Mono-ortho-chlorinated chlorobiphenyls: toxicity and induction of 7-ethoxyresorufin O-deethylase (EROD) activity in chick embryos.

Six mono-ortho-chlorinated chlorobiphenyls were compared regarding their toxicity and 7-ethoxyresorufin O-deethylase (EROD)-inducing potency in chick embryos. Three of the tested chlorobiphenyls have a chloro substituent adjacent to the ortho-chlorine, and these congeners were about ten times more potent than the three having a meta-hydrogen adjacent to the ortho-chlorine. These more toxic mono-ortho-chlorinated congeners were, however, about three orders of magnitude less toxic and less potent as EROD inducers in chick embryos than 3,3',4,4',5-pentachlorobiphenyl in a previous similar study. Malformed eyes and beaks, degenerative hepatic lesions and subcutaneous as well as pericardial edema were detected in embryos exposed to the mono-ortho-chlorine-substituted congeners, as was previously found after exposure to the most toxic non-ortho-chlorinated, coplanar chlorobiphenyls. It is concluded that the mono-ortho-chlorinated chlorobiphenyls are considerably less toxic and less potent as EROD inducers than the most toxic coplanar ones. Owing to their relatively high concentrations in technical preparations of polychlorinated biphenyls (PCBs) the mono-ortho-chlorine-substituted congeners may, however, contribute to the overall toxicity of PCBs.