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Spin trapping with DMPO indicates that superoxide formation is below the limit of detection. (a) EPR spectrum by DMPO (50 mM) in PBS with DETAPAC (250 µM); (b) superoxide and hydroxyl adducts of DMPO (50 mM) generated with HX/XO at pH 7.4 (aN = 14.1 G, aH = 11.3 G with an additional hydrogen splitting of 1.3 G for DMPO/•OOH, and aN = aH = 14.9 G for DMPO/•OH); (c) spectrum generated from 2′-Cl-2,5-Q (1 mM) and DMPO (100 mM) at pH 7.4; (d) spectrum generated from 2′-Cl-2,5-Q (1 mM), DMPO (100 mM), and 500 U SOD/mL at pH 7.4; (e) spectrum generated from 2′-Cl-2,5- H2Q (1 mM) and DMPO (100 mM) at pH 7.4; and (f) spectrum generated from 2′-Cl-2,5-H2Q (1 mM), DMPO (100 mM), and 500 U SOD/mL at pH 7.4. All experimental spectra were collected using a 1.0 G modulation amplitude.
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fig5: Spin trapping with DMPO indicates that superoxide formation is below the limit of detection. (a) EPR spectrum by DMPO (50 mM) in PBS with DETAPAC (250 µM); (b) superoxide and hydroxyl adducts of DMPO (50 mM) generated with HX/XO at pH 7.4 (aN = 14.1 G, aH = 11.3 G with an additional hydrogen splitting of 1.3 G for DMPO/•OOH, and aN = aH = 14.9 G for DMPO/•OH); (c) spectrum generated from 2′-Cl-2,5-Q (1 mM) and DMPO (100 mM) at pH 7.4; (d) spectrum generated from 2′-Cl-2,5-Q (1 mM), DMPO (100 mM), and 500 U SOD/mL at pH 7.4; (e) spectrum generated from 2′-Cl-2,5- H2Q (1 mM) and DMPO (100 mM) at pH 7.4; and (f) spectrum generated from 2′-Cl-2,5-H2Q (1 mM), DMPO (100 mM), and 500 U SOD/mL at pH 7.4. All experimental spectra were collected using a 1.0 G modulation amplitude.

Mentions: In Figure 5, spectrum “a” is a control demonstrating the absence of artifactual signals from DMPO. Spectrum “b” represents the typical DMPO/•OOH spin adduct generated from HX/XO system and trapped by DMPO; some DMPO/HO• spin adduct is also present. When 2′-Cl-2,5-Q was added to DMPO in neutral solution, both SQ•− and DMPO/HO• were detected (Figure 5c). If SOD was included, there was no significant change in the intensity of the spectrum of SQ•− (spectrum “d”), but the signal of the DMPO/HO• radical increased. Because no evidence for DMPO/•OOH radical was observed in “c” and SOD did not decrease the DMPO/HO• signal, there appears to be no significant superoxide formation from 2′-Cl-2,5-Q.

Semiquinone Radicals from Oxygenated Polychlorinated Biphenyls: Electron Paramagnetic Resonance Studies

Song Y, Wagner BA, Lehmler HJ, Buettner GR - Chem. Res. Toxicol. (2008)

Bottom Line: A guiding hypothesis in the PCB research community is that some of the detrimental health effects of some PCBs are a consequence of these oxygenated forms undergoing one-electron oxidation or reduction, generating semiquinone radicals (SQ (*-)).These radicals can enter into a futile redox cycle resulting in the formation of reactive oxygen species, that is, superoxide and hydrogen peroxide.Our data also point to futile redox cycling as being one mechanism by which oxygenated PCBs can lead to the formation of reactive oxygen species, but this is most efficient in the presence of SOD.

Affiliation: Department of Occupational and Environmental Health, The University of Iowa, Iowa City, Iowa 52242-1101, USA.

ABSTRACT
Polychlorinated biphenyls (PCBs) can be oxygenated to form very reactive hydroquinone and quinone products. A guiding hypothesis in the PCB research community is that some of the detrimental health effects of some PCBs are a consequence of these oxygenated forms undergoing one-electron oxidation or reduction, generating semiquinone radicals (SQ (*-)). These radicals can enter into a futile redox cycle resulting in the formation of reactive oxygen species, that is, superoxide and hydrogen peroxide. Here, we examine some of the properties and chemistry of these semiquinone free radicals. Using electron paramagnetic resonance (EPR) to detect SQ (*-) formation, we observed that (i) xanthine oxidase can reduce quinone PCBs to the corresponding SQ (*-); (ii) the heme-containing peroxidases (horseradish and lactoperoxidase) can oxidize hydroquinone PCBs to the corresponding SQ (*-); (iii) tyrosinase acting on PCB ortho-hydroquinones leads to the formation of SQ (*-); (iv) mixtures of PCB quinone and hydroquinone form SQ (*-) via a comproportionation reaction; (v) SQ (*-) are formed when hydroquinone-PCBs undergo autoxidation in high pH buffer (approximately >pH 8); and, surprisingly, (vi) quinone-PCBs in high pH buffer can also form SQ (*-); (vii) these observations along with EPR suggest that hydroxide anion can add to the quinone ring; (viii) H 2 O 2 in basic solution reacts rapidly with PCB-quinones; and (ix) at near-neutral pH SOD can catalyze the oxidization of PCB-hydroquinone to quinone, yielding H 2 O 2. However, using 5,5-dimethylpyrroline-1-oxide (DMPO) as a spin-trapping agent, we did not trap superoxide, indicating that generation of superoxide from SQ (*-) is not kinetically favorable. These observations demonstrate multiple routes for the formation of SQ (*-) from PCB-quinones and hydroquinones. Our data also point to futile redox cycling as being one mechanism by which oxygenated PCBs can lead to the formation of reactive oxygen species, but this is most efficient in the presence of SOD.

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