Add time:07/14/2019         Source:sciencedirect.com

The metabolism by rat hepatic microsomal cytochrome P-450-dependent monooxygenases of several model substrates that are specific for individual isoforms of cytochrome P-450 and the metabolism by these monooxygenases of two structurally related isomers of hexachlorobiophenyl was studied. The most striking result was that 2,2′,3,5,5′,6-hexachlorobiphenyl was metabolised in vitro at the rate of 4.5 pmol/mg microsomal protein per min, whereas the other isomer 2,2′,3,4,4′,6-hexachlorobiphenyl was not metabolised at detectable rates. This finding provides strong evidence for a regioselective oxidative attack by cytochrome P-450-dependent monooxygenase with preferential insertion of oxygen at meta-para unsubstituted carbon atoms. Investigations into the mechanism of this oxidative attack suggest that the ortho hydrogen atom at carbon atom C-6′ of 2,2′,3,4,4′,6-hexachlorobiphenyl was associated with a lower charge (0.075 e) compared with the meta or para hydrogen atoms at carbon atom C-3′ and C-4′ of 2,2′,3,5,5′,6-hexachlorobiphenyl (0.086 e). In addition, measurement of the main C-C bond length using MOPAC calculations and X-ray crystalographic data suggests significant differences in the bond-length distance, with the main bond lenths of 1.390, 1.385 and 1.374 Å, respectively, for bridgehead to ortho (C1-C2), for ortho to meta (C2-C3), and for meta to para bonds. These results provide evidence that the preferential meta-para oxidative attack is linked to a shorter carbon-carbon bond length and a more positive charge distribution of the corresponding hydrogen atoms.

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