Reactivity of ubiquinone and ubiquinol with superoxide and the hydroperoxyl radical: implications for in vivo antioxidant activity

Endogenous ubiquinones (UQ) such as coenzyme Q10 are essential electron carriers in the mitochondrial respiratory chain, and the reduced ubiquinol form (UQH2) is a chain-breaking antioxidant, decreasing oxidative damage caused by lipid peroxidation within mitochondria. Consequently, exogenous UQ are used as therapies to decrease mitochondrial oxidative damage. The proximal radical produced during mitochondrial oxidative stress is superoxide (O2·−) and the reaction between UQ and O2·− to form the ubisemiquinone radical anion (UQ·−) may also be important for the scavenging of O2·− by exogenous UQ. The situation in vivo is that many UQ are predominantly located in the hydrophobic membrane core, from which O2·− will be excluded but its conjugate acid, HOO·, can enter. The reactivity of UQ or UQH2 with HOO· has not been reported previously. Here a pulse radiolysis study on the reactions between UQ/UQH2 and O2·−/HOO· in water and in solvent systems mimicking the surface and core of biological membranes has been undertaken. O2·− reacts very rapidly with UQ, suggesting that this may contribute to the scavenging of O2·− in vivo. In contrast, UQH2 reacts relatively slowly with HOO·, but rapidly with other oxygen- and carbon-centered radicals, indicating that the antioxidant role of UQH2 is mainly in preventing lipid peroxidation.