Polychlorinated biphenyl isomers and congeners as inducers of both 3-methylcholanthrene- and phenobarbitone-type microsomal enzyme activity

Highly purified synthetic polychlorinated biphenyls substituted in the meta and para positions of both phenyl rings and at one ortho position were administered to male Wistar rats and the effects of these compounds on the microsomal drug-metabolising enzymes were evaluated. The in vivo effects of these compounds were determined by measuring the microsomal benzo[a]pyrene hydroxylase, dimethylaminoantipyrine N-demethylase and NADPH-cytochrome c reductase enzyme activities, the cytochrome b₅ content and the relative peak intensities and spectral shifts of the reduced microsomal cytochrome P-450 : CO and ethylisocyanide binding difference spectra. The results were compared to the effects of administering phenobarbitone (PB), 3-methylcholanthrene (MC), 2,2′,4,4′-tetrachlorobiphenyl (TCBP-II) (a PB-type inducer), 3,3′,4,4′-tetrachlorobiphenyl (TCBP-I) (an MC-type inducer), PB plus MC (coadministered) and TCBP-II + TCBP-I (coadministered) to the test animals. At dosage levels of 30 and 150 μmol · kg^-1, pretreatment with 2,3,3′,4,4′-pentachlorobiphenyl (PCBP-II), 2,3′,4,4′,5-pentachlorobiphenyl (PCBP-I), 2,3,3′,4,4′,5-hexachlorobiphenyl (HCBP-II) and 2,3,3′,4,4′,5-hexachlorobiphenyl (HCBP-III) gave hepatic microsomes with enzymic and spectral properties consistent with a mixed pattern of induction. These polychlorinated biphenyl (PCB) isomers and congeners have been identified as either major or minor components of the commercial PCB mixtures and must contribute to their activity as MC-type inducers. The only PCB isomer in this series which was not a mixed type inducer was 2,3′,4,4′,5,5′-hexachlorobiphenyl (HCBP-I) which appeared to be a PB-type inducer. This contrasted to the mixed-type activity observed for 2,3′,4,4′,5,5′-hexabromobiphenyl which was isolated from a commercial polybrominated biphenyl (PBB) mixture.