SOD inhibition of antioxidants autoxidation

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We developed a new method to measure hydrogen peroxide(H_2O_2)by HPLC-ECD.By this method, we investigated effects of superoxide dismutase(SOD) on H_2O_2 formation in aqueous solutions of antioxidants such as ubiquinols, flavonols and catechins.(1)The method to measure H_2O_2 by HPLC-ECD is convenient, selective and sensitive. It also enables us to measure H_2O_2 concentration in aqueous solutions of cigarette tar, coffee, and ascorbic acid.(2)CoQ_0H_2 and myricetin are typical examples of a ubiquinol and a flavonol, respectively. Epigallocatechin gallate(EGCg), epigallocatechin(EGC), epicatechin gallate(ECg), and epicatechin(EC) are common catechins found in green tea. These compounds show antioxidant activity. But, they also show prooxidant activity under certain conditions owing to H_2O_2 formation. We measured H_2O_2 formed in aqueous solutions of these compounds by the HPLC-ECD method. SOD inhibited the H_2O_2 formation. The inhibition of H_2O_2 formation by SOD seems contradictory to the function of SOD, because H_2O_2 is a product of its enzyme reaction. This problem is solved by postulating a chain reaction in the formation of H_2O_2 with surperoxide as the chain carrier. We investigated cytotoxicity of myricetin, EGCg and EGC by a colony formation assay with Chinese hamster V79 cells. Inhibitory effects of catalase and SOD indicate that the cytotoxic effects were attributable to H_2O_2 formation where superoxide was involved.