Some microorganisms have been shown to degrade many of the less-chlorinated PCBs (with one to four chlorines) at varying rates, but are not able to degrade more-chlorinated PCBs effectively. The chlorine-substituted positions on the biphenyl structure are very important in determining which congeners are degraded, although different microorganisms may attack different positions preferentially. Mostly, chlorine atoms at the para positions are preferentially degraded. In anaerobic conditions dechlorination of PCBs has been shown, and thus the more-chlorinated PCBs can be dechlorinated to less-chlorinated PCBs and subsequently be degraded.

In animals, the main metabolic pathway is ring hydro-xylation, mediated by the cytochrome P450 enzyme system (which can also be induced by PCBs, increasing its activity, and the rate ofmetabolism), to form hydroxy-PCBs (PCBs in which one ring is phenyl, the other phenol). The dominant oxidation sites are where there are adjacent unchlorinated positions - that is, ortho-meta or meta-para unsubstituted positions, the para position being particularly important. In general, cytochrome P450 1A1 (CYP1) can metabolize only coplanar PCBs (those with no, or only one, orho-substituted chlorine atom) whereas cytochrome P450 1A2 (CYP1A2) can metabolize coplanar and noncoplanar PCBs. Hydroxylated PCBs are not very soluble in water, but are more soluble than PCBs, and


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Figure 2 PCB concentrations in fish, seal feces, and seal blubber from a gray seal from the North Sea (captured on the east coast of the UK).

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