Review: Biological Activity, Determination and Occurrence of Planar, Mono- and Di-Ortho PCBs

Abstract

Some of the 209 polychlorinated biphenyls (PCBs) are stereochemically similar to the planar 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and, because of this similarity, exert biochemical activity and toxicity comparable to that of TCDD. The molecular dimensions and the planarity of the PCB molecule appear to be important for the interaction with the TCDD receptor. These structural requirements, in turn depend on the number of ortho chlorine atoms and the presence of two para and at least two meta chlorine atoms on the biphenyl skeleton. Planar PCBs (i.e., without ortho chlorines), their mono- and some of their di-ortho substituted congeners have inductive properties similar to 3-methylcholanthrene- or mixed-type induction of mixed-function oxidases. These congeners also appear to have the greatest toxic potential.

Planar PCBs and some of their mono- and di-ortho substituted congeners occur in very low concentrations in commercial PCB mixtures. Hence, their environmental occurrence is relatively low compared to that of abundant PCB congeners. The gas chromatographic (GC) determination of planar PCBs therefore requires relatively large sample sizes and/or the use of retention gaps or injection systems which allow the introduction of relatively large sample volumes, to obtain sufficiently low limits of detection. The isolation and identification of single PCB congeners in general, and that of less abundant ones in particular, still is rather difficult and often imprecise and/or inaccurate—even with the currently available high-resolution capillary GC columns—because of the complex chromatograms. Specific sample pretreatment techniques, using carbon chromatography or pyrenyl-coated silica columns for group separation, and/or multi-dimensional GC separations are available nowadays for the proper determination of planar PCBs.

From the limited amount of data available for individual PCBs, it appears that the environmental concentrations of the biochemically active PCBs usually are several orders of magnitude higher than those of TCDD. Hence, their environmental significance in terms of toxic potential—especially that of PCBs 105, 126 and, possibly, 118 and 156—is likely to be even greater than that of TCDD. Although some experimental evidence exists with regard to the causal link between PCB contamination and reproductive failure in fish-eating mammals, the amount of data on individual PCBs is insufficient to indicate PCB toxicity as the cause of local extinction of mammals such as, e.g. the otter and the porpoise, in northwestern Europe. The very possibility of this causal link, however, calls for immediate global monitoring action for planar and mono- and di-ortho substituted PCBs.