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Observation of an unusual electronically distorted semiquinone radical of PCB metabolites in the active site of prostaglandin H synthase-2.

ABSTRACT: The activation of the metabolites of airborne polychlorinated biphenyls (PCBs) into highly reactive radicals is of fundamental importance. We found that human recombinant prostaglandin H synthase-2 (hPGHS-2) biotransforms dihydroxy-PCBs, such as 4-chlorobiphenyl-2',5'-hydroquinone (4-CB-2',5'-H(2)Q), into semiquinone radicals via one-electron oxidation. Using electron paramagnetic resonance (EPR) spectroscopy, we observed the formation of the symmetric quartet spectrum (1:3:3:1 by area) of 4-chlorobiphenyl-2',5'-semiquinone radical (4-CB-2',5'-SQ()(-)) from 4-CB-2',5'-H(2)Q. This spectrum changed to an asymmetric spectrum with time: the change can be explained as the overlap of two different semiquinone radical species. Hindered rotation of the 4-CB-2',5'-SQ()(-) appears not to be a major factor for the change in lineshape because increasing the viscosity of the medium with glycerol produced no significant change in lineshape. Introduction of a fluorine, which increases the steric hindrance for rotation of the dihydroxy-PCB studied, also produced no significant changes. An in silico molecular docking model of 4-CB-2',5'-H(2)Q in the peroxidase site of hPGHS-2 together with ab initio quantum mechanical studies indicate that the close proximity of a negatively charged carboxylic acid in the peroxidase active site may be responsible for the observed perturbation in the spectrum. This study provides new insights into the formation of semiquinones from PCB metabolites and underscores the potential role of PGHS-2 in the metabolic activation of PCBs.

SUBMITTER: Wangpradit O

PROVIDER: S-EPMC3622869 | biostudies-literature | 2010 Dec

REPOSITORIES: biostudies-literature

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Observation of an unusual electronically distorted semiquinone radical of PCB metabolites in the active site of prostaglandin H synthase-2.

Wangpradit Orarat O Moman Edelmiro E Nolan Kevin B KB Buettner Garry R GR Robertson Larry W LW Luthe Gregor G

Chemosphere 20100916 11

The activation of the metabolites of airborne polychlorinated biphenyls (PCBs) into highly reactive radicals is of fundamental importance. We found that human recombinant prostaglandin H synthase-2 (hPGHS-2) biotransforms dihydroxy-PCBs, such as 4-chlorobiphenyl-2',5'-hydroquinone (4-CB-2',5'-H(2)Q), into semiquinone radicals via one-electron oxidation. Using electron paramagnetic resonance (EPR) spectroscopy, we observed the formation of the symmetric quartet spectrum (1:3:3:1 by area) of 4-chl ...[more]

PMID: 20843536

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Project description: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.

Dataset Information

Observation of an unusual electronically distorted semiquinone radical of PCB metabolites in the active site of prostaglandin H synthase-2.

Publications

Observation of an unusual electronically distorted semiquinone radical of PCB metabolites in the active site of prostaglandin H synthase-2.

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