Immunochemical quantitation of cytochrome P-450 isozymes and epoxide hydrolase in liver microsomes from polychlorinated or polybrominated biphenyl-treated rats. A study of structure-activity relationships

Abstract

Antibodies against cytochromes P-450a, P-450b + P-450e, P-450c, and P-450d and epoxide hydrolase were utilized in a radial immunodiffusion assay to measure the concentration of these inducible, xenobiotic-metabolizing enzymes in liver microsomes from rats treated with Aroclor 1254, fireMaster BP-6, or one of 41 individual polychlorinated and polybrominated biphenyl isomers and congeners. Cytochromes P-450b + P-450e and epoxide hydrolase varied independently from each other and from cytochromes P-450a, P450c, and P-450d in halogenated biphenyl-treated rats. Cytochromes P-450a, P-450c, and P-450d were co-induced by numerous halogenated biphenyls, but the widely different ratios of these hemoproteins observed in induced rats indicate these three isozymes are not coordinately regulated. The induction of cytochromes P-450a, P-450c, and P-450d and the repression of a form or forms of cytochrome P-450 normally present in control rats (measured indirectly from the unknown fraction of total cytochrome P-450) were apparently part of a pleiotypic response to certain toxic halogenated biphenyls. Like phenobarbital, certain halogenated biphenyl isomers and congeners preferentially induced cytochromees P-450b + P-450e (e.g. 2,4,2',4'-tetrachlorobiphenyl) whereas others like 3-methylcholanthrene, preferentially induced cytochrome P-450c (e.g. 3,4,3',4'-tetrachlorobiphenyl). However, like the commercial polychlorinated and polybrominated biphenyl mixtures, Aroclor 1254 and fireMaster BP-6, most of the halogenated biphenyls tested simultaneously induced both cytochromes P-450b + P-450e and cytochrome P-450c, albeit to varying degrees. Unprecedentedly high levels of cytochromes P-450a, P-450b + P-450e, and P-450d were observed following treatment of rats with 3,4,5,3',4',5'-hexachlorobiphenyl, 2,4,5,2',4',5'-hexachlorobiphenyl, or 3,4,5,3',4'-pentabromobiphenyl, respectively. Identified for the first time are several halogenated biphenyls, such as 2,3,4,5,4'-penta- and 2,3,4,5,6,4'-hexachlorobiphenyl, that induced epoxide hydrolase with an effectiveness commensurate with that of other potent inducers of this enzyme, such as trans-stilbene oxide and acetylaminofluorene. The results of the present study, employing immunochemical assays to measure specifically and unambiguously multiple cytochrome P-450 isozymes and epoxide hydrolase, dramatically illustrate the potency and versatility with which halogenated biphenyls induce these xenobiotic-metabolizing enzymes. The discussion of these results focuses on structure-activity relationships for halogenated biphenyls as inducers of specific cytochrome P-450 isozymes and epoxide hydrolase.