Miho Kuwabara

Abstract: Hydrogen peroxide (H2O2) formed in aqueous solutions of tea catechins such as epicatechin (EC), epicatechin gallate (ECg), epigallocatechin (EGC), and epigallocatechin gallate (EGCg) was measured by an HPLC method. The amount of H2O2 formed depended on their chemical structures, pH, temperature and incubation time. The gallyl moiety in the B-ring of the gallocatechins, namely EGC and EGCg, mainly contributed to the H2O2 formation. Superoxide dismutase inhibited oxidation of the gallocatechins and, consequently, the H2O2 formation. Cytotoxic effects were investigated by a colony formation assay with Chinese hamster V79 cells, and the effects of the gallocatechins were stronger than those of EC and ECg. The cytotoxic effects of the gallocatechins were inhibited completely by catalase and partially by superoxide dismutase. These results indicate that the gallocatechins were oxidized by superoxide, accompanied by the formation of H2O2, and their cytotoxic effects were ascribed to the H2O2 formation.

TL;DR: The lipophilicity and toxicity of the four flavonols, galangin, kaempferol, quercetin and myrice-tin, which respectively have no, one, two and three hydroxyl groups on the B-ring, were compared to show that the cytotoxicity of myricetin was attributable to the hydrogen peroxide formed by autoxidation.