Immunochemical Quantitation of Cytochrome P - 450 Isozymes and Epoxide Hydrolase in Liver Microsomes from Polychlorinated or Polybrominated Biphenyl - treated Rats

Antibodies against cytochromes P-450a, P-450b + P450e, 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, P-450c, 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 P450a, 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 cytochromes P-450b + P-450e (e.g. 2,4,2’,4’-tetrachlorobiphenyl) whereas others, like 3-methylcholanthrene, preferentially induced cytochrome P-45Oc (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 P450c, albeit to varying degrees. Unprecedentedly high levels of cytochromes P-450a, P-45Ob + P-450e, and P450d 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‘-pentaand 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 tmnsstilbene oxide and acetylaminofluorene. The results of the present study, employing immunochemical assays