Purification and characterization of a minor form of hepatic microsomal cytochrome P-450 from rats treated with polychlorinated biphenyls

Abstract

A minor form of hepatic microsomal cytochrome P-450 has been purified to apparent homogeneity from rats treated with the polychlorinated biphenyl mixture, Aroclor 1254. This newly isolated hemoprotein, cytochrome P-450_e, is inducible in rat liver by Aroclor 1254 and phenobarbital, but not by 3-methylcholanthrene. Two other hemoproteins, cytochromes P-450_b and P-450_c, have also been highly purified during the isolation of cytochrome P-450_e based on chromatographic differences among these proteins. By Ouchterlony double-diffusion analysis with antibody to cytochrome P-450_b, highly purified cytochrome P-450_e is immunochemically identical to cytochrome P-450_b but does not cross-react with antibodies prepared against other rat liver cytochromes P-450 (P-450_a, P-450_c, P-450_d) or epoxide hydrolase. Purified cytochrome P-450_e is a single protein-staining band in sodium dodecyl sulfate-polyacrylamide gels with a minimum molecular weight (52,500) slightly greater than cytochromes P-450_b or P-450_d (52,000) but clearly distinct from cytochromes P-450_a (48,000) and P-450_c (56,000). The carbon monoxide-reduced difference spectral peak of cytochrome P-450_e is at 450.6 nm, whereas the peak of cytochrome P-450_b is at 450 nm. Ethyl isocyanide binds to ferrous cytochromes P-450_e and P-450_b to yield two spectral maxima at 455 and 430 nm. At pH 7.4, the 455:430 ratio is 0.7 and 1.4 for cytochromes P-450_b and P-450_e, respectively. Metyrapone binds to reduced cytochromes P-450_e and P-450_b (absorption maximum at 445–446 nm) but not cytochromes P-450_a, P-450_c, or P-450_d. Metabolism of several substrates catalyzed by cytochrome P-450_e or P-450_b reconstituted with NADPH-cytochrome c reductase and dilauroylphosphatidylcholine was compared. The substrate specificity of cytochrome P-450_e usually paralleled that of cytochrome P-450_b except that the rate of metabolism of benzphetamine, benzo[a]pyrene, 7-ethoxycoumarin, hexobarbital, and testosterone at the 16α-position catalyzed by cytochrome P-450_e was only 15–25% that of cytochrome P-450_b. In contrast, cytochrome P-450_e catalyzed the 2-hydroxylation of estradiol-17β more efficiently (threefold) than cytochrome P-450_b. Cytochrome P-450_d, however, catalyzed the metabolism of estradiol-17β at the greatest rate compared to cytochromes P-450_a, P-450_b, P-450_c, or P-450_e. The peptide fragments of cytochromes P-450_e and P-450_b, generated by either proteolytic or chemical digestion of the hemoproteins, were very similar but not identical, indicating that these two proteins show minor structural differences.