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

Abstract

Hepatic mixed-function oxidase metabolism of the ubiquitous pollutant polychlorinated biphenyls (PCBs) is implicated in their toxification and detoxification. We used dichlorobiphenyls (DCBs) 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 to investigate this metabolism. Both isozymes are also induced by PCBs. 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 DCBs yielded 3- and 4- but no 2-monohydroxylated metabolites (3,3'-DCB also yielded a dihydroxy metabolite). Di-o-chloro-substituted DCBs were metabolized primarily by cytochrome P-450 PB-B, mono-o-chloro substituted DCBs by both isozymes approximately equivalently, and DCBs without o-chloro substituents by BNF-B primarily. Thus PB-B preferentially metabolizes noncoplanar DCBs and BNF-B coplanar DCBs. The cytochrome isozymes exhibited differing regioselectivities for DCB metabolism - PB-B hydroxylated 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. Thus the rates and regioselectivities of metabolism and thus possibly the toxicity and carcinogenicity of DCBs are dependent on the cytochrome P-450 isozymes induced.

In Vivo Oxidative Damage in Rats Is Associated with Barbiturate Response but Not Other Cytochrome P450 Inducers

“…PB is metabolized by enzymes it induces, including P450s, and the sleeping time of rats decreases after a few days of continuous treatment. ␤NF-inducible rat P450 1A1 uses ␤NF as a substrate (Vyas et al, 1983), and some of the polychlorinated biphenyls in Aroclor are also substrates for the PB-and ␤NF-inducible P450s (Kaminsky et al, 1981). CCl 4 -induced isoprostane production is exacerbated by treatment of rats with INH or PB (Morrow et al, 1992), but this is a rather extreme case in that the only product is a reactive radical itself (⅐CCl 3 ).…”

Effects of food natural products on the biotransformation of PCBs

“…There have been studies of the isoform-specific biotransformation of PCB congeners, particularly by the major Phenobarbital-inducible CYP2B isoforms and 3-methylcholanthrene-inducible CYP1A isoforms, e.g. (Kaminsky et al, 1981). Human CYP2B6 and rat CYP2B1 were the major isoforms for metabolism of PCB153 (Duignan et al, 1987;Ariyoshi et al, 1992), shown in figure 1, although it should be noted that PCB153 is relatively resistant to metabolism, and is therefore one of the more biologically persistent PCB congeners (Letcher et al, 2000).…”

Comparative Metabolism of Polychlorinated Biphenyls and Tissue Distribution of Persistent Metabolites in Rats, Hamsters, and Guinea Pigs

“…Although oxidative metabolism of PCBs by rats has been studied extensively (Kato et al, 1980;Kaminsky et al, 1981), there has been only limited comparative analysis of in vivo metabolism of PCBs between hamsters and guinea pigs (Koga and Yoshimura, 1996). The results of the present study indicated that the tissue distribution profiles of PCB residues and OH-and MeSO 2 -CBs were largely dependent on the chlorination patterns classified into four groups.…”