Metabolism of dichlorobiphenyls by highly purified isozymes of rat liver cytochrome P-450

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Abstract
Hepatic mixed-function oxidase metabolism of the ubiquitous pollutant polychlorinated biphenyls (PCB) is implicated in their toxification and detoxification. Dichlorobiphenyls (DCB) were used as models to investigate the effect of the chloro substituent sites on this metabolism experimentally and by molecular orbital calculations. Reconstituted, purified cytochrome P-450 PB-B and BNF-B, the major terminal oxidase isozymes of this system, from phenobarbital (PB)- and .beta.-naphthoflavone (BNF)-induced rats were used. Both isozymes were also induced by PCB. High-performance liquid chromatography (HPLC) was used to detect, quantify and isolate metabolites. Metabolite structures were identified by mass spectrometry, dechlorination to identifiable hydroxybiphenyls and HPLC retention times. All DCB yielded 3- and 4- but no 2-monohydroxylated metabolites (3,3''-DCB also yielded a dihydroxy metabolite). Di-o-chloro-substituted DCB were metabolized primarily by cytochrome P-450 PB-B, mono-o-chloro substituted DCB by both isozymes approximately equivalently, and DCB without o-chloro substitutents by BNF-B primarily. PB-B preferentially metabolized noncoplanar DCB and BNF-B coplanar DCB. The cytochrome isozymes exhibited differing regioselectivities for DCB metabolism, PB-B hydrolxylated unchlorinated phenyl rings and BNF-B chlorinated rings. Incorporation of epoxide hydrolase yielded DCB dihydrodiols and hydroxy metabolite patterns were consistent with those calculated from ring-opened arene oxide intermediates. The rates and regioselectivities of metabolism and possibly the toxicity and carcinogenicity of DCB depended on the cytochrome P-450 isozymes induced.