Parent Polychlorinated Biphenyls Research Articles

Metabolic activation of WHO-congeners PCB28, 52, and 101 by human CYP2A6: evidence from in vitro and in vivo experiments.

Despite extensive research on the metabolism of polychlorinated biphenyls (PCBs), knowledge gaps persist regarding their isoform-specific biotransformation pathways. This study aimed to elucidate the role of different cytochrome P450 enzymes in PCB metabolism, focusing on WHO-congeners 2,4,4'-trichlorobiphenyl (PCB28), 2,2',5,5'-tetrachlorobiphenyl (PCB52), and 2,2',4,5,5'-pentachlorobiphenyl (PCB101). Utilizing engineered HEK293 cell lines, we investigated the in vitro metabolism of these PCBs by CYP1A2, CYP2C8, CYP2C9, CYP3A4, CYP2A6, and CYP2E1, revealing robust production of hydroxylated metabolites. Our results show that CYP2A6 plays a major role in the metabolism of these congeners responsible for predominant formation of para-position hydroxylated metabolites, with concentrations reaching up to 1.61µg/L (5,89nM) for PCB28, 316.98µg/L (1,03µM) for PCB52, and 151.1µg/L (441nM) for PCB101 from a 20µM parent PCB concentration. Moreover, concentration-dependent cytotoxic and cytostatic effects induced by reactive intermediates of the PCB hydroxylation pathway were observed in HEK293CYP2A6 cells, for all three congeners tested. CYP2A6 was specifically capable of activating PCBs 28 and 101 to genotoxic metabolites which produced genetic defects which were propagated to subsequent generations, potentially contributing to carcinogenesis. In a clinical study examining CYP2A6 enzyme activity in formerly exposed individuals with elevated internal PCB levels, a participant with increased enzyme activity showed a direct association between the phenotypic activity of CYP2A6 and the metabolism of PCB28, confirming the role of CYP2A6 in the in vivo metabolism of PCB28 also in humans. These results altogether reinforce the concept that CYP2A6 plays a pivotal role in PCB congener metabolism and suggest its significance in human health, particularly in the metabolism of lower chlorinated, volatile PCB congeners.

Transformation of hydroxylated polychlorinated biphenyls by bacterial 2-hydroxybiphenyl 3-monooxygenase

Monohydroxylated PCBs (OH-PCBs) are an (eco)toxicologically significant group of compounds, as they arise from the oxidation of polychlorinated biphenyls (PCBs) and, at the same time, may exert even more severe toxic effects than their parent PCB molecules. Despite having been widely detected in environmental samples, plants, and animals, information on the fate of OH-PCBs in the environment is scarce, including on the enzymatic machinery behind their degradation. To date, only a few bacterial taxa capable of OH-PCB transformation have been reported. In this study, we aimed to obtain a deeper insight into the transformation of OH-PCBs in soil bacteria and isolated a Pseudomonas sp. strain P1B16 based on its ability to use o-phenylphenol (2-PP) which, when exposed to the Delor 103-derived OH-PCB mixture, depleted a wide spectrum of mono-, di, and trichlorinated OH-PCBs. In the P1B16 genome, a region designated as hbp was identified, which bears a set of putative genes involved in the transformation of OH-PCBs, namely hbpA encoding for a putative flavin-dependent 2-hydroxybiphenyl monooxygenase, hbpC (2,3-dihydroxybiphenyl-1,2-dioxygenase), hbpD (2-hydroxy-6-oxo-6-phenylhexa-2,4-dienoate hydrolase), and the transcriptional activator-encoding gene hbpR. The hbpA coding sequence was heterologously expressed, purified, and its substrate specificity was investigated towards the Delor 103-derived OH-PCB mixture, individual OH-PCBs, and multiple (chlorinated) phenolics. Apart from 2-PP and 2-chlorophenol, HbpA was also demonstrated to transform a range of OH-PCBs, including a 3-hydroxy-2,2′,4′,5,5′-pentachlorobiphenyl. Importantly, this is the first direct evidence of HbpA homologs being involved in the degradation of OH-PCBs. Moreover, using a P1B16-based biosensor strain, the specific induction of hbp genes by 2-PP, 3-phenylphenol, 4-phenylphenol, and the OH-PCB mixture was demonstrated. This study provides direct evidence on the specific enzymatic machinery responsible for the transformation of OH-PCBs in bacteria, with many implications in ecotoxicology, environmental restoration, and microbial ecology in habitats burdened with PCB contamination.

Long-Term Dechlorination of Polychlorinated Biphenyls (PCBs) in Taihu Lake Sediment Microcosms: Identification of New Pathways, PCB-Driven Shifts of Microbial Communities, and Insights into Dechlorination Potential.

Microbial reductive dechlorination of polychlorinated biphenyls (PCBs) is regarded as an alternative approach for in situ remediation and detoxification in the environment. To better understand the process of PCB dechlorination in freshwater lake sediment, a long-term (108 weeks) dechlorination study was performed in Taihu Lake sediment microcosms with nine parent PCB congeners (PCB5, 12, 64, 71, 105, 114, 149, 153, and 170). Within 108 weeks, the total PCBs declined by 32.8%, while parent PCBs declined by 84.8%. PCB dechlorinators preferred to attack meta- and para-chlorines, principally para-flanked meta and single-flanked para chlorines. A total of 58 dechlorination pathways were observed, and 20 of them were not in 8 processes, suggesting the broad spectrum of PCB dechlorination in the environment. Rare ortho dechlorination was confirmed to target the unflanked ortho chlorine, indicating a potential for complete dechlorination. PCBs drove the shifts of the microbial community structures, and putative dechlorinating bacteria were growth-linked to PCB dechlorination. The distinct jump of RDase genes ardA, rdh12, pcbA4, and pcbA5 was found to be consistent with the commencement of dechlorination. The maintained high level of putative dechlorinating phylum Chloroflexi (including Dehalococcoides and o-17/DF-1), genus Dehalococcoides, and four RDase genes at the end of incubation revealed the long-term dechlorination potential. This work provided insights into dechlorination potential for long-term remediation strategies at PCB-contaminated sites.

A Critical Review of Polychlorinated Biphenyls Metabolism, Metabolites, and Their Correlation with Oxidative Stress.

Polychlorinated biphenyls (PCBs) are notorious persistent organic pollutants that were banned in the last century. However, PCBs still remain ubiquitous in the ecosystem due to their persistence and bioaccumulative potency against environmental and biological degradation. Albeit there is no longer the permission of commercial production of PCBs, they were continuously released into global biota via illegal disposal of e-waste or as byproducts of industrial supplies. The role of oxidative stress is often implicated in PCBs' toxicology. PCBs, especially coplanar ones, have a high affinity toward aryl hydrocarbon receptors and inducing CYP1A1, which is considered as a source of oxidative stress. Although commercial PCBs and coplanar individual PCBs, for example, PCB 77 and 126, induced oxidative stresses have been extensively investigated, PCB metabolite-induced oxidative stress has received less attention. PCBs can undergo phase I metabolism which metabolizes the parent PCBs into hydroquinone/semiquinone/quinone metabolites as a futile redox cycle, producing downstream reactive oxygen species (ROS) as byproducts. PCBs can also undergo phase II metabolism yielding methylsulfonyl metabolites that deplete glutathione and such. PCB metabolites induce oxidative stress generally via direct production of ROS or indirect scavenge antioxidant and inhibit antioxidant enzymes, disturbing cellular redox balance. This review aims to provide a critical summary of PCBs metabolism, PCBs parents, and daughter metabolite-induced oxidative stress. We especially focus on the connection between parent PCBs and downstream metabolites, to encourage research associated with PCB metabolite-induced oxidative stress.

Cardiovascular Effects of Polychlorinated Biphenyls and Their Major Metabolites.

Background:Xenobiotic metabolism is complex, and accounting for bioactivation and detoxification processes of chemicals remains among the most challenging aspects for decision making with in vitro new approach methods data.Objectives:Considering the physiological relevance of human organotypic culture models and their utility for high-throughput screening, we hypothesized that multidimensional chemical-biological profiling of chemicals and their major metabolites is a sensible alternative for the toxicological characterization of parent molecules vs. metabolites in vitro.Methods:In this study, we tested 25 polychlorinated biphenyls (PCBs) [PCB 3, 11, 52, 126, 136, and 153 and their relevant metabolites (hydroxylated, methoxylated, sulfated, and quinone)] in concentration–response () for effects in human induced pluripotent stem cell (iPSC)-derived cardiomyocytes (CMs) and endothelial cells (ECs) (iPSC-derived and HUVECs). Functional phenotypic end points included effects on beating parameters and intracellular flux in CMs and inhibition of tubulogenesis in ECs. High-content imaging was used to evaluate cytotoxicity, mitochondrial integrity, and oxidative stress.Results:Data integration of a total of 19 physicochemical descriptors and 36 in vitro phenotypes revealed that chlorination status and metabolite class are strong predictors of the in vitro cardiovascular effects of PCBs. Oxidation of PCBs, especially to di-hydroxylated and quinone metabolites, was associated with the most pronounced effects, whereas sulfation and methoxylation of PCBs resulted in diminished bioactivity.Discussion:Risk characterization analysis showed that although in vitro derived effective concentrations exceeded the levels measured in the general population, risks cannot be ruled out due to the potential for population variability in susceptibility and the need to fill data gaps using read-across approaches. This study demonstrated a strategy for how in vitro data can be used to characterize human health risks from PCBs and their metabolites. https://doi.org/10.1289/EHP7030

Phytotoxicity and metabolic responses induced by tetrachlorobiphenyl and its hydroxylated and methoxylated derivatives in rice (Oryza sative L.)

Polychlorinated biphenyls (PCBs) and their biotransformation products, hydroxylated (OH-PCBs) and methoxylated derivatives (MeO-PCBs), have been detected in the environment and biota, especially crops. However, to date, little information is available on the phytotoxicity and metabolic responses induced by these chemicals in crops. In this study, we exposed rice (Oryza sative L.) seedlings to 2,3,4,5-tetrachlorobiphenyl (CB-61) and its hydroxylated (4′-OH-CB-61) and methoxylated derivatives (4′-MeO-CB-61) at 0, 10, 50, 100 and 500 μg/L, respectively. After exposure for 14 days, significantly growth inhibition and oxidative damage were observed, among which the toxicities of 4′-OH-CB-61 and 4′-MeO-CB-61 were greater than that of the parent PCBs. Metabolomics analysis indicated that exposure to the three chemicals induced different metabolic responses. 4′-MeO-CB-61 mainly affected the saccharide catabolism, including pyruvate metabolism, the TCA cycle, the transfer of acetyl groups into mitochondria and the Warburg effect, resulting in a greater energy consumption. Moreover, both CB-61 and 4′-OH-CB-61 promoted several amino acid metabolism and fatty acid biosynthesis, thereby alleviating the potential ROS damage. This study for the first time evaluates and reveals the phytotoxicity of OH-PCBs and MeO-PCBs at the metabolic level, which attempts to provide important information for accurately evaluating the environmental risks of PCBs from the perspective of metabolism.

Synthesis of mono- and dimethoxylated polychlorinated biphenyl derivatives starting from fluoroarene derivatives.

Polychlorinated biphenyls (PCBs) are environmental pollutants implicated in a variety of adverse health effects, including cancer and noncancer diseases in animals and humans. PCBs are metabolized to hydroxylated compounds, and some of these PCB metabolites are more toxic than the parent PCBs. Unfortunately, most PCB metabolites needed for toxicological studies are not available from commercial sources. Moreover, it is challenging to synthesize PCB metabolites because starting materials with suitable substitution patterns are not readily available. Here, we report the novel synthesis of a variety of mono- and dimethoxyarene derivatives from commercially available fluoroarenes via nucleophilic aromatic substitution with sodium methoxide. This reaction provided good to excellent yields of the desired methoxylated products. Suzuki coupling of selected mono- and dimethoxy haloarenes with chlorinated phenylboronic acids yielded methoxylated derivatives of PCB 11, 12, 25, 35, and 36 in low to good yields. Crystal structures of 3,3'-dichloro-2,5-dimethoxy-1,1'-biphenyl and 3',5-dichloro-2,3-dimethoxy-1,1'-biphenyl confirmed the substitution pattern of both compounds. This synthesis strategy provides straightforward access to a range of mono- and dimethoxylated PCB derivatives that were not readily accessible previously.

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.

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.

Effects of Polychlorinated Biphenyls (PCBs) and Their Hydroxylated Metabolites (OH-PCBs) on Arabidopsis thaliana.

Plants metabolize polychlorinated biphenyls (PCBs) into hydroxylated derivatives (OH-PCBs), which are sometimes more toxic than the parent PCBs. The objective of this research was to compare the toxicity of a suite of PCBs and OH-PCBs toward the model plant, Arabidopsis thaliana. While parent PCBs and higher-chlorinated OH-PCBs exhibited a low or nondetectable toxicity, lower-chlorinated OH-PCBs significantly inhibited the germination rate and plant growth, with inhibition concentration 50% (IC50) ranging from 1.6 to 12.0 mg L-1. The transcriptomic response of A. thaliana to 2,5-dichlorobiphenyl (2,5-DCB), and its OH metabolite, 4'-OH-2,5-DCB, was then examined using whole-genome expression microarrays (Affymetrix). Exposure to 2,5-DCB and 4'-OH-2,5-DCB resulted in different expression patterns, with the former leading to enrichment of genes involved in response to toxic stress and detoxification functions. Exposure to 2,5-DCB induced multiple xenobiotic response genes, such as cytochrome P-450 and glutathione S-transferases, potentially involved in the PCB metabolism. On the contrary, exposure to both compounds resulted in the down-regulation of genes involved in stresses not directly related to toxicity. Unlike its OH derivative, 2,5-DCB was shown to induce a transcriptomic profile similar to plant safeners, which are nontoxic chemicals stimulating detoxification pathways in plants. The differentiated induction of detoxification enzymes by 2,5-DCB may explain its lower phytotoxicity compared to 4'-OH-2,5-DCB.

Organohalogens and their hydroxylated metabolites in the blood of pigs from an open waste dumping site in south India: Association with hepatic cytochrome P450

The concentrations of polychlorinated biphenyls (PCBs), polybrominated diphenyl ethers (PBDEs), and their hydroxylated metabolites (OH–PCBs and OH–PBDEs) were measured in the blood of Eurasian wild pigs (Sus scrofa) from a municipal waste open dumping site (DS) and a reference site (RS) in South India. We showed that contamination with OH–PCBs was higher in female pigs from the DS than in all other adult pigs. The highest OH–PCB concentrations were found in piglets from the DS. Moreover, the hepatic expression levels of CYP1A and CYP2B were higher in piglets than in their dam, implying metabolism of PCBs by cytochrome P450 (CYP) enzymes. The OH–PCB congener profiles differed according to sex and collection sites, possibly because of variations in the expression levels of phase I and phase II enzymes among individual pigs, differences in the exposure sources, and maternal transfer of parent PCBs. The hepatic CYP1A expression levels were positively correlated with the blood concentrations of 4OH-CB107, 4OH-CB162, and 4OH-CB187, implying CYP1A-dependent formation of these OH–PCBs in the pig liver. We found no significant correlations between the blood concentrations of OH–PCBs and thyroid hormones (THs); however, the thyroxin (T4) levels were lower in pigs from the DS than in pigs from the RS. Our limited dataset suggest that induced CYP enzymes accelerate the metabolism of xenobiotics and endogenous molecules in pigs. Thus, besides parental compounds, the risk of hydroxylated metabolites entering wildlife and humans living in and around municipal open waste dumping sites should be considered.

Occurrence and distribution of PCB metabolites in blood and their potential health effects in humans: a review.

In recent years, attention has been directed to chemicals with possible endocrine-disrupting properties. Polychlorinated biphenyls (PCBs) and their metabolites belong to one group of environmental contaminants that have been shown to interact with the endocrine system in mammals, including humans. Although recent developments have been made in terms of determination of PCB metabolites in blood samples, still limited number of studies have been able to elucidate their profiles and toxicological and health effects in humans. This review aims to evaluate and compare the levels of hydroxylated PCBs (OH-PCBs) and methyl sulfone PCBs (MeSO2-PCBs) in blood and their relationship to parent compounds and also address the potential risks and adverse health effects in humans. Levels of OH-PCBs varied between 0.0002 and 1.6 ng g(-1) w/w in human serum/plasma from the selected literature, correlating well with ∑PCBs. In contrast, ∑OH-PCB/∑PCB ratio in animals did not show a significant correlation, which might suggest that the bioaccumulation plays an even more important role in the concentration of OH-PCBs compared to PCB metabolism. Highest levels of MeSO2-PCBs were reported in marine mammals with high selectivity retention in the liver. Health effects of PCB metabolites included carcinogenicity, reproductive impairment, and developmental neurotoxicity, being more efficiently transferred to the brain and across the placenta from mother to fetus in comparison to the parent PCBs. Based on the lack of knowledge on the occurrence and distribution of lower chlorinated OH-PCBs in humans, further studies to identify and assess the risks associated to human exposure are essential.

Rhizospere redox cycling and implications for rhizosphere biotransformation of selected polychlorinated biphenyl (PCB) congeners

Theoretically, sequential cycles of dechlorination followed by aerobic bio-oxidation are desirable to achieve complete degradation of a mixture of higher and lower chlorinated PCBs. In this research, soil was artificially contaminated with polychlorinated biphenyls (PCBs) in mixture and as single congeners, aged, and planted with two different plant species. Alternating redox cycles were created in the root zone of plants by flooding and draining the soil. Over 32 weeks, switchgrass (Panicum virgatum) and poplar (Populus deltoids × nigra DN34) planted systems that were exposed to alternate cycles of flooding performed better in reducing parent PCBs than planted systems that were not cycled (p<0.05). The cycled systems also had a higher mass of PCB transformation products than the uncycled systems. Multiple cycles were necessary to achieve significant differences between the cycled and uncycled treatments.

PCBs and OH-PCBs in Serum from Children and Mothers in Urban and Rural U.S. Communities

East Chicago, Indiana is a heavily industrialized community bisected by the Indiana Harbor and Ship Canal, which volatilizes ~7.5 kg/yr polychlorinated biphenyls (PCBs). In contrast, the rural Columbus Junction, Iowa area has no known current or past PCB industrial sources. Blood from children and their mothers from these communities were collected April 2008 to January 2009 (n = 177). Sera were analyzed for all 209 PCBs and 4 hydroxylated PCBs (OH-PCBs). Sum PCBs ranged from nondetect to 658 ng/g lw (median = 33.5 ng/g lw). Sum OH-PCBs ranged from nondetect to 1.2 ng/g fw (median = 0.07 ng/g fw). These concentrations are similar to those reported in other populations without high dietary PCB intake. Differences between the two communities were subtle. PCBs were detected in more East Chicago mothers and children than Columbus Junction mothers and children, and children from East Chicago were enriched in lower-molecular weight PCBs. East Chicago and Columbus Junction residents had similar levels of total and individual PCBs and OH-PCBs in their blood. Concentrations of parent PCBs correlated with concentrations of OH-PCBs. This is the first temporally and methodologically consistent study to evaluate all 209 PCBs and major metabolites in two generations of people living in urban and rural areas of the United States.

Nonlegacy PCBs: Pigment Manufacturing By-Products Get a Second Look

Polychlorinated biphenyls (PCBs) were commercially manufactured in the United States from about 1930 until 1979, when their production was banned under the Toxic Substances Control Act (TSCA) because of concerns about their extreme environmental persistence, ability to bioaccumulate, and adverse human health effects. PCBs were used in numerous industrial and consumer applications, most notably as insulation fluids in electrical transformers and generators but also in products including fluorescent lamp ballasts, caulk, and carbonless copy paper. These now-discontinued manufactured chemicals have received a great deal of attention in terms of research and environmental remediation. But other, lesser-known PCBs continue to be generated and released into the environment, not from intentionally created commercial products but as unintentional by-products of manufacturing processes including, according to recent studies, those used to make certain pigments used in dyes, inks, and paints. PCBs do not occur naturally, and once in the environment they can last for decades. Until recently, PCBs that were being detected in the environment were thought to come entirely from “legacy” sources. Yet developments in analytical technology have given researchers a better understanding of PCB sources, of the patterns of individual PCBs (or congeners) that are being detected environmentally, and the fate of PCBs in the environment—how they move between soil, sediment, water, and air. These advances have also enabled the detection of individual congeners at very low levels and the identification of many new and ongoing sources of PCBs beyond those resulting from historical commercial mixtures. Unintentionally produced PCBs were known to be present in inks and dyes when the U.S. Environmental Protection Agency (EPA) announced the final rule barring commercial PCB production in 1979. A rule allowing exemptions for PCBs in controlled manufacturing processes and as unintentional contaminants was promulgated under TSCA a few years later. This rule allowed for PCB concentrations of up to 50 ppm in certain products as a result of manufacturing processes.1 Recently, manufacturing by-product PCBs have been identified in wastewater, sediments, and air in numerous locations. They have also been positively identified in testing of new products colored with such pigments, so it is clear these PCBs are not occurring as a result of legacy commercial mixtures. What is emerging is an increasingly complex picture of the prevalence of nonlegacy PCBs alongside the persisting environmental presence of legacy PCBs, and a concurrent and likewise complex picture of how PCBs can affect human health at very low levels of exposure.

Polychlorinated biphenyl quinone metabolites lead to oxidative stress in HepG2 cells and the protective role of dihydrolipoic acid

Parent polychlorinated biphenyls (PCBs) have been shown to induce cellular oxidative stress. However, the effects of PCB active metabolites have not been extensively investigated. Parent PCBs are first converted to hydroquinone metabolites via cytochrome P-450-catalyzed hydroxylation, and the hydroquinone metabolites are then further oxidized into the corresponding quinone metabolites. Quinones are responsible for a wide range of toxic effects because of their high reactivity. Previous studies have suggested that reactive oxygen species (ROS) play important roles in multiple toxic mechanisms. In this context, the present study was undertaken to investigate oxidative stress resulting from treatment with PCB quinones in HepG2 cells. The protective effects resulting from co-administration of dihydrolipoic acid (DH-LA) were also investigated. We have found that exposure to PCB quinones leads to: (1) a decrease in cell viability; (2) an increase in both the total ROS production and superoxide production; (3) only 3Cl-PCBQ caused significant increase in the thiobarbituric acid reactive substances (TBARS) level; (4) an increase in SOD activity and a decrease in catalase activity; and (5) a decrease in GST activity and GSH level. We have also found that quinones possessing a higher number of chlorine atoms on the quinone ring display a greater activity and that DH-LA is an effective protective agent as it diminishes PCB quinone-induced cellular oxidative stress.

Polychlorinated biphenyls and their different level metabolites as inhibitors of glutathione S-transferase isoenzymes

Research on the effects of polychlorinated biphenyl (PCB) toxicity tends to focus on commercial PCB congeners and parent PCBs themselves. However, studies have suggested that PCB metabolites may be more interesting than the parent compounds because of their high reactivity. As a key metabolic enzyme, glutathione S-transferases (GSTs) are responsible for detoxification by catalyzing the conjugation reaction of glutathione (GSH) to xenobiotics. Inhibition of GST activity indicates reduced detoxification ability. We investigated the inhibition of chicken liver GSTs by parent PCBs and their metabolites and observed dose-dependent inhibition in vitro; inhibitory efficiency declined in the order GSH-conjugate>mono-hydroxyl ≈ quinone ≈ hydroquinone>parent PCB. Structure–inhibitory activity relationship studies indicated that with the inhibitory activity greatly increases with the number of GSH moieties or chlorine substituents on the quinone ring. However, no significant linear relationship was observed for chlorine pattern changes on the phenyl ring. The reversibility of PCB metabolite inhibition of GSTs is discussed. PCB mono-hydroxyl, hydroquinone and quinone forms showed irreversible inhibition of GSTs, which suggests a mechanism involving covalent binding to cysteine residues in the GST active site. PCB glutathionyl conjugates showed reversible GST inhibition, implying non-covalent binding. Furthermore, reactive oxygen species did not significantly affect GST activity.

Accumulation and efflux of polychlorinated biphenyls in Escherichia coli

Polychlorinated biphenyls (PCBs) are environmental pollutants that have been associated with numerous adverse health effects in human and animals. Hydroxylated PCBs (HPCBs) are the product of the oxidative metabolism of PCBs. The presence of hydroxyl groups in HPCBs makes these compounds more hydrophilic than the parent PCBs. One of the best approaches to break down and remove these contaminants is bioremediation; an environmentally friendly process that uses microorganisms to degrade hazardous chemicals into non-toxic ones. In this study, we investigated the cellular accumulation and toxicity of selected PCBs and HPCBs in Gram-negative bacteria, using Escherichia coli as a model organism. We found that none of the five PCBs tested were toxic to E. coli, presumably due to their limited bioavailability. Nevertheless, different HPCBs tested showed different levels of toxicity. Furthermore, we demonstrated that the primary multidrug efflux system in E. coli, AcrAB-TolC, facilitated the efflux of HPCBs out of the cell. Since AcrAB-TolC is constitutively expressed in E. coli and is conserved in all sequenced Gram-negative bacterial genomes, our results suggest that the efflux activities of multidrug resistant pumps may affect the accumulation and degradation of PCBs in Gram-negative bacteria.

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.

Biotransformation of PCBs in Arctic seabirds: Characterization of phase I and II pathways at transcriptional, translational and activity levels

Arctic seabirds are exposed to a wide range of halogenated organic contaminants (HOCs). Exposure occurs mainly through food intake, and many pollutants accumulate in lipid-rich tissues. Little is known about how HOCs are biotransformed in arctic seabirds. In this study, we characterized biotransformation enzymes in chicks of northern fulmars ( Fulmarus glacialis) and black-legged kittiwakes ( Rissa tridactyla) from Kongsfjorden (Svalbard, Norway). Phase I and II enzymes were analyzed at the transcriptional, translational and activity levels. For gene expression patterns, quantitative polymerase chain reactions (qPCR), using gene-sequence primers, were performed. Protein levels were analyzed using immunochemical assays of western blot with commercially available antibodies. Liver samples were analyzed for phase I and II enzyme activities using a variety of substrates including ethoxyresorufin (cytochrome (CYP)1A1/1A2), pentoxyresorufin (CYP2B), methoxyresorufin (CYP1A), benzyloxyresorufin (CYP3A), testosterone (CYP3A/CYP2B), 1-chloro-2,4-nitrobenzene (CDNB) (glutathione S-transferase (GST)) and 4-nitrophenol (uridine diphosphate glucuronyltransferase (UDPGT)). In addition, the hydroxylated (OH-) polychlorinated biphenyls (PCBs) were analyzed in the blood, liver and brain tissue, whereas the methylsulfone (MeSO 2-) PCBs were analyzed in liver tissue. Results indicated the presence of phase I (CYP1A4/CYP1A5, CYP2B, and CYP3A) and phase II (GST and UDPGT) enzymes at the activity, protein and/or mRNA level in both species. Northern fulmar chicks had higher enzyme activity than black-legged kittiwake chicks. This in combination with the higher ΣOH-PCB to parent PCB ratios suggests that northern fulmar chicks have a different biotransformation capacity than black-legged kittiwake chicks.