Nonplanar PCBs

In the rabbit, the nonplanar PCB 2,2, 5,5 -tetrachlorobiphenyl (2,2, 5,5 -TCB) is converted into the 3, 4 -epoxide by monooxygenase attack on the meta-para position, and rearrangement yields two monohydroxymetabolites with substitution in the meta and para positions (Sundstrom et al. 1976). The epoxide is also transformed into a dihydrodiol by epoxide hydrolase attack (see Chapter 2, Section 2.3.2.4). This latter conversion is inhibited by 3,3,3-trichloropropene-l,2-oxide (TCPO), thus providing strong confirmatory evidence for the formation of an unstable epoxide in the primary oxidative attack (Forgue et al. 1980). 

In the examples given, there is good evidence for the formation of an unstable epoxide intermediate in the production of monohydroxymetabolites. However, there is an ongoing debate about the possible operation of other mechanisms of primary oxidative attack that do not involve epoxide formation, for example, in the production of 2 OH 3,3, 4,4 -TCB (Figure 6.3). As mentioned earlier, P450s of gene family 1 (CYP 1) tend to be specific for planar substrates, including coplanar PCBs they do not appear to be involved in the metabolism of nonplanar PCBs. On the other hand. 

P450s of gene family 2 (CYP 2) are more catholic, and can metabolize both planar and nonplanar PCBs. 

Feltz, K.P., D.E. Tillitt, R.W. Gale, and PH. Peterman. 1995. Automated HPLC fractionation of PCDDs and PCDFs and planar and nonplanar PCBs on C18-dispersed PX-21 carbon. Environ. Sci. Technol. 29 709-718. 

However, nonplanar PCBs do resemble estrogens. Figure 5.27 compares an -OH-substituted PCB to estradiol. The resemblance is common especially... 

Since the retention times of mono-ortho- and non-ortf/osubstituted PCBs are usually greater than those of other isomers, coplanar PCBs can coelute with nonplanar PCBs of higher chlorine number. The marked overlap of isomer groups with respect to elution is illustrated by the chromatograms of a mixture of 13... 

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