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.
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