Encyclopedia of Environmental Science and Engineering, Volume I and II

870 PCBs AND ASSOCIATED AROMATICS

FIGURE 12 Mono-ortho-chlorobiphenyls.

FIGURE 11

Ah receptor with much lower affinity, but elicit dioxin-like

responses. The di-ortho-coplanar PCBs are very insoluble

so that receptor-binding activity cannot be readily measured,

but at sufficiently high doses, dioxin-like responses have

been observed. PCB congeners which lack a para-chloro

substituent also produce weak dioxin-like effects.

The main mechanism of PCB metabolism in birds

and mammals involves the insertion of oxygen into two

adjacent carbon atoms on the ring by hepatic microsomal

oxidases (HMOs) to form a transient arene oxide inter-

mediate which then becomes degraded to hydroxylated

PCBs. For example, the metabolism of 4,4’dichlorobiphenyl

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