Effects of thiol antioxidants on the atropselective oxidation of 2,2′,3,3′,6,6′-hexachlorobiphenyl (PCB 136) by rat liver microsomes

Abstract

Chiral polychlorinated biphenyl (PCB) congeners, such as PCB 136, are atropselectively metabolized to various hydroxylated PCB metabolites (HO-PCBs). The present study investigates the effect of two thiol antioxidants, glutathione and N-acetyl-cysteine (NAC), on profiles and chiral signatures of PCB 136 and its HO-PCB metabolites in rat liver microsomal incubations. Liver microsomes prepared from rats pretreated with phenobarbital were incubated with PCB 136 (5 μM) in the presence of the respective antioxidant (0–10 mM), and levels and chiral signatures of PCB 136 and its HO-PCB metabolites were determined. Three metabolites, 5-136 (2,2′,3,3′,6,6′-hexachlorobiphenyl-5-ol), 4-136 (2,2′,3,3′,6,6′-hexachlorobiphenyl-4-ol) and 4,5-136 (2,2′,3,3′,6,6′-hexachlorobiphenyl-4,5-diol), were detected in all incubations, with 5-136 being the major metabolite. Compared to microsomal incubations without antioxidant, levels of 4,5-136 increased with increasing antioxidant concentration, whereas levels of PCB 136 and both mono-HO-PCBs were not affected by the presence of either antioxidant. PCB 136, 4-136 and 5-136 displayed significant atropisomeric enrichment; however, the direction and extent of the atropisomeric enrichment was not altered in the presence of an antioxidant. Because 4,5-136 can either be conjugated to a sulfate or glucuronide metabolite that is readily excreted or further oxidized a potentially toxic PCB 136 quinone, the effect of both thiol antioxidants on 4,5-136 formation suggests that disruptions of glutathione homeostasis may alter the balance between both metabolic pathways and, thus, PCB 136 toxicity in vivo.

Gas Chromatographic Enantiomer Separation of Polychlorinated Biphenyls (PCBs): Methods, Metabolisms, Enantiomeric Composition in Environmental Samples and their Interpretation

“…The following recent case study was chosen for a deeper investigation of the consequences of this hypothesis for enantioselective PCB metabolism. The study of Wu and Lehmler provided a nearly complete set of parameters, which allowed retrospective data analysis. In this study, liver microsomes from rats (pretreated with phenobarbital) were incubated with PCB 136 (5 μM) alone and also in the additional presence of antioxidants (0.01–10 mM of glutathione or N‐acetyl‐cysteine (NAC)).…”

“…In this study, liver microsomes from rats (pretreated with phenobarbital) were incubated with PCB 136 (5 μM) alone and also in the additional presence of antioxidants (0.01–10 mM of glutathione or N‐acetyl‐cysteine (NAC)). In each case, nonracemic PCB 136 (stronger prevalence of (−)‐PCB 136), as well as nonracemic metabolites (i.e., 5‐OH‐PCB 136, 4‐OH‐PCB‐136, and 4,5‐diOH‐PCB 136), were detected, whose shared concentrations amounted to 62–65 % of the parent compound . Higher antioxidant levels increased the relative abundance of 4,5‐diOH‐PCB 136 in the OH‐PCBs metabolite group from ∼3 to ∼12 %.…”

“…Higher antioxidant levels increased the relative abundance of 4,5‐diOH‐PCB 136 in the OH‐PCBs metabolite group from ∼3 to ∼12 %. However, neither the overall share of OH‐PCBs (∼65 % PCB 136), nor the EF of PCB 136 (0.43–0.47), nor its metabolites (deviation ∼0.1 from racemate) were altered under the impact of the two antioxidants …”

“…The data of Wu and Lehmler enabled the following retrospective evaluation of the enantioselective fate of PCB 136. Recovery of PCB 136 and OH‐PCBs during sample clean‐up was ∼93 or ∼94 % (no difference between parent compound and metabolites).…”

Chemoselective reactions and their applications in metabolite isolation and analysis.

“…Since publication of this work, several articles have been published describing oxidation behavior of thiol-containing amino acids with glutathione, quantitative analysis of thiols in biofluids, and antioxidant behavior of thiols [164][165][166]. Clearly the analysis of thiol metabolites is of great interest.…”