Hydroxylated Polychlorinated Biphenyls Research Articles

In silico exploration of hydroxylated polychlorinated biphenyls as estrogen receptor β ligands by 3D-QSAR, molecular docking and molecular dynamics simulations

Hydroxylated polychlorinated biphenyls (HO-PCBs), as the major metabolites of PCBs, have been reported to act as estrogen receptor β (ERβ) agonists. However, the chemical–biological interactions governing their activities toward ERβ have not been elucidated. Therefore, three dimensional quantitative structure–activity relationship (3D-QSAR), molecular docking and molecular dynamics (MD) simulations, to the best of our knowledge, for the first time were performed to explore the correlation between the structures and activities. The best 3D-QSAR model presented higher predictive ability (R 2 cv=0.543, R 2 pred=0.5793/R 2 cv=0.543, R 2 pred=0.6795) based on comparative molecular field analysis (CoMFA) and comparative similarity indices analysis (CoMSIA), respectively. At the same time, the derived contour maps indicated the important structural features required for improving the activity. Furthermore, molecular docking studies and MD simulations predicted the binding mode and the interactions between the ligand and the receptor. All the results would lead to a better understanding of the specific mechanism of HO-PCBs on estrogen receptor β (ERβ). Communicated by Ramaswamy H. Sarma

PCB 28 metabolites elimination kinetics in human plasma on a real case scenario: Study of hydroxylated polychlorinated biphenyl (OH-PCB) metabolites of PCB 28 in a highly exposed German Cohort

Polychlorinated biphenyls (PCBs) are suspected of carcinogenic, neurotoxic and immunotoxic effects in animals and humans. Although background levels of PCBs have been slowly decreased after their ban, they are still among the most persistent and ubiquitous pollutants in the environment, remaining the subject of great concern. PCB 28 is a trichlorinated PCB found in high concentrations not only in human plasma but also in indoor air in Europe, yet little is known about its metabolic pathway and potential metabolites in humans. The present study aims to elucidate the kinetics of metabolite formation and elimination by analyzing four hydroxylated PCBs (OH-PCBs) in human plasma as potential metabolites of the PCB 28 congener. For this purpose, the study was conducted in plasma samples of highly PCB-exposed individuals (N=268), collected from 2010 to 2014 as a representation of a real case scenario with longitudinal data. OH-PCBs have been predicted, synthesized in the course of this study and further identified and quantitated in human plasma. This is the first time that previously unknown PCB 28 metabolites have been measured in human plasma and half-lives have been estimated for PCB metabolites, which could then provide further understanding in the toxicological consequences of exposure to PCBs in humans.

Toxicity of hydroxylated polychlorinated biphenyls (HO-PCBs) using the bioluminescent assay Microtox(®).

Hydroxylated polychlorinated biphenyls (HO-PCBs) are toxic contaminants which are produced in the environment by biological or abiotic oxidation of PCBs. The toxicity of a suite of 23 mono-hydroxylated derivatives of PCBs and 12 parent PCBs was determined using the bacterial bioluminescent assay Microtox(®). All HO-PCBs tested exhibited higher toxicity than the corresponding parent PCB, with effect concentration 50% (EC50) ranging from 0.07 to 133mg L(-1). The highest toxicities were recorded with 4-hydroxylated derivatives of di-chlorinated biphenyls (EC50=0.07-0.36mg L(-1)) and 2-hydroxylated derivatives of tri-chlorinated biphenyls carrying a chlorine substituent on the phenolic ring (EC50=0.34-0.48mg L(-1)). The toxicity of HO-PCBs generally decreased when the degree of chlorination increased. Consistently with this observation, a significant positive correlation was measured between toxicity (measured by EC50) and octanol-water partition coefficient (pK ow) for the HO-PCBs under study (Pearson's correlation coefficient, r=0.74), which may be explained by the lower solubility and bioavailability generally associated with higher hydrophobicity. This study is the first one which assessed the toxicity of a suite of PCBs and HO-PCBs using the bioluminescent assay Microtox(®), showing an inverse correlation between toxicity and hydrophobicity.

Molecular docking and 3D-QSAR studies on the glucocorticoid receptor antagonistic activity of hydroxylated polychlorinated biphenyls

The glucocorticoid receptor (GR) antagonistic activities of hydroxylated polychlorinated biphenyls (HO-PCBs) were recently characterised. To further explore the interactions between HO-PCBs and the GR, and to elucidate structural characteristics that influence the GR antagonistic activity of HO-PCBs, molecular docking and three-dimensional quantitative structure–activity relationship (3D-QSAR) studies were performed. Comparative molecular similarity indices analysis (CoMSIA) was performed using both ligand- and receptor-based alignment schemes. Results generated from the receptor-based model were found to be more satisfactory, with q2 of 0.632 and r2 of 0.931 compared with those from the ligand-based model. Some internal validation strategies (e.g. cross-validation analysis, bootstrapping analysis and Y-randomisation) and an external validation method were used respectively to further assess the stability and predictive ability of the derived model. Graphical interpretation of the model provided some insights into the structural features that affected the GR antagonistic activity of HO-PCBs. Molecular docking studies revealed that some key residues were critical for ligand–receptor interactions by forming hydrogen bonds (Glu540) and hydrophobic interactions with ligands (Ile539, Val543 and Trp577). Although CoMSIA sometimes depends on the alignment of the molecules, the information provided is beneficial for predicting the GR antagonistic activities of HO-PCB homologues and is helpful for understanding the binding mechanisms of HO-PCBs to GR.

Modeling and predicting pKa values of mono-hydroxylated polychlorinated biphenyls (HO-PCBs) and polybrominated diphenyl ethers (HO-PBDEs) by local molecular descriptors

Hydroxylated polychlorinated biphenyls (HO-PCBs) and polybrominated diphenyl ethers (HO-PBDEs) are attracting considerable concerns because of their multiple endocrine-disrupting effects and wide existence in environment and organisms. The hydroxyl groups enable these chemicals to be ionizable, and dissociation constant, pKa, becomes an important parameter for investigating their environmental behavior and biological activities. In this study, a new pKa prediction model was developed using local molecular descriptors. The dataset contains 21 experimental pKa values of HO-PCBs and HO-PBDEs. The optimized geometries by ab initio HF/6-31G∗∗ algorithm were used to calculate the site-specific molecular readiness to accept or donate electron charges. The developed model obtained good statistical performance, which significantly outperformed commercial software ACD and SPARC. Mechanism analysis indicates that pKa values increase with the charge-limited donor energy EQocc on hydroxyl oxygen atom and decrease with the energy-limited acceptor charge QEvac on hydroxyl hydrogen atom. The regression model was also applied to calculate pKa values for all 837 mono-hydroxylated PCBs and PBDEs in each class, aiming to provide basic data for the ecological risk assessment of these chemicals.

Bioaccumulation of hydroxylated polychlorinated biphenyls and pentachlorophenol in the serum of northern elephant seal pups (Mirounga angustirostris)

Northern elephant seals (NES) (Mirounga angustirostris) from the Año Nuevo State Reserve (CA, USA) were sampled at 1-, 4-, 7- and 10-week post-weaning. Concentrations of hydroxylated polychlorinated biphenyls (HO-PCBs) and their parent PCBs were measured in the serum of each individual. The ΣHO-PCB concentrations in the serum increased significantly between early and late fast (from 282±20 to 529±31pg/mL). This increase might result from a mobilisation of HO-PCBs transferred from the mother during gestation and/or lactation and stored in the pup's liver. Food deprivation has been shown to exacerbate biotransformation capacities in mammals, birds and fish. The HO-penta-CBs was the predominant homologue group, followed by HO-hexa-CBs and HO-hepta-CBs. No preferential pathway for the metabolism of HO-PCBs (HO-direct insertion or NIH-shift of a chlorine atom) could be evidenced. The concentrations of pentachlorophenol (PCP) in the serum of weaned NES increased from 103±7pg/mL at early fast to 246±41pg/mL at late fast, which is within the range of PCP concentrations usually encountered in marine mammals.

Discovery of Hydroxylated Polychlorinated Biphenyls (OH-PCBs) in Sediment from a Lake Michigan Waterway and Original Commercial Aroclors

Hydroxylated polychlorinated biphenyls (OH-PCBs) were measured in surficial sediment from Indiana Harbor and Ship Canal (IHSC), East Chicago, IN and five original Monsanto Aroclors. These compounds were measured using gas chromatography with tandem mass spectrometry (GC-MS/MS) and certified standards that allowed us to identify 65 individual or coeluting congeners. Concentrations in the sediment ranged from 0.20 to 26 ng/g dry weight. Profiles of most samples were similar and were dominated by mono- to penta-chlorinated OH-PCBs. Interestingly, most of the samples strongly resembled the OH-PCB profiles of Aroclors 1221, 1242, 1248, and 1254, yet 25% of OH-PCBs measured in the sediment were not detected in Aroclors. A strong positive correlation was found between ΣOH-PCB and ΣPCB (p < 0.0001) and also between many individual OH-PCB:PCB pairs (p < 0.05). Analysis of OH-PCB:PCB pairs suggest PCB degradation is unlikely as a source of OH-PCBs in IHSC sediment. We are the first to report levels of OH-PCBs in sediment and Aroclors, and our discovery is significant because it is likely that OH-PCB contamination exists in sediment anywhere that PCB contamination from Aroclors is present.

Molecular docking, molecular dynamics simulation, and structure-based 3D-QSAR studies on the aryl hydrocarbon receptor agonistic activity of hydroxylated polychlorinated biphenyls

The binding interactions between hydroxylated polychlorinated biphenyls (HO-PCBs) and the aryl hydrocarbon receptor (AhR) are suspected of causing toxic effects. To understand the binding mode between HO-PCBs and AhR, and to explore the structural characteristics that influence the AhR agonistic activities of HO-PCBs, the combination of molecular docking, three-dimensional quantitative structure–activity relationship (3D-QSAR), and molecular dynamics (MD) simulations was performed. Using molecular docking, the HO-PCBs were docked into the binding pocket of AhR, which was generated by homology modeling. Comparative molecular similarity index analysis (CoMSIA) models were subsequently developed from three different alignment rules. The optimum 3D-QSAR model showed good predictive ability (q2=0.583, R2=0.913) and good mechanism interpretability. The statistical reliability of the CoMSIA model was also validated. In addition, molecular docking and MD simulations were applied to explore the binding modes between the ligands and AhR. The results obtained from this study may lead to a better understanding of the interaction mechanism between HO-PCBs and AhR.

Molecular Modeling and Molecular Dynamics Simulation Studies on the Interactions of Hydroxylated Polychlorinated Biphenyls with Estrogen Receptor-β

Endocrine-disrupting chemicals have attracted great concern. As major metabolites of polychlorinated biphenyls (PCBs), hydroxylated polychlorinated biphenyls (HO-PCBs) may disrupt estrogen hormone status because of their structural similarity to estrogen endogenous compounds. However, interactions between HO-PCBs and estrogen receptors (ERs) are not fully understood. In the present work, a molecular modeling study combining molecular docking, molecular dynamics simulations, and binding free energy calculations was performed to characterize the interactions of three HO-PCBs (4'-HO-PCB50, 2'-HO-PCB65, and 4'-HO-PCB69) having much different estrogenic activities with ERβ. Docking results showed that binding between ligands and ERβ was stabilized by hydrogen bond and hydrophobic interactions. The binding free energies of three ligands with ERβ were calculated, and further binding free energy decomposition analysis indicated that the dominating driving force of the binding between the ligands and ERβ was the van der Waals interaction. Some key residues, such as Leu298, Phe356, Gly472, His475, and Leu476, played important roles in ligand-receptor interactions by forming hydrophobic and hydrogen bond interactions with ligands. The results may be beneficial to increase understanding of the interactions between HO-PCBs and ERβ.

In silico study on hydroxylated polychlorinated biphenyls as androgen receptor antagonists

Hydroxylated polychlorinated biphenyls (HO-PCBs), major metabolites of PCBs, may have the potential to disrupt androgen hormone homeostasis. However, there is a lack of systematic investigation into the intermolecular interaction mechanism between HO-PCBs and the androgen receptor (AR). In this study, the combination of three-dimensional quantitative structure–activity relationship (3D–QSAR), molecular docking, and molecular dynamics (MD) simulations was performed to elucidate structural characteristics that influence the anti-androgen activity of HO-PCBs, and to provide a better understanding of the binding modes between HO-PCBs and AR. A predictive comparative molecular field analysis (CoMFA) model was developed with good robustness and predictive ability. Graphical interpretation of the model provided some insights into the structural features that affect the anti-androgen activity of HO-PCBs. The hydrogen bond interaction with Gln711, and hydrophobic interactions with residues in the hydrophobic pocket played important roles in the binding of ligand with receptor. These results are expected to be beneficial to predict anti-androgen activities of other HO-PCBs and provided possible clues for further elucidation of the binding mechanism of HO-PCBs with AR.

Molecular docking, molecular dynamics simulation, and structure-based 3D-QSAR studies on estrogenic activity of hydroxylated polychlorinated biphenyls

Hydroxylated polychlorinated biphenyls (HO-PCBs), major metabolites of PCBs, have been reported to present agonist or antagonist interactions with estrogen receptor α (ERα) and induce ER-mediated responses. In this work, a multistep framework combining molecular docking, molecular dynamics (MD) simulations, and structure-based three-dimensional quantitative structure–activity relationship (3D-QSAR) studies were performed to explore the influence of structural features on the estrogenic activities of HO-PCBs, and to investigate the molecular mechanism of ERα–ligand interactions. The CoMSIA (comparative molecular similarity indices analysis) model was developed from the conformations obtained from molecular docking. The model exhibited statistically significant results as the cross-validated correlation coefficient q2 was 0.648, the non-cross-validated correlation coefficient r2 was 0.968, and the external predictive correlation coefficient rpred2 was 0.625. The key amino acid residues were identified by molecular docking, and the detailed binding modes of the compounds with different activities were determined by MD simulations. The binding free energies correlated well with the experimental activity. An energetic analysis, MM-GBSA energy decomposition, revealed that the van der Waals interaction was the major driving force for the binding of compounds to ERα. The hydrogen bond interactions between the ligands and residue His524 help to stabilize the conformation of ligands at the binding pocket. These results are expected to be beneficial to predict estrogenic activities of other HO-PCB congeners and helpful for understanding the binding mechanism of HO-PCBs and ERα.

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.

Distribution of PCBs, Their Hydroxylated Metabolites, and Other Phenolic Contaminants in Human Serum from Two European Countries

Human exposure to mixtures of polychlorinated biphenyls (PCBs) may result in the formation of different profiles of hydroxylated PCBs (HO-PCBs), as a consequence of different exposures or dissimilar metabolism of parent compounds. Therefore, we investigated the levels and profiles of PCBs and HO-PCBs in human serum samples collected from two European countries with different degrees of pollution. There was no significant difference between the levels of sum PCBs measured in each set of samples, with a median concentration of 3100 pg/mL for Romanian samples (n = 53) and 3380 pg/mL for Belgian samples (n = 22). However, the median concentrations recorded for sum HO-PCBs were almost double in Belgian (310 pg/mL) compared to Romanian (175 pg/mL) samples. The detection frequency recorded for HO-PCBs in Belgian samples was also significantly higher compared to Romanian samples. The main contributors to the sum HO-PCBs in the Belgian samples were 4HO-CB107 > 4HO-CB146 > 4HO-CB187 (76% from the sum HO-PCBs) and 4HO-CB187 > 4HO-CB146 > 3'HO-CB138 (66% from the sum of HO-PCBs) in the Romanian samples. The HO-PCB profile showed that the higher chlorinated HO-PCBs had a higher contribution in the Romanian samples compared to the Belgian ones. This suggests that differences in the PCB profiles between populations can lead to the formation of different HO-PCB metabolite profiles presenting thus different risks for populations. No clear preferential mechanism of HO-PCB metabolite formation (HO-direct insertion vs. 1,2-shift of a chlorine atom) could be highlighted for investigated samples. The main chlorinated phenolic compound found in the Belgian samples was pentachlorophenol (PCP) which accounted for up to 85% of the total quantified phenolics, whereas in the Romanian samples, PCP accounted for only 35%.

Thyroid Function and Plasma Concentrations of Polyhalogenated Compounds in Inuit Adults

BackgroundSeveral ubiquitous polyhalogenated compounds (PHCs) have been shown to alter thyroid function in animal and in vitro studies. So far, epidemiologic studies have focused on the potential effect of a small number of them, namely, polychlorinated biphenyls (PCBs) and some organochlorines (OCs), without paying attention to other important PHCs.ObjectivesWe investigated the relationship between exposure to several PHCs and thyroid hormone homeostasis in Inuit adults from Nunavik.MethodsWe measured thyroid parameters [thyroid-stimulating-hormone (TSH), free thyroxine (fT4), total triiodothyronine (tT3), and thyroxine-binding globulin (TBG)] and concentrations of 41 contaminants, including PCBs and their metabolites, organochlorine pesticides (OCPs), polybrominated diphenyl ethers (PBDEs), perfluorooctanesulfonate (PFOS), and a measure of dioxin-like compounds, detected in plasma samples from Inuit adults (n = 623).ResultsWe found negative associations between tT3 concentrations and levels of 14 PCBs, 7 hydroxylated PCBs (HO-PCBs), all methylsulfonyl metabolites of PCBs (MeSO2-PCBs), and 2 OCPs. Moreover, we found negative associations between fT4 levels and hexachlorobenzene Concentrations. TBG concentrations were inversely related to 8 PCBs, 5 HO-PCBs, and 3 OCPs. Exposure to BDE-47 was positively related to tT 3, whereas PFOS concentrations were negatively associated with TSH, tT3, and TBG and positively with fT4 concentrations.ConclusionExposure to several PHCs was associated with modifications of the thyroid parameters in adult Inuit, mainly by reducing tT3 and TBG circulating concentrations. The effects of PFOS and BDE-47 on thyroid homeostasis require further investigation because other human populations display similar or higher concentrations of these chemicals.

Predatory Bird Species Show Different Patterns of Hydroxylated Polychlorinated Biphenyls (HO-PCBs) and Polychlorinated Biphenyls (PCBs)

Hydroxylated metabolites of polychlorinated biphenyls (HO-PCBs) have previously been associated with endocrine disrupting effects. Since metabolic capacity may differ among species, we investigated the levels and profiles of HO-PCBs and PCBs in livers of four predatory bird species from Belgium. Maximum concentrations for sum HO-PCBs were found in the common buzzard (Buteo buteo) up to 13 700 pg/g wet weight (ww). The most prominent HO-PCB congener in all bird species was 4-HO-CB 187 (up to 6420 pg/g ww in buzzard liver), followed by 4-HO-CB148 in the buzzard (up to 1820 pg/g ww), sparrowhawk (Accipiter nisus), and grey heron (Ardea cinerea), and by 3'-HO-CB138 in long eared owl (Asio otus) and in one grey heron (up to 985 pg/g ww and 3450 pg/g ww, respectively). The mean profile of the grey heron differed from the other species with 3'-HO-CB138 and 4-HO-CB163 contributing more to the sum HO-PCBs. This indicates that aquatic and terrestrial predatory bird species may show differences in their HO-PCBs profiles. Variation in the diet and species-specific accumulation and metabolism of PCBs are probably the most important causes for these differences. Correlations between HO-PCBs and their parent PCBs were only found significant for buzzards.

Determination of Hydroxylated Polychlorinated Biphenyls (HO-PCBs)in Blood Plasma by High-Performance Liquid Chromatography-Electrospray Ionization-Tandem Quadrupole Mass Spectrometry

Hydroxylated metabolites of polychlorinated biphenyls (HO-PCBs) and pentachlorophenol (PCP) are halogenated phenolic compounds, and they are increasingly common as environmental contaminants mainly in the blood of wildlife and humans. A methodology based on high-performance liquid chromatography (reversed-phase)-electrospray (negative) ionization-tandem quadrupole mass spectrometry (LC-ESI(-)-MS-MS) in the select ion monitoring or multiple reaction monitoring modes was developed for HO-PCB and PCP determination in blood plasma and serum. Among 11 environmentally relevant HO-PCB congeners and PCP spiked to fetal calf serum, quantitative assessments, including matrix effects on ESI(-) suppression/ enhancement, showed process (recovery) efficiencies of 73% to 89% without internal standard (IS) correction, and 88% to 103% with IS correction, and method limits of quantification ranging from 1 to 50 pg/g (wet weight). Using the developed LC-ESI(-)-MS methodology in comparison with GC-MS and GC-ECD based approaches, similar results were found for HO-PCB identification and quantification in the plasma of polar bear (Ursus maritimus) from the Canadian arctic. LC-ESI(-)-MS identified four HO-PCB congeners [4'-HO-2,2',4,6,6'-pentachlorobiphenyl (4'-HO-CB104), 4-HO-2,3,3',4',5-pentachlorobiphenyl (4-HO-CB107), 4-HO-2,3,3',5,5',6-hexachlorobiphenyl (4-HO-CB165) and 3'-HO-2,2',3,4,4',5,5'-heptachlorobiphenyl (3'-HO-CB180)], and 14 additional tetra- to hepta-chlorinated HO-PCBs isomers in the polar bear plasma.

Increased [ [formula omitted]]Phorbol Ester Binding in Rat Cerebellar Granule Cells and Inhibition of [formula omitted] Buffering in Rat Cerebellum by Hydroxylated Polychlorinated Biphenyls

Our previous structure–activity relationship (SAR) studies indicated that the effects of polychlorinated biphenyls (PCBs) on neuronal Ca 2+ homeostasis and protein kinase C (PKC) translocation were associated with the extent of coplanarity. Chlorine substitutions at ortho position on the biphenyl, which increase the non-coplanarity, are characteristic of the most active congeners in vitro. In the present study, we investigated the effects of selected hydroxylated PCBs, which are major PCB metabolites identified in mammals, on the same measures where PCBs had differential effects based on structural configuration. These measures include PKC translocation as determined by [ 3H ]phorbol ester ([ 3H ]PDBu) binding in cerebellar granule cells, and Ca 2+ sequestration as determined by 45Ca 2+ uptake by microsomes isolated from adult rat cerebellum. All the selected hydroxy-PCBs with ortho-chlorine substitutions increased [ 3H ]PDBu binding in a concentration-dependent manner and the order of potency as determined by E 50 (concentration that increases control activity by 50%) is 2′,4′,6′-trichloro-4-biphenylol (32 ± 4 μM), 2′,5′-dichloro-4-biphenylol (70 ± 9 μM), 2,2′,4′,5,5′-pentachloro-4-biphenylol (80 ± 7 μM) and 2,2′,5′-trichloro-4-biphenylol (93 ± 14 μM). All the selected hydroxy-PCBs inhibited microsomal 45Ca 2+ uptake to a different extent. Among the hydroxy-PCBs selected, 2′,4′,6′-trichloro-4-biphenylol is the most active in increasing [ 3H ]PDBu binding as well as inhibiting microsomal 45Ca 2+ uptake. 3,5-Dichloro-4-biphenylol and 3,4′,5-trichloro-4-biphenylol did not increase [ 3H ]PDBu binding, but inhibited microsomal 45Ca 2+ uptake. This effect was not related to ionization of these two hydroxy-PCBs. Hydroxylated PCBs seemed to be as active as parent PCBs in vitro. These studies indicate that PCB metabolites such as hydroxy-PCBs might contribute significantly to the neurotoxic responses of PCBs.

Synthesis of hydroxylated PCB metabolites with the Suzuki-coupling

An improved synthesis of hydroxylated polychlorinated biphenyls (PCBs) which are structurally related to the major hydroxy PCB congeners identified in human plasma is described. The coupling of (chlorinated) aryl boronic acids with bromochloro anisoles using the standard conditions of the Suzuki coupling gave the desired hydroxylated PCB metabolites in good to excellent yields. The approach offers the advantage of high selectivity and good yields compared to conventional methods such as the Cadogan reaction and allows the use of less toxic starting materials.

Different competition of thyroxine binding to transthyretin and thyroxine-binding globulin by hydroxy-PCBs, PCDDs and PCDFs

In an earlier study several hydroxylated polychlorinated biphenyls (PCBs), dibenzo- p-dioxins (PCDDs) and dibenzofurans (PCDFs) competitively displaced [ 125I]thyroxine (T 4) from transthyretin with different potencies. Transthyretin is the major T 4 transport protein in plasma of rodents. In man, however, thyroxine-binding globulin transports most of the T 4 in blood. In this study, hydroxylated PCBs, PCDDs and PCDFs were tested in an in vitro competitive binding assay, using purified human thyroxine-binding globulin and [ 125I]T 4 as the displaceable radioligand. None of the tested hydroxylated PCBs, PCDDs and PCDFs inhibited [ 125I]T 4 binding to thyroxine-binding globulin. In addition, some T 4 derived compounds, e.g., tyrosine, mono-iodotyrosine, di-iodotyrosine and tri-iodophenol were tested on both transthyretin and thyroxine-binding globulin to investigate possible differences in structural characteristics determining T 4 binding to thyroxine-binding globulin and transthyretin. The T 4 derived compounds also did not inhibit [ 125I]T 4 binding to thyroxine-binding globulin as tested in the in vitro assay. However, tri-iodophenol and to a lesser extent di-iodotyrosine inhibited [ 125I]T 4-transthyretin binding. These results indicate a marked difference in T 4 binding to thyroxine-binding globulin,or transthyretin. The hydroxylated PCBs, PCDDs and PCDFs can inhibit T 4 binding to transthyretin, but not to thyroxine-binding globulin, and thus may cause different effects in rodents and man.