Immunochemical study on the contributions of two molecular species of microsomal cytochrome P-450 to the metabolism of benzo(a)pyrene by rat liver microsomes

Abstract

The roles of two species of cytochrome P-450, the major cytochrome P-450 components of liver microsomes of phenobarbital-treated rats (PB-P-450) and 3-methylcholanthrene-treated rats (MC-P-448), were studied in the metabolism of benzo(a)pyrene in rat liver microsomes in vitro. Benzo(a)pyrene was incubated with polychlorinated biphenyl-treated rat liver microsomes, in which PB-P-450 and MC-P-448 constituted about 45 and 24% of the total cytochrome P-450, respectively. Then the metabolites were separated into those soluble in ethyl acetate and in water, and those covalently bound to protein. Using high-pressure liquid chromatography, the ethyl acetate-soluble metabolites were separated into three major groups, phenols, quinones, and dihydrodiols, including peaks of three unknown materials. Addition of anti-MC-P-448 immunoglobulin to the reaction mixture completely inhibited the formation of all ethyl acetate-soluble metabolites. In contrast, anti-PB-P-450 immunoglobulin did not inhibit the formations of 4,5-dihydro-4,5-dihydroxybenzo(a)pyrene and 3-hydroxybenzo(a)pyrene; partially inhibited the formations of 7,8-dihydro-7,8-dihydroxybenzo(a)pyrene, 9, 10-dihydro-9, 10-dihydroxybenzo(a)pyrene, and the three unknown materials; and caused 30 to 40% enhancement of the formations of 9-hydroxy-benzo(a)pyrene and benzo(a)pyrene-3,6-dione and 80% enhancement of that of benzo(a)pyrene-1,6-dione. Antibody against MC-P-448, but not against PB-P-450, also caused 75% inhibition of the formation of water-soluble metabolites and 85% inhibition of formation of benzo(a)pyrene metabolites covalently bound to protein. These results show that MC-P-448 is important in the metabolism of benzo(a)pyrene.