Polychlorinated biphenyls (PCBs) continue to pose a health concern because of their predominance in the diet and air as well as in environmental samples and humans. PCB congeners with 3 or 4 chlorine substituents in ortho position have been associated with neurodevelopmental disorders. Hydroxylated metabolites (OH-PCBs) of these PCBs are also potentially toxic to the developing brain. Metabolism studies have mainly focused on animal models. However, preliminary data from this dissertation work have revealed PCB metabolism differences between laboratory animal models and humans in terms of metabolite profiles, chiral signatures. More concerning, biotransformation of chiral PCBs is poorly investigated in humans. The objective of this dissertation research was to study the biotransformation of chiral and prochiral PCBs to chiral hydroxylated metabolites in humans and rats and to identify individual human P450 enzymes involved in the metabolism of these PCBs. I chose chiral PCB congeners 2,2',3,4',6-pentachlorobiphenyl (PCB 91); 2,2',3,5',6-pentachlorobiphenyl (PCB 95), 2,2',3,3',4,6'-hexachlorobiphenyl (PCB 132) and 2,2',3,3',6,6'-hexachlorobiphenyl (PCB 136) for this investigation because they are environmentally relevant and their metabolism has been studied in rodents and other laboratory animal species (Kania-Korwel et al., 2016a). Prochiral PCB congeners 2,2′,4,6′-tetrachlorobiphenyl (PCB 51) and 2,2′,4,5,6′-pentachlorobiphenyl (PCB 102) were selected because their considerable presence in technical PCB mixtures.
To test the hypothesis that P450 enzyme and species differences mediate the congener-specific enantioselective metabolism of chiral PCBs to hydroxylated metabolites, I sought to establish structure-metabolism relationships by studying the enantioselective metabolism of structurally diverse chiral PCBs by human liver microsomes (HLMs). Racemic PCB 91, PCB 95 and PCB 132 were incubated in vitro with pooled or individual donor HLMs at 37 °C, and levels and chiral signatures of the parent PCB and its hydroxylated metabolites were determined by high-resolution gas chromatography equipped with time-of-flight mass spectrometry (GC/TOF-MS) or electron capture detection (GC-ECD). Hydroxylated metabolites formed were identified and metabolic schemes for these PCBs proposed. I found inter-individual differences in the formation of OH-PCBs by individual donor HLMs. Comparison of the metabolite profiles of PCB 91, PCB 95, PCB 132 and PCB 136 (PCB 136 metabolism by HLMs was investigated by other researchers) revealed congener-specific differences in the oxidation of PCBs by human cytochrome P450 enzymes. PCB 91 and PCB 132 were mainly hydroxylated in meta position, with the 1,2-shift metabolites being the major metabolites formed from both PCB congeners by HLMs. In contrast, PCB 95 and PCB 136 were primarily hydroxylated in the para position. Moreover, we determined human P450 isoforms involved in the metabolism of neurotoxic PCBs using in silico and in vitro approaches. In silico predictions suggested that chiral PCBs are metabolized by CYP1A2, CYP2A6, CYP2B6, CYP2E1, and CYP3A4. Experimentally we found that CYP2A6, CYP2B6 and to a minor extent CYP2E1 were the enzymes involved in the metabolism of these chiral PCBS.
We also investigated nonchiral sources of chiral OH-PCBs by studying the P450- and species-dependent biotransformation of prochiral PCB 51 and PCB 102 to chiral OH-PCB metabolites. Prochiral PCB 51 and PCB 102 were incubated with liver microsomes prepared from male Sprague-Dawley rats pretreated with various inducers of P450 enzymes including phenobarbital (PB), dexamethasone (DEX), isoniazid (INH), β-naphthoflavone (BNF), clofibric acid (CFA) or corn oil (CO); and untreated male cynomolgus monkeys, Hartley albino guinea pigs, New Zealand rabbits, golden Syrian hamsters; and untreated female Beagle dogs. PCB 51 and PCB 102 were metabolized to 2,2',4,6'-tetrachlorobiphenyl-3'-ol (OH-PCB 51) and 2,2',4,5,6'-pentachlorobiphenyl-3'-ol (OH-PCB 102), respectively. The formation of both metabolites was P450 isoforms- and species-dependent. Moreover, OH-PCB 51 and OH-PCB 102 were chiral and were formed enantioselectively in all microsomes investigated.
Taken together, my findings demonstrate (1) considerable inter-individual variability in the congener-specific metabolism of PCBs to OH-PCBs; (2) the enantioselective formation of OH-PCBs by human CYP2A6, CYP2B6, and CYP2E1; and (3) that chiral PCB metabolites are formed enantioselectively from prochiral PCB congeners. Interestingly, the metabolism of PCBs by CYP2A6 appears to involve arene oxide intermediates, as suggested by the formation of 1,2-shift products as major metabolites of PCB 91 and PCB 132. In contrast, 1,2-shift products are minor PCB metabolites formed in rodents. Therefore extrapolation of hepatic metabolism across species may not be consistent and these differences should be considered in future toxicity and risk assessment studies.