The ecotoxicology of coplanar polychlorinated biphenyls

Abstract

Polychlorinated biphenyls (PCBs) have been recognized for over 25 years as global environmental contaminants. However, many PCB congeners may be relatively harmless, while a small group of PCB congeners are highly toxic to biota. The toxic coplanar PCB congeners are chlorinated at meta positions and at one or none of the ortho positions on the biphenyl ring, thus resembling 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in molecular configuration. In vitro and in vivo toxicity tests with rodents, fish, and birds have shown that the coplanar PCB congener 126 is almost as toxic as TCDD. Several coplanar PCBs (e.g., 77, 126, 105, 118) are present in biota at parts per billion concentrations, which is orders of magnitude higher than concentrations of TCDD. Thus, coplanar PCBs may account for over 95% of the dioxinlike toxic activity affecting biota, such as fish-eating birds in the Great Lakes. There is some evidence that the toxicokinetics of coplanar PCBs in organisms differs from that of other PCB homologues. If coplanar PCBs are more persistent than their homologues, they could become enriched in biota as they pass up through the food chain (i.e., trophic enrichment), or as overall PCB levels decline with time (i.e., temporal enrichment). Overall, the available data do not support the concept of trophic or temporal enrichment in the environment.Key words: polychlorinated biphenyls, coplanar, toxic equivalents, kinetics, mixed function oxidase, 7-ethoxyresorufin-O-deethylase.