2,2',3,3',6,6'-Hexachlorobiphenyl (PCB 136) atropisomers interact enantioselectively with hepatic microsomal cytochrome P450 enzymes

Chiral polychlorinated biphenyls are biotransformed enantioselectively by mammalian cytochrome P-450 isozymes to form hydroxylated metabolites

In vitro incubations of purified rat cytochrome P-450 (CYP) 2B1 and human CYP 2B6 were performed to determine if CYP isozymes biotransform polychlorinated biphenyls (PCBs) enantioselectively. Enantioselective metabolism of chiral PCBs 45, 84, 91, 95, 132, and 136 and production of hydroxylated PCB metabolites (OH-PCBs) were observed, while no changes in PCB 183 atropisomer composition were observed for either isozyme. Enantiomer fractions (EFs) of parent PCBs, individually incubated as racemates at 25 ng/mL initial concentration, with rat CYP 2B1 ranged from 0.353 to 0.822. Enantioselectivity was also observed for PCBs 45 (EF = 0.437) and 132 (EF = 0.537) incubated at that concentration with human CYP 2B6. Both atropisomers of chiral PCBs appeared to be biotransformed simultaneously by rat CYP 2B1, except for (+)-PCB 132, but at different rates. Hydroxylated PCBs were identified using gas chromatography-high resolution mass spectrometry for all chiral PCBs enantioselectively transformed by CYPs. These metabolites did not correspond to any commercially available authentic standards, supporting the hypothesis that many unidentified OH-PCBs detected in wildlife may have arisen from in vivo biotransformation of chiral PCBs. A rough estimate suggested that more than half of the total congener metabolized by rat CYP 2B1 was converted to OH-PCBs. Similar concentration decreases were observed for congeners incubated with human CYP 2B6, but less OH-PCBs were formed. Formation of OH-PCBs via an enantioselective OH insertion mechanism was suggested, and may be a source of the unidentified OH-PCBs currently found in the environment.

Gas chromatographic separation of methoxylated polychlorinated biphenyl atropisomers

Several polychlorinated biphenyls (PCBs) and their hydroxylated metabolites display axial chirality. Here we describe an enantioselective, gas chromatographic separation of methylated derivatives of hydroxylated (OH-)PCB atropisomers (MeO-PCB) using a chemically bonded beta-cyclodextrin column (Chirasil-Dex). The atropisomers of several MeO-PCBs could be separated on this column with resolutions ranging from 0.42 to 0.87 under isothermal or temperature-programmed conditions. In addition, the enantiomeric fraction of OH-PCB 136 metabolites was determined in male and female rats treated with racemic PCB 136. The methylated derivatives of two OH-PCB 136 metabolites showed an enantiomeric enrichment in liver tissue, whereas PCB 136 itself was near racemic.