Polychlorinated biphenyls as inducers of hepatic microsomal enzymes: Effects of di-ortho substitution

All of the 13 possible polychlorinated biphenyl (PCB) isomers and congeners substituted at both para positions, at least two meta positions (but not necessarily on the same ring) and at two ortho positions have been synthesized and tested as rat hepatic microsomal enzyme inducers. The effects of these compounds were evaluated by measuring microsomal benzo[a]pyrene (B[a]P) hydroxylase, 4-chlorobiphenyl (4-CBP) hydroxylase, 4-dimethylaminoantipyrine (DMAP) N-demethylase and NADPH-cytochrome c reductase activities, the cytochrome b₅ content and the relative peak intensities and spectral shifts of the carbon monoxide(CO)- and ethylisocyanide(EIC)-difference spectra of ferrocytochrome P-450. The results were compared to the effects of administering phenobarbitone (PB), 3-methylcholanthrene (MC) and PB plus MC (coadministered). At dose levels of 150 μmol · kg^-1, all of the PCB congeners, except 2,3′,4,4′,5′,6-hexachlorobiphenyl, significantly enhanced hepatic microsomal cytochrome P-450 content, B[a]P hydroxylase and/or DMAP N-demethylase activities compared to the control (corn oil-treated) animals. Only 5 of these compounds, namely 2,3,4,4′,5,6-hexa-, 2,2′,3,3′,4,4′-hexa-, 2,2′,3′,4,4′,5-hexa-, 2,3,3′,4,4′,6-hexa- and 2,2′,3,3′,4,4′,5-heptachlorobiphenyl, enhanced microsomal B[a]P hydroxylase, 4-CBP hydroxylase, NADPH-cytochrome c reductase and DMAP N-demethylase activities in a manner consistent with a mixed pattern of induction. The results suggest that PCB isomers and congeners substituted at both para positions, at least two meta positions, at two ortho positions and containing a 2,3,4-trichloro substitution pattern on one ring are mixed-type inducers; in addition the effects of 2,3,4,4′,5,6-hexachlorobiphenyl were also consistent with a mixed pattern of induction.