Editor's Highlight: Congener-Specific Disposition of Chiral Polychlorinated Biphenyls in Lactating Mice and Their Offspring: Implications for PCB Developmental Neurotoxicity

Chiral polychlorinated biphenyl (PCB) congeners have been implicated by laboratory and epidemiological studies in PCB developmental neurotoxicity. These congeners are metabolized by cytochrome P450 (P450) enzymes to potentially neurotoxic hydroxylated metabolites (OH-PCBs). The present study explores the enantioselective disposition and toxicity of 2 environmentally relevant, neurotoxic PCB congeners and their OH-PCB metabolites in lactating mice and their offspring following dietary exposure of the dam. Female C57BL/6N mice (8-weeks old) were fed daily, beginning 2 weeks prior to conception and continuing throughout gestation and lactation, with 3.1 µmol/kg bw/d of racemic 2,2',3,5',6-pentachlorobiphenyl (PCB 95) or 2,2',3,3',6,6'-hexachlorobiphenyl (PCB 136) in peanut butter; controls received vehicle (peanut oil) in peanut butter. PCB 95 levels were higher than PCB 136 levels in both dams and pups, consistent with the more rapid metabolism of PCB 136 compared with PCB 95. In pups and dams, both congeners were enriched for the enantiomer eluting second on enantioselective gas chromatography columns. OH-PCB profiles in lactating mice and their offspring were complex and varied according to congener, tissue and age. Developmental exposure to PCB 95 versus PCB 136 differentially affected the expression of P450 enzymes as well as neural plasticity (arc and ppp1r9b) and thyroid hormone-responsive genes (nrgn and mbp). The results suggest that the enantioselective metabolism of PCBs to OH-PCBs may influence neurotoxic outcomes following developmental exposures, a hypothesis that warrants further investigation.

Genetic differences in the aryl hydrocarbon receptor and CYP1A2 affect sensitivity to developmental polychlorinated biphenyl exposure in mice: relevance to studies of human neurological disorders

Polychlorinated biphenyls (PCBs) are persistent organic pollutants that remain a human health concern with newly discovered sources of contamination and ongoing bioaccumulation and biomagnification. Children exposed during early brain development are at highest risk of neurological deficits, but highly exposed adults reportedly have an increased risk of Parkinson's disease. Our previous studies found allelic differences in the aryl hydrocarbon receptor and cytochrome P450 1A2 (CYP1A2) affect sensitivity to developmental PCB exposure, resulting in cognitive deficits and motor dysfunction. High-affinity Ahr ^b Cyp1a2(-/-) mice were most sensitive compared with poor-affinity Ahr ^d Cyp1a2(-/-) and wild-type Ahr ^b Cyp1a2(+/+) mice. Our follow-up studies assessed biochemical, histological, and gene expression changes to identify the brain regions and pathways affected. We also measured PCB and metabolite levels in tissues to determine if genotype altered toxicokinetics. We found evidence of AHR-mediated toxicity with reduced thymus and spleen weights and significantly reduced thyroxine at P14 in PCB-exposed pups. In the brain, the greatest changes were seen in the cerebellum where a foliation defect was over-represented in Cyp1a2(-/-) mice. In contrast, we found no difference in tyrosine hydroxylase immunostaining in the striatum. Gene expression patterns varied across the three genotypes, but there was clear evidence of AHR activation. Distribution of parent PCB congeners also varied by genotype with strikingly high levels of PCB 77 in poor-affinity Ahr ^d Cyp1a2(-/-) while Ahr ^b Cyp1a2(+/+) mice effectively sequestered coplanar PCBs in the liver. Together, our data suggest that the AHR pathway plays a role in developmental PCB neurotoxicity, but we found little evidence that developmental exposure is a risk factor for Parkinson's disease.

Identification of lipidomic markers of chronic 3,3',4,4',5-pentachlorobiphenyl (PCB 126) exposure in the male rat liver

Exposure to PCB 126, an environmentally relevant aryl hydrocarbon receptor agonist, is an environmental factor causing hepatic steatosis in rodent models; however, the lipidome of PCB 126-exposed rats has not been investigated in-depth. The objective of the present study was therefore to characterize dose-dependent changes in the lipid profile in the liver of male Sprague-Dawley rats exposed to PCB 126. Rats were exposed for three month to intraperitoneal injections of 0.01, 0.05 and 0.2μmol/kg bw PCB 126 in corn oil. Control animals were exposed in parallel and received corn oil alone. Lipids were extracted from whole liver homogenate and levels of polar lipids and fatty acids incorporated into triglycerides (FATAGs) were determined with tandem mass spectrometry using electrospray ionization. PCB 126 exposure increased the hepatic content of polar lipids and FATAGs. Protein adjusted levels of several polar lipid classes, in particular phosphatidylserine levels, decreased, whereas FATAGs levels typically increased with increasing PCB 126 dose. Sensitive, dose-dependent endpoints of PCB 126 exposure included an increase in levels of adrenic acid incorporated into triglycerides and changes in levels of certain ether-linked phospholipid and 1-alkyl/1-alkenyldiacylglycerol species, as determined using partial least square discriminant analysis (PLS-DA) and ANOVA. These changes in the composition of polar lipids and fatty acid in the liver of PCB 126 exposed rats identified several novel markers of PCB 126-mediated fatty liver disease that need to be validated in further studies.

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.

Identification of a novel hydroxylated metabolite of 2,2',3,5',6-pentachlorobiphenyl formed in whole poplar plants

Polychlorinated biphenyls (PCBs) are a group of persistent organic pollutants consisting of 209 congeners. Oxidation of several PCB congeners to hydroxylated PCBs (OH-PCBs) in whole poplar plants has been reported before. Moreover, 2,2',3,5',6-pentachlorobiphenyl (PCB95), as a chiral congener, has been previously shown to be atropselectively taken up and transformed in whole poplar plants. The objective of this study was to determine if PCB95 is atropselectively metabolized to OH-PCBs in whole poplar plants. Two hydroxylated PCB95s were detected by high-performance liquid chromatography-mass spectrometry in the roots of whole poplar plants exposed to racemic PCB95 for 30 days. The major metabolite was confirmed to be 4'-hydroxy-2,2',3,5',6-pentachlorobiphenyl (4'-OH-PCB95) by gas chromatography-mass spectrometry (GC-MS) using an authentic reference standard. Enantioselective analysis showed that 4'-OH-PCB95 was formed atropselectively, with the atropisomer eluting second on the Nucleodex β-PM column (E2-4'-OH-PCB95) being slightly more abundant in the roots of whole poplar plants. Therefore, PCB95 can at least be metabolized into 4'-OH-PCB95 and another unknown hydroxylated PCB95 (as a minor metabolite) in whole poplar plants. Both atropisomers of 4'-OH-PCB95 are formed, but E2-4'-OH-PCB95 has greater atropisomeric enrichment in the roots of whole poplar plants. A comparison with mammalian biotransformation studies indicates a distinctively different metabolite profile of OH-PCB95 metabolites in whole poplar plants. Our observations suggest that biotransformation of chiral PCBs to OH-PCBs by plants may represent an important source of enantiomerically enriched OH-PCBs in the environment.

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.

Metabolism and metabolites of polychlorinated biphenyls

Abstract The metabolism of polychlorinated biphenyls (PCBs) is complex and has an impact on toxicity, and thereby on the assessment of PCB risks. A large number of reactive and stable metabolites are formed in the processes of biotransformation in biota in general, and in humans in particular. The aim of this document is to provide an overview of PCB metabolism, and to identify the metabolites of concern and their occurrence. Emphasis is given to mammalian metabolism of PCBs and their hydroxyl, methylsulfonyl, and sulfated metabolites, especially those that persist in human blood. Potential intracellular targets and health risks are also discussed.

PCB 136 atropselectively alters morphometric and functional parameters of neuronal connectivity in cultured rat hippocampal neurons via ryanodine receptor-dependent mechanisms

We recently demonstrated that polychlorinated biphenyl (PCB) congeners with multiple ortho chlorine substitutions sensitize ryanodine receptors (RyRs), and this activity promotes Ca²⁺-dependent dendritic growth in cultured neurons. Many ortho-substituted congeners display axial chirality, and we previously reported that the chiral congener PCB 136 (2,2',3,3',6,6'-hexachlorobiphenyl) atropselectively sensitizes RyRs. Here, we test the hypothesis that PCB 136 atropisomers differentially alter dendritic growth and other parameters of neuronal connectivity influenced by RyR activity. (-)-PCB 136, which potently sensitizes RyRs, enhances dendritic growth in primary cultures of rat hippocampal neurons, whereas (+)-PCB 136, which lacks RyR activity, has no effect on dendritic growth. The dendrite-promoting activity of (-)-PCB 136 is observed at concentrations ranging from 0.1 to 100 nM and is blocked by pharmacologic RyR antagonism. Neither atropisomer alters axonal growth or cell viability. Quantification of PCB 136 atropisomers in hippocampal cultures indicates that atropselective effects on dendritic growth are not due to differential partitioning of atropisomers into cultured cells. Imaging of hippocampal neurons loaded with Ca²⁺-sensitive dye demonstrates that (-)-PCB 136 but not (+)-PCB 136 increases the frequency of spontaneous Ca²⁺ oscillations. Similarly, (-)-PCB 136 but not (+)-PCB 136 increases the activity of hippocampal neurons plated on microelectrode arrays. These data support the hypothesis that atropselective effects on RyR activity translate into atropselective effects of PCB 136 atropisomers on neuronal connectivity, and suggest that the variable atropisomeric enrichment of chiral PCBs observed in the human population may be a significant determinant of individual susceptibility for adverse neurodevelopmental outcomes following PCB exposure.

Cytochrome p450 mRNA expression in the rodent brain: species-, sex-, and region-dependent differences

Cytochrome P450 (P450) enzymes play a critical role in the activation and detoxication of many neurotoxic chemicals. Although research has largely focused on P450-mediated metabolism in the liver, emerging evidence suggests that brain P450s influence neurotoxicity by modulating local metabolite levels. As a first step toward better understanding the relative role of brain P450s in determining neurotoxic outcome, we characterized mRNA expression of specific P450 isoforms in the rodent brain. Adult mice (male and female) and rats (male) were treated with vehicle, phenobarbital, or dexamethasone. Transcripts for CYP2B, CYP3A, CYP1A2, and the orphan CYP4X1 and CYP2S1 were quantified in the liver, hippocampus, cortex, and cerebellum by quantitative (real-time) polymerase chain reaction. These P450s were all detected in the liver with the exception of CYP4X1, which was detected in rat but not mouse liver. P450 expression profiles in the brain varied regionally. With the exception of the hippocampus, there were no sex differences in regional brain P450 expression profiles in mice; however, there were marked species differences. In the liver, phenobarbital induced CYP2B expression in both species. Dexamethasone induced hepatic CYP2B and CYP3A in mice but not rats. In contrast, brain P450s did not respond to these classic hepatic P450 inducers. Our findings demonstrate that P450 mRNA expression in the brain varies by region, regional brain P450 profiles vary between species, and their induction varies from that of hepatic P450s. These novel data will be useful for designing mechanistic studies to examine the relative role of P450-mediated brain metabolism in neurotoxicity.

Chlordane and heptachlor are metabolized enantioselectively by rat liver microsomes

Chlordane, heptachlor, and their metabolites are chiral persistent organic pollutants that undergo enantiomeric enrichment in the environment. This study investigated the enantioselective metabolism of both chlordane isomers and heptachlor, major components of technical chlordane, by liver microsomes prepared from male rats treated with corn oil (CO) or inducers of CYP2B (PB; phenobarbital) and CYP3A enzymes (DX; dexamethasone), isoforms induced by chlordane treatment. The extent of the metabolism of all three parent compounds was dependent on the microsomal preparation used and followed the rank order PB > DX > CO. The mass balances ranged from 49 to 130% of the parent compound added to the microsomal incubations. Both cis- and trans-chlordane were enantioselectively metabolized to oxychlordane (EF = 0.45-0.89) and 1,2-dichlorochlordene (EF = 0.42-0.90). Heptachlor was metabolized enantioselectively, with heptachlor epoxide B (EF = 0.44-0.54) being the only metabolite. Interestingly, the direction on the enrichment for oxychlordane, 1,2-dichlorochlordene, and heptachlor epoxide differed depending on the microsomal preparation. These findings demonstrate that the direction and extent of the enantioselective metabolism of both chlordane isomers and heptachlor is P450 isoform-dependent and can be modulated by the induction of P450 enzymes.

Biological and tumor-promoting effects of dioxin-like and non-dioxin-like polychlorinated biphenyls in mouse liver after single or combined treatment

To assess the impact of a mixture containing dioxin-like and non-dioxin-like polychlorinated biphenyls (PCBs), male mice were initiated with N-nitroso-diethylamine and subsequently treated with PCB126, an Ah-Receptor agonist, and PCB153, acting via activation of the constitutive androstane receptor. The two congeners were given at two dose levels: the low dose was adjusted to induce ~150-fold increases in cytochrome P450 (Cyp)1a1 (PCB126) and Cyp2b10 mRNAs (PCB153), and the high dose was chosen as twice the low dose. To keep the liver PCB levels constant, mice were given initial loading doses followed by weekly maintenance doses calculated on the basis of the PCBs' half-lives. Mice were treated with the individual congeners (low and high dose) or with a mixture consisting of the low doses of the 2 PCBs. The following results were obtained: (1) the 2 PCBs produced dose-dependent increases in Cyp1a1 and Cyp2b10 mRNA, protein, and activity when given individually; (2) combined treatment caused more than additive effects on Cyp1a1 mRNA expression, protein level, and ethoxyresurofin activity; (3) changes in the levels of several proteins were detected by proteome analysis in livers of PCB-treated mice; (4) besides these biological responses, the individual PCBs caused no significant increase in the number of glucose-6-phospatase (G6Pase)-deficient neoplastic lesions in liver, whereas a moderate significant effect occurred in the combination group. These results suggest weak but significant response-additive effects of the 2 PCBs when given in combination. They also suggest that the Cyp biomarkers tend to overestimate the carcinogenic response produced by the PCBs in mouse liver.

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.

Stereoselective formation of mono- and dihydroxylated polychlorinated biphenyls by rat cytochrome P450 2B1

Changes in atropisomer composition of chiral polychlorinated biphenyls (PCBs) and their mono- and dihydroxylated 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. Nonracemic 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 nonracemic 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.

Assigning atropisomer elution orders using atropisomerically enriched polychlorinated biphenyl fractions generated by microsomal metabolism

Changes in the enantiomeric fraction of chiral polychlorinated biphenyls (PCBs) are a powerful tool to investigate the movement of PCBs in the environment, for example as part of source apportionment and ecological studies. Environmental studies typically employ a series of cyclodextrin-based gas chromatography columns to separate all environmentally relevant PCB congeners. The elution order of most PCB atropisomers has not been established on different enantioselective columns due to the unavailability of analytical standards. To overcome this limitation, the current study generated atropisomerically enriched fractions of chiral PCBs with rat liver microsomes. Subsequently, the enrichment profile of the enriched PCB fractions was used to determine the elution order of PCB atropisomers on selected enantioselective gas chromatography columns. While the elution order of PCB 95, 131, 132, 136, 149 and 176 atropisomers was identical on all enantioselective columns investigated, an inversion of the elution order was observed for PCB 45, 84, 91 and 174 atropisomers on a few columns. These results demonstrate that atropisomerically enriched fractions obtained from microsomal metabolism can be used to unambiguously establish the relative elution order of the atropisomers of PCBs and potentially other environmental pollutant, especially if pure enantiomers are not available.

2,2',3,5',6-Pentachlorobiphenyl (PCB 95) and its hydroxylated metabolites are enantiomerically enriched in female mice

Epidemiological and laboratory studies link polychlorinated biphenyls and their metabolites to adverse neurodevelopmental outcomes. Several neurotoxic PCB congeners are chiral and undergo enantiomeric enrichment in mammalian species, which may modulate PCB developmental neurotoxicity. This study measures levels and enantiomeric enrichment of PCB 95 and its hydroxylated metabolites (OH-PCBs) in adult female C57Bl/6 mice following subchronic exposure to racemic PCB 95. Tissue levels of PCB 95 and OH-PCBs increased with increasing dose. Dose-dependent enantiomeric enrichment of PCB 95 was observed in brain and other tissues. OH-PCBs also displayed enantiomeric enrichment in blood and liver, but were not detected in adipose and brain. In light of data suggesting enantioselective effects of chiral PCBs on molecular targets linked to PCB developmental neurotoxicity, our observations highlight the importance of accounting for PCB and OH-PCB enantiomeric enrichment in the assessment of PCB developmental neurotoxicity.

Gas chromatographic analysis with chiral cyclodextrin phases reveals the enantioselective formation of hydroxylated polychlorinated biphenyls by rat liver microsomes

Chiral PCB congeners are major components of PCB mixtures and undergo enantioselective biotransformation to hydroxylated (OH-)PCBs by cytochrome P450 enzymes. While it is known that biotransformation results in an enantiomeric enrichment of the parent PCB, it is currently unknown if OH-PCBs are formed enantioselectively. The present study screened seven commercial capillary gas chromatography columns containing modified β- or γ-cyclodextrins for their potential to separate the atropisomers of methylated derivatives of OH-PCB. The atropisomers of 3-, 4- and 5-methoxy derivatives were at least partially separated on one or more columns. A subsequent biotransformation study was performed with rat liver microsomes to assess if hydroxylated metabolites are formed enantioselectively from PCBs 91, 95, 132, and 149. The OH-PCBs were extracted from the microsomal incubations, derivatized with diazomethane and analyzed as the respective methoxylated (MeO-)PCB derivatives using selected columns. The 5-hydroxylated metabolites of PCBs 91, 95, 132, and 149 were the major metabolites, which is consistent with PCB's biotransformation by cytochrome P450 2B enzymes. All 5-hydroxylated metabolites displayed a clear, congener-specific enantiomeric enrichment. Overall, this study demonstrates for the first time that chiral PCBs, such as PCB 91, 95, 132, and 149, are enantioselectively metabolized to OH-PCBs by cytochrome P450 enzymes.

2,2',3,3',6,6'-Hexachlorobiphenyl (PCB 136) is enantioselectively oxidized to hydroxylated metabolites by rat liver microsomes

Developmental exposure to multiple ortho-substituted polychlorinated biphenyls (PCBs) causes adverse neurodevelopmental outcomes in laboratory animals and humans by mechanisms involving the sensitization of Ryanodine receptors (RyRs). In the case of PCB 136, the sensitization of RyR is enantiospecific, with only (-)-PCB 136 being active. However, the role of enantioselective metabolism in the developmental neurotoxicity of PCB 136 is poorly understood. The present study employed hepatic microsomes from phenobarbital (PB)-, dexamethasone (DEX)- and corn oil (VEH)-treated male Sprague-Dawley rats to investigate the hypothesis that PCB 136 atropisomers are enantioselectively metabolized by P450 enzymes to potentially neurotoxic, hydroxylated PCB 136 metabolites. The results demonstrated the time- and isoform-dependent formation of three metabolites, with 5-OH-PCB 136 (2,2',3,3',6,6'-hexachlorobiphenyl-5-ol) being the major metabolite. The formation of 5-OH-PCB 136 increased with the activity of P450 2B enzymes in the microsomal preparation, which is consistent with PCB 136 metabolism by rat P450 2B1. The minor metabolite 4-OH-PCB 136 (2,2',3,3',6,6'-hexachlorobiphenyl-4-ol) was produced by a currently unidentified P450 enzyme. An enantiomeric enrichment of (-)-PCB 136 was observed in microsomal incubations due to the preferential metabolism of (+)-PCB 136 to the corresponding 5-OH-PCB 136 atropisomer. 4-OH-PCB 136 displayed an enrichment of the atropisomer formed from (-)-PCB 136; however, the enrichment of this metabolite atropisomer did not affect the enantiomeric enrichment of the parent PCB because 4-OH-PCB 136 is only a minor metabolite. Although the formation of 5- and 4-OH-PCB 136 atropisomers increased with time, the enantioselective formation of the OH-PCB metabolites resulted in constant enantiomeric enrichment, especially at later incubation times. These observations not only demonstrate that the chiral signatures of PCBs and their metabolites in wildlife and humans are due to metabolism by P450 enzymes but also suggest that the enantioselective formation of neurotoxic PCB 136 metabolites, such as 4-OH-PCB 136, may play a role in the developmental neurotoxicity of PCBs.

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.

Time course of congener uptake and elimination in rats after short-term inhalation exposure to an airborne polychlorinated biphenyl (PCB) mixture

Despite the continued presence of PCBs in indoor and ambient air, few studies have investigated the inhalation route of exposure. While dietary exposure has declined, inhalation of the semivolatile, lower-chlorinated PCBs has risen in importance. We measured the uptake, distribution, and time course of elimination of inhaled PCB congeners to characterize the pulmonary route after short-term exposure. Vapor-phase PCBs were generated from Aroclor 1242 to a nose-only exposure system and characterized for congener levels and profiles. Rats were exposed via inhalation acutely (2.4 mg/m3 for 2 h) or subacutely (8.2 mg/m3, 2 hx10 days), after which pulmonary immune responses and PCB tissue levels were measured. Animals acutely exposed were euthanized at 0, 1, 3, 6, and 12 h post exposure to assess the time course of PCB uptake and elimination. Following rapid absorption and distribution, PCBs accumulated in adipose tissue but decayed in other tissues with half-lives increasing in liver (5.6 h)<lung (8.2 h)<brain (8.5 h)<blood (9.7 h). PCB levels were similar in lung, liver, and adipose tissue, lower in brain, and lowest in blood. Inhalation of the airborne PCB mixture contributed significantly to the body burden of lower-chlorinated congeners. Congeners detected in tissue were mostly ortho-substituted ranging from mono- to pentachlorobiphenyls. Selective uptake and elimination led to accumulation of a distinct congener spectrum in tissue. Minimal evidence of toxicity was observed.

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.

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.

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

Several PCB congeners, present in commercial PCB formulations, are chiral. These PCBs can undergo enantiomeric enrichment in many animal species and in humans due to currently uncharacterized enantioselective biotransformation processes. To investigate if certain cytochrome P-450 enzymes (CYPs), such as CYP2B's, are responsible for this enantiomeric enrichment, we investigated the enantioselective disposition of (+/-)-PCB 136 in female mice after induction of different CYP enzymes by pretreatment with corn oil alone, beta-naphthoflavone (CYP1A's), phenobarbital (CYP2B's), or dexamethasone (2B's and 3A's), followed by oral PCB administration. PCB 136 levels were significantly lower in phenobarbital- and, to a lesser extent, in dexamethasone-pretreated animals, presumably due to the induction of PCB 136 metabolizing enzymes. Although (+)-PCB 136 was enriched in all tissues, none of the pretreatments altered the enantioselective disposition of PCB 136 in a manner that suggests a particular CYP subfamily as the cause of the enrichment of (+)-PCB 136. Fecal PCB levels and enantiomeric fraction values changed over time in a manner consistent with slower digestive motility in the mice pretreated with phenobarbital and dexamethasone. Overall, this study does not support the hypothesis that metabolism by CYP2B enzymes is responsible for the enrichment of (+)-PCB 136 in mice.

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.

Dose-dependent enantiomeric enrichment of 2,2',3,3',6,6'-hexachlorobiphenyl in female mice

Nineteen of the 209 possible polychlorinated biphenyl (PCB) congeners are chiral and stable to racemization at ambient temperature. Chiral PCB congeners are important components of technical and environmental PCB mixtures, and some are highly toxic. Both environmental and laboratory studies have shown that these chiral PCB congeners undergo enantiomeric enrichment in many species; however, the processes and factors influencing the extent of this enantiomeric enrichment are poorly understood. We hypothesized that the exposure levels are an important factor affecting the extent of enantiomeric enrichment. To test this hypothesis, we investigated the levels and enantiomeric fractions of (+/-)-PCB 136 in selected tissues, feces, and urine of female C57Bl/6 mice 3 d after oral administration of 2.5, 10, or 50 mg/kg body weight of (+/-)-PCB 136. The PCB 136 tissue levels typically increased with increasing dose. The extent of the enrichment of (+)-PCB 136 in tissues and feces, however, decreased with increasing dose, an observation that suggests a saturation of the disposition process responsible for the enantiomeric enrichment. Overall, the present study demonstrates that in addition to species, exposure source, exposure frequency, and other factors, levels of PCB exposure are an important determinant of the enantiomeric enrichment of PCBs in mice and, most likely, other species.

Differences in the isomer composition of perfluoroctanesulfonyl (PFOS) derivatives

Perfluorooctanesulfonyl (PFOS)-based materials and related compounds are an emerging group of environmental pollutants. Perfluorooctanesulfonyl fluoride, the key intermediate for the production of these materials, was manufactured by an electrochemical fluorination process that resulted in complex mixtures containing linear and branched PFOS derivatives and other perfluorinated compounds. This study uses 19F-NMR spectroscopy to investigate differences in the composition between commercial samples of PFOS and PFBS (perfluorobutanesulfonyl) derivatives. While PFBS derivatives, which are under evaluation as substitutes for PFOS-based materials, contained no detectable levels of branched impurities, all PFOS derivatives contained significant levels of branched and other impurities. Analysis of the NMR data reveals that PFOS fluorides typically have a higher content of internally branched and similar levels of isopropyl branched PFOS isomers compared to PFOS potassium salts. Furthermore, the isomer distribution of PFOS derivatives may vary depending on their source. These findings suggest that it is important to determine the isomer composition of PFOS samples used in both environmental and toxicological studies.

Effect of potassium perfluorooctanesulfonate, perfluorooctanoate and octanesulfonate on the phase transition of dipalmitoylphosphatidylcholine (DPPC) bilayers

Perfluorooctanesulfonic acid (PFOS) is a persistent environmental pollutant that may cause adverse effects by inhibiting pulmonary surfactant. To gain further insights in this potential mechanism of toxicity, we investigated the interaction of PFOS potassium salt with dipalmitoylphosphatidylcholine (DPPC) - the major component of pulmonary surfactant - using steady-state fluorescence anisotropy spectroscopy and DSC (differential scanning calorimetry). In addition, we investigated the interactions of two structurally related compounds, perfluorooctanoic acid (PFOA) and octanesulfonic acid (OS) potassium salt, with DPPC. In the fluorescence experiments a linear depression of the main phase transition temperature of DPPC (T(m)) and an increased peak width was observed with increasing concentration of all three compounds, both using 1,6-diphenyl-1,3,5-hexatriene (DPH) and 1-(4-trimethylammoniumphenyl)-6-phenyl-1,3,5-hexatriene p-toluenesulfonate (TMA-DPH) as fluorescent probes. PFOS caused an effect on T(m) and peak width at much lower concentrations because of its increased tendency to partition onto DPPC bilayers, i.e., the partition coefficients decrease in the K(PFOS)>K(PFOA)>>K(OS). Similar to the fluorescence anisotropy measurements, all three compounds caused a linear depression in the onset of the main phase transition temperature and a significant peak broadening in the DSC experiments, with PFOS having the most pronounced effect of the peak width. The effect of PFOS and other fluorinated surfactants on DPPC in both mono- and bilayers may be one mechanism by which these compounds cause adverse biological effects.

Enantioselective disposition of PCB 136 (2,2',3,3',6,6'-hexachlorobiphenyl) in C57BL/6 mice after oral and intraperitoneal administration

Studies of xenobiotic disposition in rodents often employ experimental designs using differing routes of administration. In an effort to investigate the effects of route of administration on enantioselective disposition of xenobiotics, a chiral polychlorinated biphenyl (PCB), racemic PCB 136, was administered as a single dose (50 mg/kg body weight) to male or female C57BL/6 mice either orally or via intraperitoneal injection. Mice were sacrificed after either 3 or 6 days, and blood and organs were collected for PCB analysis. Intraperitoneal injection of PCB 136 produced statistically higher PCB levels in blood and organs than did the oral administration. Tissue levels were higher after 3 days than those after 6 days. Enantioselective analysis showed that (+)-PCB 136 was enriched in most organs, with the most pronounced enrichment found in the liver and the brain of animals dosed orally or by intraperitoneal injection, respectively. Significantly higher retained enantiomeric fractions of PCB 136 were found in the oral treatment groups compared with those found in intraperitoneal treatment groups, possibly as a result of the lower PCB levels in oral treatment groups. Therefore, the choice of administration route may well have implications for the enantioselective disposition of PCB 136 and other chiral substances.

Distribution of chiral PCBs in selected tissues in the laboratory rat

The enantiomeric enrichment of polychlorinated biphenyl (PCB) atropisomers has been reported in both wildlife and in humans. The biological processes causing this enrichment are only poorly investigated, a fact that limits the use of enantiomeric fractions (EFs) as a tool to study various processes of environmental relevance. To further understand these enantioselective processes, this study investigates the tissue distribution and EFs of some PCB atropisomers after administration of PCB mixtures to immature male Sprague-Dawley rats. The mixtures selected for this study, Aroclor 1254 and an environmental mixture extracted from Chlorofen-contaminated soil, are qualitatively different and are known to induce different groups of hepatic enzymes. Animals were sacrificed 6 days after dosing, PCBs were extracted, and, whenever possible, the EFs of PCBs 84, 91, 95, 149, 174, and 176 were determined by chiral gas chromatography. The EFs of PCB 95 (adipose tissue, liver, and skin) and PCB 149 (adipose tissue, liver, skin, and blood) in tissues from Aroclor 1254-treated animals differed significantly from EFs in the Aroclor standard, while only EFs of PCB 95 (blood) and PCB 174 (adipose tissue) in tissues from soil-extract-treated animals were different from those of the Chlorofen soil extract. PCB 149 in tissues from soil-extract-treated animals underwent no statistically significant enantiomeric enrichment. These differences in the EFs clearly suggest that the enantioselective enrichment of PCB atropisomers may correlate with exposure history, and with the induction of hepatic enzymes, and that EFs may be useful chemical markers of physiologic and biochemical changes following exposure to PCBs.

Congener-specific tissue distribution of aroclor 1254 and a highly chlorinated environmental PCB mixture in rats

Polychlorinated biphenyl (PCB) mixtures were synthesized and marketed in Eastern European countries, but little is known abouttheir composition, distribution, ortoxicity. PCB-contaminated soil from the former production site of the Polish PCB mixture Chlorofen was collected, and the PCBs were extracted. An in vivo study was performed to investigate the PCB tissue distribution and biochemical effects of this soil extract in immature male Sprague-Dawley rats. Rats were administered 0.05 mmol/kg soil-extracted PCBs or Aroclor 1254 and sacrificed 7 days later, and congener-specific PCB profiles in selected tissues were determined. Distribution of SigmaPCBs (sum of 120 congeners) in tissues was primarily a function of lipid content of the tissues, except for the spleen, which retained more PCB than other tissues. Multivariate analysis of the PCB congener data showed that (a) congener profiles in tissues had changed, as compared to the parent mixture; (b) disposition and redistribution of individual congeners in vivo differed between both mixtures; and (c) more highly chlorinated congeners were retained in the spleens of both treatment groups. Differences in the induction of cytochrome P-450 1A and 2B subfamilies reflected the homologue composition of the respective mixtures and predict a different toxicity profile for Chlorofen than for Aroclor 1254.

Synthesis of polychlorinated biphenyls and their metabolites with a modified Suzuki-coupling

A modified procedure for the synthesis of polychlorinated biphenyls (PCBs) utilizing the Suzuki-coupling, a palladium-catalyzed cross-coupling reaction, is described. The coupling of (chlorinated) benzene boronic acids with bromochlorobenzenes, using Pd(dppf)(2)Cl(2) (dppf = 1,1'-bis(diphenylphosphino)ferrocene) as the catalyst and aqueous sodium carbonate as the base, gave the desired PCB congeners in moderate to good yields. Eleven PCB congeners, including environmentally important PCB congeners and metabolites, were synthesized using this modified procedure. This new catalyst Pd(dppf)(2)Cl(2) offers the advantage of being less air-sensitive and has a longer shelf life compared to Pd(PPh(4))(4). Three new (di-)methoxylated PCB congeners were synthesized using the same procedure by either coupling a chlorinated benzene boronic acid with a bromo (di-)methoxybenzene or by coupling a (di-)methoxy benzene boronic acid with a chlorinated bromobenzene. The dimethoxylated PCB congeners were readily converted into the respective dihydroxylated PCB derivatives using boron tribromide in dichloromethane. This approach offers the advantage of high selectivity and moderate to good yields compared to conventional methods such as the Cadogan reaction and allows the use of less toxic starting materials.