Covalent binding of PCB metabolites to lipids: route of formation and characterization

Dioxin-like PCB 126 Increases Systemic Inflammation and Accelerates Atherosclerosis in Lean LDL Receptor-Deficient Mice

Exposure to dioxins and related persistent organic pollutants likely contributes to cardiovascular disease (CVD) risk through multiple mechanisms including the induction of chronic inflammation. Epidemiological studies have shown that leaner individuals may be more susceptible to the detrimental effects of lipophilic toxicants because they lack large adipose tissue depots that can accumulate and sequester these pollutants. This phenomenon complicates efforts to study mechanisms of pollutant-accelerated atherosclerosis in experimental animal models where high-fat feeding and adipose expansion limit the bioavailability of lipophilic pollutants. Here, we investigated whether a model dioxin-like pollutant, PCB 126, could increase inflammation and accelerate atherosclerosis in Ldlr-/- mice fed a low-fat atherogenic diet. We fed Ldlr-/- mice the Clinton/Cybulsky diet (10% kcal fat, 0.15% cholesterol) and sacrificed mice at 8, 10, or 12 weeks postPCB (2 doses of 1 μmol/kg) or vehicle gavage. To characterize this novel model, we examined the effects of PCB 126 on markers of systemic inflammation, hematological indices, fatty livers, and atherosclerotic lesion size. Mice exposed to PCB 126 exhibited significantly increased plasma inflammatory cytokine levels, increased circulating biomarkers of CVD, altered platelet, and red blood cell counts, increased accumulation of hepatic fatty acids, and accelerated atherosclerotic lesion formation in the aortic root. PCB 126 also increased circulating neutrophils, monocytes, and macrophages as determined by flow cytometry analysis. Exposure to dioxin-like PCB 126 increases inflammation and accelerates atherosclerosis in mice. This low-fat atherogenic diet may provide a useful tool to study the mechanisms linking exposure to lipophilic pollutants to increased risk of CVD.

Cytochrome c adducts with PCB quinoid metabolites

Polychlorinated biphenyls (PCBs) are a group of 209 individual congeners widely used as industrial chemicals. PCBs are found as by-products in dye and paint manufacture and are legacy, ubiquitous, and persistent as human and environmental contaminants. PCBs with fewer chlorine atoms may be metabolized to hydroxy- and dihydroxy-metabolites and further oxidized to quinoid metabolites both in vitro and in vivo. Specifically, quinoid metabolites may form adducts on nucleophilic sites within cells. We hypothesized that the PCB-quinones covalently bind to cytochrome c and, thereby, cause defects in the function of cytochrome c. In this study, synthetic PCB quinones, 2-(4'-chlorophenyl)-1,4-benzoquinone (PCB3-pQ), 4-4'-chlorophenyl)-1,2-benzoquinone (PCB3-oQ), 2-(3', 5'-dichlorophenyl)-1,4-benzoquinone, 2-(3',4', 5'-trichlorophenyl)-1,4-benzoquinone, and 2-(4'-chlorophenyl)-3,6-dichloro-1,4-benzoquinone, were incubated with cytochrome c, and adducts were detected by liquid chromatography-mass spectrometry (LC-MS) and matrix-assisted laser desorption/ionization time of flight mass spectrometry (MALDI TOF). Sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) was employed to separate the adducted proteins, while trypsin digestion and liquid chromatography-tandem mass spectrometry (LC-MS/MS) were applied to identify the amino acid binding sites on cytochrome c. Conformation change of cytochrome c after binding with PCB3-pQ was investigated by SYBYL-X simulation and cytochrome c function was examined. We found that more than one molecule of PCB-quinone may bind to one molecule of cytochrome c. Lysine and glutamic acid were identified as the predominant binding sites. Software simulation showed conformation changes of adducted cytochrome c. Additionally, cross-linking of cytochrome c was observed on the SDS-PAGE gel. Cytochrome c was found to lose its function as electron acceptor after incubation with PCB quinones. These data provide evidence that the covalent binding of PCB quinone metabolites to cytochrome c may be included among the toxic effects of PCBs.

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.

Identification of hydroxylated metabolites of 3,3',4,4'-tetrachlorobiphenyl and metabolic pathway in whole poplar plants

Polychlorinated biphenyls (PCBs) can be metabolized to hydroxylated polychlorinated biphenyls (OH-PCBs) as reported in a number of animal studies. However, there are few studies on OH-PCBs in vivo in whole plants. In order to explore the formation of OH-PCBs in whole plants in detail, poplars (Populus deltoides×nigra, DN34) were exposed to 3,3',4,4'-tetrachlorobiphenyl (CB77) in hydroponic solution. Poplars are widely used in phytoremediation applications and the complete genome has been sequenced. In this research, a HPLC-MS method was developed to directly determine the hydroxylated metabolites of CB77 (OH-CB77s), avoiding the experimental errors introduced by derivatization pretreatments required by gas chromatography-based methods. Three potential hydroxylated metabolites of CB77, including 6-hydroxy-3,3',4,4'-tetrachlorobiphenyl (6OH-CB77), 5-hydroxy-3,3',4,4'-tetrachlorobiphenyl (5OH-CB77) and 4'-hydroxy-3,3',4,5'-tetrachlorobiphenyl (4'OH-CB79), were determined in poplar tissues. The major product, 6OH-CB77, was detected in the roots, bottom bark, bottom wood, middle bark and middle wood for the whole poplar plants, but the minor product, 5OH-CB77, was detected only in the poplar roots. The concentration of 6OH-CB77 was about 10 times greater than that of 5OH-CB77 in the roots. However, the major mammalian metabolite, 4'OH-CB79 was not detected in any of the samples. The results suggest that the hydroxylated metabolic pathway of CB77 is via an epoxide intermediate in poplar.

Some dietary fibers increase elimination of orally administered polychlorinated biphenyls but not that of retinol in mice

Dietary fiber supplementation can increase the size and nutrient absorption capacities of the small intestine in some mammals, but does this increase the risk of accumulating environmental contaminants? This study addressed this question by feeding mice diets containing various types of fiber at 0 or 100 g/kg (cellulose, lactosucrose, polydextrose, indigestible dextrin, soy polysaccharide, rice bran and chitosan) for 10 wk. During the final 2 wk, the mice were fed retinol and a dose of Arochlor 1254 [polychlorinated biphenyls (PCB)] estimated to be 5% of the median lethal dose. Accumulation was determined using whole blood samples collected on days 1, 3 and 7 as well as eight tissues (whole blood, small and large intestine, liver, gall bladder, mesentery, kidney and brain). Elimination of Arochlor 1254 and retinol was determined using daily collections of feces and urine. The patterns of accumulation and elimination differed between Arochlor 1254 and retinol, among tissues, and among mice fed diets with various amounts and types of fiber. Dietary fiber supplementation did not decrease accumulation of PCB. However, the diet with chitosan increased fecal excretion of Arochlor 1254 compared to the fiber-free diet (P<0.05). The diets with fermentable fiber (polydextrose, indigestible dextrin and soy polysaccharides) increased urinary excretion of PCB compared to the diets with water-insoluble fiber (cellulose, rice bran and chitosan; P<0.05). The most efficacious diets for minimizing accumulation of environmental contaminants and accelerating elimination likely include a combination of soluble and insoluble fiber, but the specific types, proportions and amounts remain to be determined.

Decabromodiphenyl ether in the rat: absorption, distribution, metabolism, and excretion

Among the group of polybrominated diphenyl ethers used as flame-retardants, the fully brominated diphenyl ether, decabromodiphenyl ether (decaBDE), is the most commonly used. Despite the large usage of decaBDE, neither the metabolic pathways nor the absorption have been addressed, and there are very few studies on its toxicology. In this work, it is shown that after a single oral dose of 14C-labeled decaBDE to rats, at least 10% of the decaBDE dose is absorbed. The major excretion route in conventional rats is via feces that contained 90% of the decaBDE dose. The excretion in bile was close to 10% of the dose and represented mainly metabolites. It cannot be excluded that greater than 10% of the oral dose had been absorbed since 65% of the radioactivity excreted in feces was metabolites. The highest concentrations on a lipid weight basis were found in plasma and blood-rich tissues, and the adipose tissue had the lowest concentration of decaBDE. After derivatization of a phenolic fraction, gas chromatography-mass spectrometry (GC/MS) analyses indicated that metabolites with five to seven bromine atoms had formed, and they possessed a guaiacol structure (a hydroxy and a methoxy group) in one of the rings. In addition, traces of nonabrominated diphenyl ethers and monohydroxylated metabolites were found by GC/MS. Metabolites, characterized by their chemical properties, were interpreted to be covalently bound to macromolecules, either proteins or lipids. In addition, water solubility was suggested. The metabolic pathway was indicated to include a reactive intermediate.