The metabolism of benzo[a]pyrene phenols by rat liver microsomal fractions

The oxidative metabolism of benzoa]pryrene (Ba]P) phenols catalyzed by liver microsomes in vitro leads to multiple products. High-pressure liquid chromatography analysis of the organic-soluble products formed indicates that regardless of the animal pretreatment regime, 3-hydroxy-Ba]P is metabolized to the 3,6-quinone and to a hydroxylated derivative tentatively identified as 3,9-dihyroxy-Ba]P. However, the distribution of products obtained with 9-hydroxy-Ba]P varied with animal pretreatment. A maximum of three distinct metabolites was obtained when the 9-phenol was metabolized in vitro with microsomes from phenobarbital-pretreated rats and the tentative 3,9-dihydroxy derivative was a common metabolite for all pretreatment regimes. Physical characterization, including mass spectrometry, indicates that all three products have an extra oxygen atom incorporated into their molecular structure from molecular oxygen. Studies utilizing specific inhibitors of the cytochrome P-450-dependent monooxygenase clearly suggest that the formation of dihydroxy or phenol-oxide derivatives is catalyzed by the hemoprotein, cytochrome P-450. These metabolites of the benzoa]pyrene phenols are most likely related to the putative phenol-oxides of benzoa]pyrene which have been demonstrated to alkylate DNA and protein. Repetitive scan difference spectrophotometric analysis of incubation mixtures containing rat liver microsomes, 3- or 9-hydroxy-Ba]P, NADPH, and oxygen shows the conversion of the phenols into products which absorb in the region from 400 to 500 nm. During and after the steady state of the reaction, it can be seen that certain of the hydroxy compounds produced are in equilibrium with their respective quinone form and may be involved in an oxygen-coupled redox cycle.