2,2′,3,3′,6,6′-Hexachlorobiphenyl (PCB 136) Atropisomers Interact Enantioselectively with Hepatic Microsomal Cytochrome P450 Enzymes

Abstract

2,2',3,3',6,6'-Hexachlorobiphenyl (PCB 136) is a chiral and highly neurotoxic PCB congener of environmental relevance. (+)-PCB 136 was previously shown to be enriched in tissues from mice treated with racemic PCB 136. We investigated the spectral interactions of (+)-, (-)-, and (+/-)-PCB 136 with mouse and rat hepatic microsomal cytochrome P450 (P450) enzymes to test the hypothesis that enantioselective binding to specific P450 enzymes causes the enrichment of (+)-PCB 136 in vivo. Hepatic microsomes prepared from C57BL/6 mice or Long Evans rats treated with beta-naphthoflavone or 3-methylcholanthrene, phenobarbital, and dexamethasone (prototypical inducers of CYP1A, CYP2B, and CYP3A, respectively) were used to determine first, whether the (+)-PCB 136 atropisomer binds to hepatic microsomal P450 enzymes to a greater extent than does the (-)-PCB 136 atropisomer and second, whether P450 enzymes of one subfamily bind the two PCB 136 atropisomers more efficiently than do P450 enzymes of other subfamilies. Increasing concentrations of (+)-, (-)-, or (+/-)-PCB 136 were added to hepatic microsomes, and the difference spectrum and maximal absorbance change, a measure of PCB binding to P450 enzymes, were measured. A significantly larger absorbance change was observed with (+)-PCB 136 than with (-)-PCB 136 with all four hepatic microsomal preparations in mice and rats, indicating that (+)-PCB 136 interacted with microsomal P450 enzymes to a greater degree than did (-)-PCB 136. In addition, binding of the PCB 136 atropisomers was greatest in microsomes from PB-treated mice and rats and was inhibited by CYP2B antibodies, indicating the involvement of CYP2B enzymes. Together, these results suggest preferential binding of (+)-PCB 136 to P450 enzymes (such as CYP2B and CYP3A) in hepatic microsomes, an observation that may explain the enantioselective enrichment of the (+)-PCB 136 atropisomer in tissues of mice.

Cytochrome P450 mRNA Expression in the Rodent Brain: Species-, Sex-, and Region-Dependent Differences

“…Male and female C57BL/6 mice (7-8 weeks) were obtained from the Jackson Laboratory (Bar Harbor, ME) and randomly assigned to one of four groups: 1) phenobarbital (PB; Sigma-Aldrich, St. Louis, MO) at 102 mg/kg/day in saline or 2) saline at 20 ml/kg/day, i.p., for 3 consecutive days; 3) dexamethasone (DEX; Sigma-Aldrich) at 50 mg/kg/day in corn oil (CO) or 4) CO (Fisher Scientific, Pittsburg, PA) at 10 ml/kg/day, i.p., for 4 consecutive days (KaniaKorwel et al, 2008). Male Sprague-Dawley rats (8 weeks) were purchased from Harlan, Inc. (Indianapolis, IN), acclimated for 1 week, then randomly assigned to one of four groups: 1) PB at 102 mg/kg/day in saline or 2) saline at 5 ml/kg/day, i.p., for 3 consecutive days; 3) DEX at 50 mg/kg/day in CO or 4) CO vehicle control at 5 ml/kg/day, i.p., for 4 consecutive days (Kania-Korwel et al, 2008). Animals were euthanized 24 hours after the last treatment by CO 2 asphyxiation followed by cervical dislocation.…”

“…We also assessed the influence of sex and species on P450 expression profiles in the brain using the male mouse as the reference. We studied CYP2B (mouse CYP2B10/rat CYP2B1/2), CYP3A (mouse CYP3A11/rat CYP3A2), and CYP1A2 because these isoforms have been implicated in the metabolism of polychlorinated biphenyls (Kania-Korwel et al, 2008Curran et al, 2011), polybrominated diphenyl ethers (Erratico et al, 2013;Feo et al, 2013), and organophosphorus pesticides (Tang et al, 2001;Foxenberg et al, 2007). Two orphan P450s, CYP4X1 and CYP2S1 (Guengerich et al, 2010), were also included in this study because s This article has supplemental material available at dmd.aspetjournals.org.…”

Experimental and Theoretical Evidence for Diastereomer- and Enantiomer-Specific Accumulation and Biotransformation of HBCD in Maize Roots

“…The individual enantiomers of chiral compounds can interact enantioselectively with enzymes and biological receptors in organisms. 5,26 For example, Kania-Korwel et al 27 found a greater binding of (+)-PCB 136 to P450 isozymes CYP2B and CYP3A than (−)-PCB 136 in mouse and rat in vivo. The (−)-o,p,-DDT enantiomer was found to be an active estrogen mimic at the human estrogen receptor, whereas (+)-o,p,-DDT had negligible activity.…”

Chirality as an Environmental Forensics Tool

“…In vitro experiments showed that (þ)-PCB 136 bound more strongly to mouse CYP isozymes than (À)-PCB 136 [277]. Binding was inhibited by CYP2B antibodies, indicating that CYP2B isozymes were involved [277]. However, enantioselectivity of PCBs did not change in mice exposed in vivo to CYP2B or CYP3A inducers [278], indicating that enantioselective bioprocessing of PCBs was not due solely to CYP-mediated biotransformation.…”

“…In female mice, enrichment of (þ)-PCB 136 decreased with increasing dose (2-50 mg/kg body weight) of the racemate, suggesting a saturation of the enantioselective processes in the mice [276]. In vitro experiments showed that (þ)-PCB 136 bound more strongly to mouse CYP isozymes than (À)-PCB 136 [277]. Binding was inhibited by CYP2B antibodies, indicating that CYP2B isozymes were involved [277].…”