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Effects of epigallocatechin 3-O-gallate on cellular antioxidative system in HepG2 cells.

To elucidate the interactions of catechins with the cellular antioxidative system, human hepatoma HepG2 cells were incubated in a serum-free medium with catechins, and the level of thiobarbituric acid-reactive substances (TBARS) as a marker of lipid peroxidation was determined, as well as the contents of alpha-tocopherol (alpha-Toc) and glutathione (GSH) and the activities of superoxide dismutase (SOD) and glutathione peroxidase (GSH-Px). TBARS was promptly decreased by the incubation with epigallocatechin 3-O-gallate (EGCG), and 12h later TBARS in the cells with 10microM EGCG was about 15% (p < 0.05) of that in the controls (without catechins). Epigallocatechin, epicatechin 3-O-gallate, and epicatechin also had an antioxidative activity, but a higher concentration was required to induce the same effect as EGCG. In the cells incubated with EGCG, the consumption of alpha-Toc and the formation of the oxidized form of GSH were suppressed. Although EGCG showed no effects on the Cu/Zn-SOD activity, the Mn-SOD activity in the cells was enhanced (p < 0.05) by the incubation with EGCG. Moreover, the GSH-Px activity was maintained at a higher level (p < 0.05) in the cells with EGCG, compared with that in the controls. When the cells were preincubated with EGCG, the cytotoxicity of H2O2 was significantly reduced. Furthermore, the decrease of cellular alpha-Toc content induced by exposure to H2O2 was prevented by the pretreatment of EGCG. These results suggest that EGCG taken up into HepG2 cells is preferentially used as an antioxidant, rather than alpha-Toc and GSH, to suppress lipid peroxidation and to protect these cells from oxidative damages.