The quinone/semiquinone/hydroquinone triad (Q/SQ •− /H 2 Q) represents a class of compounds that has great importance in a wide range of biological processes. The half-cell reduction potentials of these redox couples in aqueous solutions at neutral pH, E°′, provide a window to understanding the thermodynamic and kinetic characteristics of this triad and their associated chemistry and biochemistry in vivo. Substituents on the quinone ring can significantly influence the electron density "on the ring" and thus modify E°′ dramatically. E°′ of the quinone governs the reaction of semiquinone with dioxygen to form superoxide. At near-neutral pH the pK a 's of the hydroquinone are outstanding indicators of the electron density in the aromatic ring of the members of these triads (electrophilicity) and thus are excellent tools to predict half-cell reduction potentials for both the one-electron and two-electron couples, which in turn allow estimates of rate constants for the reactions of these triads. For example, the higher the pK a 's of H 2 Q, the lower the reduction potentials and the higher the rate constants for the reaction of SQ •− with dioxygen to form superoxide. However, hydroquinone autoxidation is controlled by the concentration of di-ionized hydroquinone; thus, the lower the pK a 's the less stable H 2 Q to autoxidation. Catalysts, e.g., metals and quinone, can accelerate oxidation processes; by removing superoxide and increasing the rate of formation of quinone, superoxide dismutase can accelerate oxidation of hydroquinones and thereby increase the flux of hydrogen peroxide. The principal reactions of quinones are with nucleophiles via Michael addition, for example, with thiols and amines. The rate constants for these addition reactions are also related to E°′. Thus, pK a 's of a hydroquinone and E°′ are central to the chemistry of these triads.

The reactions of glutathione (GSH) with polychlorinated biphenyl (PCB) quinones having different degrees of chlorination on the quinone ring were examined. EPR spectroscopy and MS revealed 2 types of reactions yielding different products: (i) a nonenzymatic, nucleophilic displacement of chlorine on the quinone ring yielding a glutathiylated conjugated quinone and (ii) Michael addition of GSH to the quinone, a 2-electron reduction, yielding a glutathiylated conjugated hydroquinone. The pKa of parent hydroquinone decreased by 1 unit as the degree of chlorination increased. This resulted in a corresponding increase in the oxidizability of these chlorinated hydroquinones. The reaction with oxygen appears to be first-order each in ionized hydroquinone and dioxygen, yielding hydrogen peroxide stoichiometrically. The generation of semiquinone radicals, superoxide, and hydroxyl radicals was observed by EPR; however, the mechanisms and yields vary depending on the degree of the chlorination of hydroquinone/quinone and the presence or absence of GSH. Our discovery that chlorinated quinones undergo a rapid, nonenzymatic dechlorination upon reaction with GSH opens a different view on mechanisms of metabolism and the toxicity of this class of compounds.dechlorination ͉ EPR ͉ superoxide ͉ semiquinone ͉ hydrogen peroxide P olychlorinated biphenyls (PCBs) are ubiquitous environmental pollutants (1, 2). Many PCBs are poorly biodegradable and thus accumulate and are amplified through the food chain (3). Several congeners, including lower chlorinated PCBs, act as tumor promoters (4). Mechanisms involving reactive metabolites have been proposed in the initiating action of PCBs. Lower-chlorinated biphenyls can be metabolized by cytochrome P450 1A1, 1A2, 2B1/2B2, via arene oxides to mono-and dihydroxylated intermediates and further to quinones (5, 6). Quinones are reactive electrophiles, which can readily undergo Michael addition with a multitude of intracellular nucleophiles, such as amino acids, glutathione (GSH), proteins, and nucleic acids. Quinones can also be reduced to highly reactive semiquinone radicals, which in turn, lead to the formation of reactive oxygen species (ROS), causing oxidative stress and toxicity (7).GSH is the major nonprotein sulfhydryl in cells (8). As a nucleophile, it will conjugate with electrophiles both enzymatically and nonenzymatically; conjugation with various xenobiotics and/or their metabolic intermediates typically converts them into less toxic products. These reactions can be complex because of the possible involvement of a variety of free radicals (9, 10). Here, we evaluate the redox properties and mechanisms of the reactions of GSH with PCB quinones. We have discovered that certain chlorinated quinones not only undergo Michael addition reactions with GSH, but also GSH can nonenzymatically displace chlorine on the quinone ring. Results and DiscussionGlutathione can react with quinone rings via Michael addition forming corresponding hydroquinones (11, 12), Q ϩ GSH 3 GS-H 2 Q.[1]These hydroquinon...

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 (≈>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) H2O2 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 H2O2. 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.

Several polychlorinated biphenyls (PCBs) and their hydroxylated metabolites display axial chirality. Here we describe an enantioselective, gas chromatographic separation of methylated derivatives of hydroxylated (OH-)PCB atropisomers (MeO-PCB) using a chemically bonded β-cyclodextrin column (Chirasil-Dex). The atropisomers of several MeO-PCBs could be separated on this column with resolutions ranging from 0.42-0.87 under isothermal or temperature-programmed conditions. In addition, the enantiomeric fraction of OH-PCB 136 metabolites was determined in male and female rats treated with racemic PCB 136. The methylated derivatives of two OH-PCB 136 metabolites showed an enantiomeric enrichment in liver tissue, whereas PCB 136 itself was near racemic.

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