Oxidation of Polychlorinated Biphenyls by Liver Tissue Slices from Phenobarbital-Pretreated Mice Is Congener-Specific and Atropselective

Abstract

Mouse models are powerful tools to study the developmental neurotoxicity of polychlorinated biphenyls (PCBs); however, studies of the oxidation of chiral PCB congeners to potentially neurotoxic hydroxylated metabolites (OH-PCBs) in mice have not been reported. Here we investigate the atropselective oxidation of chiral PCB 91 (2,2',3,4',6-pentachlorobiphenyl), PCB 95 (2,2',3,5',6-pentachlorobiphenyl), PCB 132 (2,2',3,3',4,6'-hexachlorobiphenyl), PCB 136 (2,2',3,3',6,6'-hexachlorobiphenyl) and PCB 149 (2,2',3,4',5',6-hexachlorobiphenyl) to OH-PCBs in liver tissue slices prepared from female mice. The metabolite profile of PCB 136 typically followed the rank order 5-OH-PCB > 4-OH-PCB > 4,5-OH-PCB, and metabolite levels increased with PCB concentration and incubation time. A similar OH-PCB profile was observed with the other PCB congeners, with 5-OH-PCB:4-OH-PCB ratios ranging from 2 to 12. More 5-OH-PCB 136 was formed in liver tissue slices obtained from animals pretreated with phenobarbital (P450 2B inducer) or, to a lesser extent, dexamethasone (P450 2B and 3A enzyme inducer) compared to tissue slices prepared from vehicle-pretreated animals. The apparent rate of 5-OH-PCBs formation followed the approximate rank order PCB 149 > PCB 91 > PCB 132 ~ PCB 136 > PCB 95. Atropselective gas chromatography revealed a congener-specific atropisomeric enrichment of major OH-PCB metabolites. Comparison of our results with published OH-PCB patterns and chiral signatures (i.e., the direction and extent of the atropisomeric enrichment) from rat liver microsomal revealed drastic differences between both species, especially following induction of P450 2B enzymes. These species differences in the metabolism of chiral PCBs should be considered in developmental neurotoxicity studies of PCBs.

Chiral polychlorinated biphenyls: absorption, metabolism and excretion—a review

“…A convenient nomenclature for PCB metabolites has been proposed by Maervoet et al and, as shown for PCB 136 in Fig 3, will be used throughout this manuscript (Maervoet et al 2004). The oxidation of C-PCBs, in particular those with a 2,3,6-trichloro substitution pattern in one phenyl ring, to HO-PCBs has been studied extensively using recombinant enzymes (Lu et al 2013, Lu and Wong 2011, Waller et al 1999, Warner et al 2009), hepatic microsomes (Kania-Korwel et al 2011, Kania-Korwel and Lehmler 2013, Schnellmann et al 1983, Wu et al 2014, Wu et al 2011), isolated hepatocytes (Vickers et al 1986) and, liver, hippocampus and skin slices (Garner et al 2006, Wu et al 2013a, Wu et al 2013b) obtained from mammalian species. To the best of our knowledge, the oxidation of C-PCBs in non-mammalian species, such as amphibians, fish or avian species, has not been investigated to date.…”

“…The 4,5-epoxide intermediate of C-PCBs with a 2,3,6-trichloro substitution pattern can rearrange to form both 4- or 5-hydroxylated PCB metabolites. In addition, a 1,2-shift (or NIH-shift) (Guroff et al 1967, Jerina and Daly 1974) of the 3-chloro substituent can lead to the formation of HO-PCBs with a 2,4,6-trichloro-3-hydroxy substitution pattern as minor metabolites (Kania-Korwel et al 2012, Kania-Korwel et al 2011, Wu et al 2013a, Wu et al 2014). Studies with recombinant P450 enzymes demonstrate that CYP2B isoforms, such as rat CYP2B1 (Lu et al 2013, Waller et al 1999, Warner et al 2009), human CYP2B6 (Warner et al.…”

“…Several studies using recombinant P450 enzymes (Lu et al 2013, Warner et al 2009), liver microsomes (Kania-Korwel et al 2011, Kania-Korwel and Lehmler 2013, Wu et al 2014, Wu et al 2011) or liver tissue slices (Wu et al 2013a, Wu et al 2013b) demonstrate that PCBs are atropselectively metabolized to HO-CPBs by P450 enzymes, thus resulting in an atropisomeric enrichment of PCBs and HO-PCBs in the microsomal incubation (Table 2). The atropselective biotransformation of C-PCBs to HO-PCB metabolites by recombinant rat and human cytochrome P450 enzymes has been studies in some detail (Lu et al 2013, Warner et al 2009).…”

“…Moreover, the atropisomeric enrichment of E 2 -PCB 95 in the in vitro metabolism experiments is consistent with the atropisomeric enrichment of PCB 95 observed in rats treated with an Aroclor 1254 mixture (Kania-Korwel et al 2006). Similarly, in vitro metabolism studies (Wu et al 2013a) accurately predict the direction of the atropisomeric enrichment of PCB 136 (Kania-Korwel et al 2010, Kania-Korwel et al 2008a, Kania-Korwel et al 2007, Kania-Korwel et al 2008d, Milanowski et al 2010) and PCB 95 (Kania-Korwel et al 2012, Kania-Korwel et al 2010, Milanowski et al 2010) in mice in vivo . Incubations with human CYP2B6 result in biotransformation of PCBs 45, 91 and 132; however, only the biotransformation of PCBs 45 and 132 is enantioselective.…”

“…Several recent metabolism studies demonstrate that the atropselective formation of HO-PCBs is also species dependent (Kania-Korwel et al 2008b, Wu et al 2013a, Wu et al 2014, Wu et al 2013b, Wu et al 2011). For example, both 5-HO-PCB136 and 4-HO-PCB136 are atropselectively formed by liver microsomes from humans, dogs, monkeys, guinea pigs, mice, hamsters and rabbits (Fig.…”