maxima at 450 nm. A time-dependent reduction of cytochrome P-450 was also observed. The levels of these components and rates of microsomal benzoa]pyrene (BP) metabolism were highest in the digestive gland, and were very similar between species. In M. edulis there was a suggested seasonal variation in BP metabolism but no population differences in this activity or in levels of other components. Digestive gland microsomal metabolites of BP identified by HPLC retention and UV spectroscopy included BP-1.6-quinone, BP-3,6-quinone and BP-6,12-quinone, which comprised 65% of the total metabolites, and dihydrodiols and phenols, the latter products consistent with cytochrome P-450 monooxygenation and expoxide hydrolase function. However, the inconsistent dependence of BP metabolism on NADPH, and inconsistent inhibition by CO suggest that catalyst(s) additional to cytochrome P-450 may be acting in BP metabolism. Based on these results and the prominent quinone formation, we speculate that peroxidative mechanism(s) may be involved. The role of peroxidative as well as well as monooxygenase reactions in the in-vivo disposition and effects of foreign chemicals in bivalves, and also the major function of cytochrome P-450 in these bivalves, remain to be established.Some of these results have appeared in preliminary form; J. J. Stegeman, Sea Grant Annual Report, Words Hole Oceanographic Institution, p 15, 1981