Enantiomeric enrichment of 2,2',3,3',6,6'-hexachlorobiphenyl (PCB 136) in mice after induction of CYP enzymes

The disposition of polychlorinated biphenyls (PCBs) differs between germ-free and conventional mice

The disposition of toxicants, such as polychlorinated biphenyls (PCBs), in germ-free (GF) vs. conventional (CV) mice has received little attention to date. Here, we investigate PCB levels in three-month-old female CV and GF mice exposed orally daily for 3 days to 0, 6, or 30 mg/kg body weight of the Fox River Mixture (FRM), an environmental PCB mixture. We euthanized animals 24 h after the final dose. PCB profiles in tissues differed from the FRM profile but were similar in tissues across all 4 PCB exposure groups. PCB levels in CV but not GF mice followed the difference in PCB dose. Importantly, PCB levels were higher in CV than GF mice exposed to the same dose. Hepatic cytochrome P450 enzyme or lipid levels did not explain these trends in PCB tissue levels. Thus, toxicity studies with CV and GF animals need to assess the toxicokinetics of the toxicant investigated. CAPSULE: PCB levels are typically higher in conventional than germ-free mice exposed to the same dose of PCBs.

Gut Microbiome Critically Impacts PCB-induced Changes in Metabolic Fingerprints and the Hepatic Transcriptome in Mice

Polychlorinated biphenyls (PCBs) are ubiquitously detected and have been linked to metabolic diseases. Gut microbiome is recognized as a critical regulator of disease susceptibility; however, little is known how PCBs and gut microbiome interact to modulate hepatic xenobiotic and intermediary metabolism. We hypothesized the gut microbiome regulates PCB-mediated changes in the metabolic fingerprints and hepatic transcriptome. Ninety-day-old female conventional and germ-free mice were orally exposed to the Fox River Mixture (synthetic PCB mixture, 6 or 30 mg/kg) or corn oil (vehicle control, 10 ml/kg), once daily for 3 consecutive days. RNA-seq was conducted in liver, and endogenous metabolites were measured in liver and serum by LC-MS. Prototypical target genes of aryl hydrocarbon receptor, pregnane X receptor, and constitutive androstane receptor were more readily upregulated by PCBs in conventional conditions, indicating PCBs, to the hepatic transcriptome, act partly through the gut microbiome. In a gut microbiome-dependent manner, xenobiotic, and steroid metabolism pathways were upregulated, whereas response to misfolded proteins-related pathways was downregulated by PCBs. At the high PCB dose, NADP, and arginine appear to interact with drug-metabolizing enzymes (ie, Cyp1-3 family), which are highly correlated with Ruminiclostridium and Roseburia, providing a novel explanation of gut-liver interaction from PCB-exposure. Utilizing the Library of Integrated Network-based Cellular Signatures L1000 database, therapeutics targeting anti-inflammatory and endoplasmic reticulum stress pathways are predicted to be remedies that can mitigate PCB toxicity. Our findings demonstrate that habitation of the gut microbiota drives PCB-mediated hepatic responses. Our study adds knowledge of physiological response differences from PCB exposure and considerations for further investigations for gut microbiome-dependent therapeutics.

Gut Microbiota Modulates Interactions Between Polychlorinated Biphenyls and Bile Acid Homeostasis

The gut microbiome is increasingly recognized as a second genome that contributes to the health and diseases of the host. A major function of the gut microbiota is to convert primary bile acids (BAs) produced from cholesterol in the liver into secondary BAs that activate distinct host receptors to modulate xenobiotic metabolism and energy homeostasis. The goal of this study was to investigate to what extent oral exposure to an environmentally relevant polychlorinated biphenyl (PCBs mixture), namely the Fox River mixture, impacts gut microbiome and BA homeostasis. Ninety-day-old adult female conventional (CV) and germ-free (GF) C57BL/6 mice were orally exposed to corn oil (vehicle), or the Fox River mixture at 6 or 30 mg/kg once daily for 3 consecutive days. The PCB low dose profoundly increased BA metabolism related bacteria Akkermansia (A.) muciniphila, Clostridium (C.) scindens, and Enterococcus in the large intestinal pellet (LIP) of CV mice (16S rRNA sequencing/qPCR). This correlated with a PCB low dose-mediated increase in multiple BAs in serum and small intestinal content (SIP) in a gut microbiota-dependent manner (UPLC-MS/MS). Conversely, at PCB high dose, BA levels remained stable in CV mice correlated with an increase in hepatic efflux transporters and ileal Fgf15. Interestingly, lack of gut microbiota potentiated the PCB-mediated increase in taurine conjugated α and β muricholic acids in liver, SIP, and LIP. Pearson's correlation identified positive correlations between 5 taxa and most secondary BAs. In conclusion, PCBs dose-dependently altered BA homeostasis through a joint effort between host gut-liver axis and intestinal bacteria.

Atropselective Disposition of 2,2',3,4',6-Pentachlorobiphenyl (PCB 91) and Identification of Its Metabolites in Mice with Liver-Specific Deletion of Cytochrome P450 Reductase

Hepatic cytochrome P450 enzymes metabolize chiral polychlorinated biphenyls (PCBs) to hydroxylated metabolites (OH-PCBs). Animal models with impaired metabolism of PCBs are one approach to study how the atropselective oxidation of PCBs to OH-PCBs contributes to toxic outcomes, such as neurodevelopmental disorders, following PCB exposure. We investigated the disposition of PCB 91, a para-substituted PCB congener, in mice with a liver-specific deletion of the cytochrome P450 reductase (cpr) gene (KO mice). KO mice and wild-type (WT) mice were exposed orally to racemic PCB 91 (30 mg/kg b.w.). Levels and enantiomeric fractions of PCB 91 and its hydroxylated metabolites were determined in tissues 3 days after PCB exposure and in excreta on days 1-3 after PCB exposure. PCB 91, but not OH-PCB levels were higher in KO compared to WT mice. The elevated fat and protein content in the liver of KO mice resulted in the hepatic accumulation of PCB 91. OH-PCBs were detected in blood, liver, and excreta samples of KO and WT mice. 2,2',3,4',6-Pentachlorobiphenyl-5-ol (5-91) was the major metabolite. A considerable percent of the total PCB 91 dose (%TD) was excreted with the feces as 5-91 (23%TD and 31%TD in KO and WT mice, respectively). We tentatively identified glucuronide and sulfate metabolites present in urine samples. The PCB 91 atropisomer eluting first on the chiral column (E₁-PCB 91) displayed genotype-dependent atropisomeric enrichment, with a more pronounced atropisomeric enrichment observed in WT compared to KO mice. E₁-atropisomers of 5-91 and 2,2',3,4',6-pentachlorobiphenyl-4-ol (4-91) were enriched in blood and liver, irrespective of the genotype; however, the extent of the enrichment of E₁-5-91 was genotype dependent. These differences in atropselective disposition are consistent with slower metabolism of PCB 91 in KO compared to WT mice and the accumulation of the parent PCB in the fatty liver of KO mice.

Toxicokinetics of Chiral PCB 136 and Its Hydroxylated Metabolites in Mice with a Liver-Specific Deletion of Cytochrome P450 Reductase

Exposure to polychlorinated biphenyls (PCBs) has been implicated in adverse human health effects, including developmental neurotoxicity. Several neurotoxic PCBs are chiral and undergo atropisomeric enrichment in vivo due to atropselective metabolism by cytochrome P450 enzymes. Here we study how the liver-specific deletion of the cytochrome P450 reductase ( cpr) gene alters the toxicokinetics of 2,2',3,3',6,6'-hexachlorobiphenyl (PCB 136) in mice. Male and female mice with a liver-specific deletion of cpr (KO) and congenic wild-type (WT) mice were exposed to a single oral dose of racemic PCB 136 (6.63 mg/kg). Levels and chiral signatures of PCB 136 and its hydroxylated metabolites were determined 1-48 h after PCB exposure in whole blood. Blood levels of PCB 136 were typically higher in M-WT compared to F-WT mice. At the later time points, F-KO mice had significantly higher PCB 136 levels than F-WT mice. 2,2',3',4,6,6'-Hexachlorobiphenyl-3-ol (3-150), 2,2',3,3',6,6'-hexachlorobiphenyl-4-ol (4-136), 2,2',3,3',6,6'-hexachlorobiphenyl-5-ol (5-136), and 4,5-dihydroxy-2,2',3,3',6,6'-hexachlorobiphenyl (4,5-136) were detected in blood, with 5-136 and 4-136 being major metabolites. At later time points, the sum of HO-PCB (∑HO-PCB) levels exceeded PCB 136 levels in the blood; however, higher ∑HO-PCB than PCB 136 levels were observed later in KO than WT mice. PCB 136 and its major metabolites displayed atropisomeric enrichment in a manner that depended on the time point, sex, and genotype. Toxicokinetic analysis revealed sex and genotype-dependent differences in toxicokinetic parameters for PCB 136 atropisomers and its metabolites. The results suggest that mice with a liver-specific deletion of the cpr gene can potentially be used to assess how an altered metabolism of neurotoxic PCB congeners affects neurotoxic outcomes following exposure of the offspring to PCBs via the maternal diet.

Changes to polychlorinated biphenyl (PCB) signatures and enantiomer fractions across different tissue types in Guillemots

Two Guillemot carcases were dissected, each providing 12 discrete tissue samples and 3 samples of partially digested food. One hundred and five PCBs from the 209 PCBs determined by GCxGC-ToFMS were detected. The relative proportions of individual PCBs did not vary greatly within tissue types, although the PCB profile from undigested food could be distinguished. Enantiomer fractions (EFs) were determined for CB-95, CB-136 and CB-149 by GC-HRqToFMS. EFs in the partially digested food were near racemic, with high levels of enrichment for E1 CB-95 in the kidneys and liver (EF of 0.80 and 0.84 respectively). This provides some of the clearest evidence to date that fractionation takes place in the organs where metabolic biotransformation and elimination of PCBs occurs. Our findings also confirm the ability of non-lethal sampling techniques, such as collection of small (<1 g) blood samples, to provide PCB signatures that are representative of an individual organism.

Sources and toxicities of phenolic polychlorinated biphenyls (OH-PCBs)

Polychlorinated biphenyls (PCBs), a group of 209 congeners that differ in the number and position of chlorines on the biphenyl ring, are anthropogenic chemicals that belong to the persistent organic pollutants (POPs). For many years, PCBs have been a topic of interest because of their biomagnification in the food chain and their environmental persistence. PCBs with fewer chlorine atoms, however, are less persistent and more susceptible to metabolic attack, giving rise to chemicals characterized by the addition of one or more hydroxyl groups to the chlorinated biphenyl skeleton, collectively known as hydroxylated PCBs (OH-PCBs). In animals and plants, this biotransformation of PCBs to OH-PCBs is primarily carried out by cytochrome P-450-dependent monooxygenases. One of the reasons for infrequent detection of lower chlorinated PCBs in serum and other biological matrices is their shorter half-lives, and their metabolic transformation, resulting in OH-PCBs or their conjugates, such as sulfates and glucuronides, or macromolecule adducts. Recent biomonitoring studies have reported the presence of OH-PCBs in human serum. The occurrence of OH-PCBs, the size of this group (there are 837 mono-hydroxyl PCBs alone), and their wide spectra of physical characteristics (pKa's and log P's ranging over 5 to 6 orders of magnitude) give rise to a multiplicity of biological effects. Among those are bioactivation to electrophilic metabolites that can form covalent adducts with DNA and other macromolecules, interference with hormonal signaling, inhibition of enzymes that regulate cellular concentrations of active hormones, and interference with the transport of hormones. This new information creates an urgent need for a new perspective on these often overlooked metabolites.

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

Seventy eight out of the 209 possible polychlorinated biphenyl (PCB) congeners are chiral, 19 of which exist under ambient conditions as stable rotational isomers that are non-superimposable mirror images of each other. These congeners (C-PCBs) represent up to 6 % by weight of technical PCB mixtures and undergo considerable atropisomeric enrichment in wildlife, laboratory animals, and humans. The objective of this review is to summarize our current knowledge of the processes involved in the absorption, metabolism, and excretion of C-PCBs and their metabolites in laboratory animals and humans. C-PCBs are absorbed and excreted by passive diffusion, a process that, like other physicochemical processes, is inherently not atropselective. In mammals, metabolism by cytochrome P450 (P450) enzymes represents a major route of elimination for many C-PCBs. In vitro studies demonstrate that C-PCBs with a 2,3,6-trichlorosubstitution pattern in one phenyl ring are readily oxidized to hydroxylated PCB metabolites (HO-PCBs) by P450 enzymes, such as rat CYP2B1, human CYP2B6, and dog CYP2B11. The oxidation of C-PCBs is atropselective, thus resulting in a species- and congener-dependent atropisomeric enrichment of C-PCBs and their metabolites. This atropisomeric enrichment of C-PCBs and their metabolites likely plays a poorly understood role in the atropselective toxicity of C-PCBs and, therefore, warrants further investigation.

Toxicokinetics of chiral polychlorinated biphenyls across different species--a review

Nineteen polychlorinated biphenyls (chiral or C-PCBs) exist as two stable rotational isomers (atropisomers) that are non-superimposable mirror images of each other. C-PCBs are released into the environment as racemic (i.e., equal) mixtures of both atropisomers and undergo atropisomeric enrichment due to biological, but not abiotic, processes. In particular, toxicokinetic studies provide important initial insights into atropselective processes involved in the disposition (i.e., absorption, distribution, biotransformation, and excretion) of C-PCBs. The toxicokinetic of C-PCBs is highly congener and species dependent. In particular, at lower trophic levels, abiotic processes play a predominant role in C-PCB toxicokinetics. Biotransformation plays an important role in the elimination of C-PCBs in mammals. The elimination of C-PCB follows the approximate order mammals > birds > amphibians > fish, mostly due to a corresponding decrease in metabolic capacity. A few studies have shown differences in the toxicokinetics of C-PCB atropisomers; however, more work is needed to understand the toxicokinetics of C-PCBs and the underlying biological processes. Such studies will not only contribute to our understanding of the fate of C-PCBs in aquatic and terrestrial food webs but also facilitate our understanding of human exposures to C-PCBs.

Effect of pregnancy on the disposition of 2,2',3,5',6-pentachlorobiphenyl (PCB 95) atropisomers and their hydroxylated metabolites in female mice

Chiral PCBs, such as PCB 95, are developmental neurotoxicants that undergo atropisomeric enrichment in nonpregnant adult mice. Because pregnancy is associated with changes in hepatic cytochrome P450 enzyme activity as well as lipid disposition and metabolism, this study investigates the effect of pregnancy on the maternal disposition of chiral PCBs. Female C57BL/6 mice (8 weeks old) were dosed daily beginning 2 weeks prior to conception and continuing throughout gestation and lactation (56 days total) with racemic PCB 95 (0, 0.1, 1.0, or 6.0 mg/kg body wt/day) in peanut butter. Levels and chiral signatures of PCB 95 and its hydroxylated metabolites (OH-PCBs) were determined in adipose, blood, brain, and liver. Tissue levels of PCB 95 increased 4- to 12-fold with increasing dose, with considerable enrichment of the second eluting atropisomer in all tissues (EF range 0.11 to 0.26). OH-PCBs displayed atropisomeric enrichment in blood and liver but were not detected in adipose and brain. Levels of PCB 95 and its metabolites were 2- to 11-fold lower in pregnant dams relative to those previously reported in nonpregnant age-matched female mice; however, PCB 95 and OH-PCB profiles and chiral signatures were similar between both studies. In contrast, human brain samples contained racemic PCB 95 residues (EF = 0.50). These results demonstrate that changes in cytochrome P450 enzyme activity and lipid disposition during pregnancy reduce the PCB body burden in dams but do not affect metabolite profiles or chiral signatures. The differences in chiral signatures between mice and humans suggest species-specific differences in atropisomeric disposition, the toxicological significance of which remains to be determined.

Hepatic metabolism affects the atropselective disposition of 2,2',3,3',6,6'-hexachlorobiphenyl (PCB 136) in mice

To understand the role of hepatic vs extrahepatic metabolism in the disposition of chiral PCBs, we studied the disposition of 2,2',3,3',6,6'-hexachlorobiphenyl (PCB 136) and its hydroxylated metabolites (HO-PCBs) in mice with defective hepatic metabolism due to the liver-specific deletion of cytochrome P450 oxidoreductase (KO mice). Female KO and congenic wild type (WT) mice were treated with racemic PCB 136, and levels and chiral signatures of PCB 136 and HO-PCBs were determined in tissues and excreta 3 days after PCB administration. PCB 136 tissue levels were higher in KO compared to WT mice. Feces was a major route of PCB metabolite excretion, with 2,2',3,3',6,6'-hexachlorobiphenyl-5-ol being the major metabolite recovered from feces. (+)-PCB 136, the second eluting PCB 136 atropisomers, was enriched in all tissues and excreta. The second eluting atropisomers of the HO-PCBs metabolites were enriched in blood and liver; 2,2',3,3',6,6'-hexachlorobiphenyl-5-ol in blood was an exception and displayed an enrichment of the first eluting atropisomers. Fecal HO-PCB levels and chiral signatures changed with time and differed between KO and WT mice, with larger HO-PCB enantiomeric fractions in WT compared to KO mice. Our results demonstrate that hepatic and, possibly, extrahepatic cytochrome P450 (P450) enzymes play a role in the disposition of PCBs.

Oxidation of polychlorinated biphenyls by liver tissue slices from phenobarbital-pretreated mice is congener-specific and atropselective

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 the induction of P450 2B enzymes. These species differences in the metabolism of chiral PCBs should be considered in developmental neurotoxicity studies of PCBs.

Metabolism of 2,2',3,3',6,6'-hexachlorobiphenyl (PCB 136) atropisomers in tissue slices from phenobarbital or dexamethasone-induced rats is sex-dependent

1. Chiral polychlorinated biphenyls (PCBs) such as PCB 136 enantioselectively sensitize the ryanodine receptor (RyR). In light of recent evidence that PCBs cause developmental neurotoxicity via RyR-dependent mechanisms, this suggests that enantioselective PCB metabolism may influence the developmental neurotoxicity of chiral PCBs. However, enantioselective disposition of PCBs has not been fully characterized. 2. The effect of sex and cytochrome P450 (P450) enzyme induction on the enantioselective metabolism of PCB 136 was studied using liver tissue slices prepared from naïve control (CTL), phenobarbital (PB; CYP2B inducer) or dexamethasone (DEX; CYP3A inducer) pretreated adult Sprague-Dawley rats. PCB 136 metabolism was also examined in hippocampal slices derived from untreated rat pups. 3. In liver tissue slices, hydroxylated PCB (OH-PCB) profiles depended on sex and inducer pretreatment, and OH-PCB levels followed the rank orders male > female and PB > DEX > CTL. In contrast, the enantiomeric enrichment of PCB 136 and its metabolites was independent of sex and inducer pretreatment. Only small amounts of PCB 136 partitioned into hippocampal tissue slices and no OH-PCB metabolites were detected. 4. Our results suggest that enantioselective metabolism, sex and induction status of P450 enzymes in the liver may modulate the neurotoxic outcomes of developmental exposure to chiral PCBs.

Absolute configuration of atropisomeric polychlorinated biphenyl 183 enantiomerically enriched in human samples

Polychlorinated biphenyls (PCBs) are still of serious concern as a potential health hazard due to their persistency and bioacumulation. Of 209 possible PCB congeners, with varying number and position of chlorine atom(s), 19 are chiral. These are mostly highly chlorinated and tend to remain longer against the biological decompositions, suffering biological deracemization in the environment. In this work, we have unequivocally determined the absolute configurations of important chiral PCBs 183 and 171, as well as 132, through the combined theoretical and experimental investigations of the chiroptical properties (circular dichroism and optical rotation), which will be valuable in elucidating the mechanism of biological enantiomer enrichment of PCBs in the environment.

Assessment of the disposition of chiral polychlorinated biphenyls in female mdr 1a/b knockout versus wild-type mice using multivariate analyses

Polychlorinated biphenyls (PCBs) are present in the environment as complex mixtures, which make it challenging to identify PCB congeners that may be subject to active transport processes. Here we employ a transgenic mouse model in combination with multivariate analyses to investigate if chiral PCBs 91, 95, 132, 136, 149, 174, 176 and 183 are subject to active (enantioselective) transport by multidrug resistance (MDR) transporters. A synthetic PCB mixture containing these congeners was administered orally to female FVB or mdr1a/1b knockout mice. Due to the short half-life of chiral PCB congeners, mice were euthanized after 24h and PCB concentrations and enantiomeric fractions were determined in selected tissues and excreta. Principal component analysis did not reveal differences between wild-type and mdr1a/1b knockout mice. However, Hotelling T(2)-test revealed significantly lower PCB concentrations and a more pronounced enantiomeric enrichment in the adipose tissue of mdr1a/1b knockout mice. These differences are due to higher body weights and higher fecal fat contents of mdr1a/1b knockout mice. Analysis of the enantiomeric fractions of PCBs 91, 95, 136, 149 and 174 showed a significant enantiomeric enrichment for all five congeners in wild-type and mdr1a/1b knockout mice. Overall, by studying a PCB mixture in a transgenic mouse model in combination with a multivariate data reduction approach, PCBs 91, 95, 136, 149 and 174 could be excluded as substrates of multidrug resistance transporters 1a/b.

Clearance of polychlorinated biphenyl atropisomers is enantioselective in female C57Bl/6 mice

Changes in the enantiomeric composition of polychlorinated biphenyls (PCBs) can not only be used to investigate environmental and biological transport processes, but also have human health implications because of enantiospecific adverse health effects. To further understand differences in the disposition of PCB atropisomers in vivo, the present study investigates the toxicokinetics of PCB atropisomers in female C57Bl/6 mice after oral administration of a mixture of several PCBs, including racemic PCBs 91, 95, 132, 136, 149, 174, and 176. On the Chirasil-Dex column, an enrichment of the second eluting atropisomers was generally observed, whereas only the first eluting atropisomers E1-PCB 95, (-)-PCB 132, and (-)-PCB 149 had half-lives that were distinctively longer compared to the second eluting atropisomers. The bioavailability normalized clearance of first eluting atropisomers in blood was faster compared to that of second eluting atropisomers. The opposite trend was observed for the accumulation factors in adipose tissue, which is consistent with the slower clearance of the first eluting atropisomer. The only exception was PCB 174, which showed no differences in the toxicokinetic parameters of both atropisomers. Together, the differences in the toxicokinetics of PCB atropisomers point toward enantioselective biotransformation processes as the origin of PCB's enantiomeric enrichment in mammals and, possibly, humans.

Chiral polychlorinated biphenyl transport, metabolism, and distribution: a review

Chirality can be exploited to gain insight into enantioselective fate processes that may otherwise remain undetected because only biological, but not physical and chemical transport and transformation processes in an achiral environment will change enantiomer compositions. This review provides an in-depth overview of the application of chirality to the study of chiral polychlorinated biphenyls (PCBs), an important group of legacy pollutants. Like other chiral compounds, individual PCB enantiomers may interact enantioselectively (or enantiospecifically) with chiral macromolecules, such as cytochrome P-450 enzymes or ryanodine receptors, leading to differences in their toxicological effects and the enantioselective formation of chiral biotransformation products. Species and congener-specific enantiomer enrichment has been demonstrated in environmental compartments, wildlife, and mammals, including humans, typically due to a complex combination of biotransformation processes and uptake via the diet by passive diffusion. Changes in the enantiomer composition of chiral PCBs in the environment have been used to understand complex aerobic and anaerobic microbial transformation pathways, to delineate and quantify PCB sources and transport in the environment, to gain insight into the biotransformation of PCBs in aquatic food webs, and to investigate the enantioselective disposition of PCBs and their methylsulfonyl PCBs metabolites in rodents. Overall, changes in chiral signatures are powerful, but currently underutilized tools for studies of environmental and biological processes of PCBs.

Enantiomeric specificity of (-)-2,2',3,3',6,6'-hexachlorobiphenyl toward ryanodine receptor types 1 and 2

Polychlorinated biphenyls (PCBs) with unsymmetrical chlorine substitutions and multiple orthosubstitutions that restrict rotation around the biphenyl bond may exist in two stable enantiomeric forms.Stereospecific binding and functional modification of specific biological signaling targets have not been previously described for PCB atropisomers. We report that (-)-2,2',3,3',6,6'-hexachlorobiphenyl [(-)-PCB 136] enhances the binding of [3H]ryanodine to high-affinity sites on ryanodine receptors type 1(RyR1) and type 2 (RyR2) (EC50 values ~0.95 microM), whereas (+)-PCB 136 is inactive at < or =10 microM.(-)-PCB 136 induces a rapid release of Ca2+ from microsomal vesicles by selective sensitization of RyRs, an effect not antagonized by (+)-PCB 136. (-)-PCB 136 (500nM) enhances the activity of reconstituted RyR1 channels 3-fold by stabilizing the open and destabilizing the closed conformational states. The enantiomeric specificity is also demonstrated in intact HEK 293 cells expressing RyR1 where exposure to (-)-PCB 136 (100 nM; 12 h) sensitizes responses to caffeine, whereas (+)-PCB 136 does not. These data show enantiomeric specificity of (-)-PCB 136 toward a broadly expressed family of microsomal Ca2+ channels that may extend to other chiral noncoplanar PCBs and related structures.Evidence for enantioselective enrichment of PCBs in biological tissues that express RyR1 and RyR2channels may provide new mechanistic leads about their toxicological impacts on human health

Simultaneous extraction and clean-up of polychlorinated biphenyls and their metabolites from small tissue samples using pressurized liquid extraction

A pressurized liquid extraction-based method for the simultaneous extraction and in situ clean-up of polychlorinated biphenyls (PCBs), hydroxylated (OH)-PCBs and methylsulfonyl (MeSO(2))-PCBs from small (<0.5 g) tissue samples was developed and validated. Extraction of a laboratory reference material with hexane-dichloromethane-methanol (48:43:9, v/v) and Florisil as fat retainer allowed an efficient recovery of PCBs (78-112%; RSD: 13-37%), OH-PCBs (46+/-2%; RSD: 4%) and MeSO(2)-PCBs (89+/-21%; RSD: 24%). Comparable results were obtained with an established analysis method for PCBs, OH-PCBs and MeSO(2)-PCBs.

Gas chromatographic separation of methoxylated polychlorinated biphenyl atropisomers

Several polychlorinated biphenyls (PCBs) and their hydroxylated metabolites display axial chirality. Here we describe an enantioselective, gas chromatographic separation of methylated derivatives of hydroxylated (OH-)PCB atropisomers (MeO-PCB) using a chemically bonded beta-cyclodextrin column (Chirasil-Dex). The atropisomers of several MeO-PCBs could be separated on this column with resolutions ranging from 0.42 to 0.87 under isothermal or temperature-programmed conditions. In addition, the enantiomeric fraction of OH-PCB 136 metabolites was determined in male and female rats treated with racemic PCB 136. The methylated derivatives of two OH-PCB 136 metabolites showed an enantiomeric enrichment in liver tissue, whereas PCB 136 itself was near racemic.

2,2',3,3',6,6'-Hexachlorobiphenyl (PCB 136) atropisomers interact enantioselectively with hepatic microsomal cytochrome P450 enzymes

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.