Hydrogen peroxide-supported oxidation of benzo [a]pyrene by rat liver microsomal fractions

In the presence of liver microsomes from phenobarbital-pretreated rats, hydrogen peroxide oxidized benzo [a]pyrene to a number of biologically significant products at a rate that was approximately 20 per cent as fast as that seen by us and others with NADPH and oxygen. As with NADPH-dependent reactions [J. Capdevila, R. W. Estabrook, and R. A. Prough, Archs. Biochem. Biophys. 200, 186 (1980)], the hydrogen peroxide-dependent reactions resulted in the production of relatively large quantities of dihydrodiols as metabolites. This was in marked contrast to the product distribution observed when cumene hydroperoxide was utilized as a cosubstrate (foregoing reference). The formation of the various organic-soluble metabolites was dependent on the presence of functional liver microsomal cytochrome P-450 in the reaction mixture. Approximately 48 per cent of the benzo[a]pyrene metabolites, however, was observed to be bound to microsomal protein, and inhibition of cytochrome P-450 function, by metyrapone or N-octylamine did not affect the extent of covalent binding of the hydrocarbon to the microsomal protein. The differences noted during benzo[a]pyrene metabolism using hydrogen peroxide strongly suggest that at least two distinct mechanisms exist to account for the oxidation of the hydrocarbon, i.e. epoxidation and one-electron oxidation reactions.