Antioxidative Function of Plant Polyphenols in Mammalian Cells

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(1) Cytotoxicity and DNA single-strand breaks caused by H_2O_2 were assessed by a colony formation assay and a DNA precipitation assay, respectively, with Chinese hamster V79 cells. In both assays, caffeic acid ethyl ester showed protective effects. The structure-activity relationship showed that the omicron-dihydroxy structure of caffeic acid ethyl ester was essential for the protective effects.(2) We assessed inhibitory effects of curcumin and its chemically modified homologues against H_2O_2-induced cytotoxicity toward Chinese hamster long fibroblasts V79 cells with a colony formation assay. Among 4 curcuminoids, dihydroxycurcumin and dihydroxytetrahydrocurcumin suppressed H_2O_2-induced cytotoxicity. On the contrary, neither curcumin nor tetrahydrocurcumin showed any suppressive effects. These results support out empirical rule that omicron-dihydroxy (catechol) moiety is essential for the inhibitory effects.(3) We established a simple method to quantify hydrogen peroxide (H_2O_2) generated during the aerobic heating process of (+) -catechin solution Using this method, we found that nonenzymatic H_2O_2 formation from catechin depended on pH,temperature, incubation time, and the presence of O_2 in the solution. The formation of oxidezed products of (+) -catechin, which was estimatied by measuring the absorbance of the solution at 430 nm, also depended on these factors. The H_2O_2 formation was inhibited by various kinds of superoxide dismutase (SOD) with almost the same dose dependency. Although the oxidation of (+) -catechin was enhanced by superoxide (O_2-), neither catalase nor H_2O_2 had any effects on the oxidation. These results suggest that O_2-, rather than H_2O_2, participated in the autoxidation of (+) -catechin, which was coupled with the H_2O_2 formation.