Antioxidant and pro-oxidant effects of green tea extracts in oxygen radical absorbance capacity assay

Green tea extracts (GTEs) [water (GTE-W) and 75% ethanol (GTE-E)] were investigated to characterize their propensities to act as antioxidants or as pro-oxidants by analyzing oxygen radical absorbance capacity (ORAC) and scavenging capacity for hydroxyl radical. When 2,2'- azobis(2-amidinopropane) dihydrochloride was used for the generation of peroxyl radicals, both GTE-W and GTE-E exhibited strong concentration-dependent scavenging activity through donating protons, which could be explained by their reducing property. When hydroxyl radicals were generated through the addition of Cu2+ and H2O2, GTE-W and GTE-E exhibited antioxidant activity or pro-oxidant activity, depending on their concentrations, which might be attributed to the metal chelating activity, the scavenging activity on hydroxyl radical, and/or the pro-oxidant activity to generate some reactive oxygen species. When Cu2+ without H2O2 was used as an oxidant in the assay, the copper-initiated pro-oxidant activities of GTE-W and GTE-W was consistent with the availability of (-)-epigallocatechin and (-)-epicatechin to generate hydrogen peroxide and/or hydroxyl radical. The pro-oxidant activity of GTE-W and GTE-E was demonstrated by the deoxyribose assay. These results indicate that both GTE-W and GTE-E can have pro-oxidant activity at lower concentrations and antioxidant activity at higher concentrations in the ORAC and deoxyribose assays using generated hydroxyl radicals.