Stereoselective Formation of Mono- and Dihydroxylated Polychlorinated Biphenyls by Rat Cytochrome P450 2B1

Abstract

Changes in atropisomer composition of chiral polychlorinated biphenyls (PCBs) and their mono- and di- hydroxylated metabolites (OH- and diOH-PCBs) via rat cytochrome P450 2B1 (CYP2B1) mediated biotransformation were investigated in vitro. Rat CYP2B1 could stereoselectively biotransform chiral PCBs to generate meta-OH-PCBs as the major metabolites after 60 min incubations. Non-racemic enantiomer fractions (EFs: concentration ratios of the (+)-atropisomer or the first-eluting atropisomer over the total concentrations of two atropisomers) of 5-OH-PCBs, were 0.17, 0.20, 0.85, 0.77 and 0.41 for incubations with PCBs 91, 95, 132, 136 and 149, respectively. CYP-mediated stereoselective formation of diOH-PCBs from OH-PCBs was observed for the first time. After 60 min stereoselective biotransformation, the EFs of both 4-OH-PCB 95 and 5-OH-PCB 95 changed from racemic (i.e., 0.50) to 0.62 and 0.46, respectively. These transformations generated statistically non-racemic 4,5-diOH-PCB 95, with EFs of 0.53 and 0.58 for 4-OH-PCB 95 and 5-OH-PCB 95 incubations, respectively. Biotransformation of PCBs 91 and 136 also generated 4,5-diOH-PCB 91 and 4,5-diOH-PCB 136, respectively. These in vitro results were consistent with that observed for stereoselective PCB biotransformation by rat liver microsomes and in vivo. Biotransformation interference between two atropisomers of PCB 136 was investigated for the first time in this study. The biotransformation process of (−)-PCB 136 was significantly disrupted by the presence of (+)-PCB 136, but not the other way around. Thus, stereoselective metabolism of chiral PCBs and OH-PCBs by CYPs is a major mechanism for atropisomer composition change of PCBs and their metabolites in the environment, with the degree of composition change dependent, at least in part, on stereoselective interference of atropisomers with each other at the enzyme level.

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 low affinity reaction had an apparent K m of 645 μM and a V max of 367 pmoles/min/mg hepatocyte. Several recent studies using pure PCB 132 and 136 atropisomers suggest that the respective atropisomers interfere with each other’s metabolism by CYP2B1 (Lu et al 2013, Lu and Wong 2011), and further studies of the enzyme kinetics of pure PCB atropisomers are needed.…”

“…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. 2009) and dog CYP2B11 (Waller et al 1999), are involved in the formation of meta hydroxylated C-PCBs.…”

Enantioselective accumulation of chiral polychlorinated biphenyls in lotus plant (Nelumbonucifera spp.)

“…A major mechanism for changing EFs of PCBs is from stereoselective metabolism by CYPs [33]. For example, enantioselective biotransformation of chiral PCBs in the plant leaves may possibly be due to metabolism by CYP enzymes there [20].…”

“…The formation of neurotoxic metabolites like 4-OH-PCB 136 may play a role in the developmental neurotoxicity of PCBs [7]. PCBs 91, 95 and 136 can be stereoselectively biotransformed to OH-PCB and dihydroxylated metabolites (diOH-PCB) by rat CYP2B1 in vitro, with significantly higher biotransformation activities of (+)-PCB 91 (E1-PCB 91), (+)-PCB 95 (E2-PCB 95), and (+)-PCB 136 but no apparent stereoselectivity for PCB 149 [33]. The formation rate of 5-OH-PCBs formation is congener-specific for these four PCBs.…”