Scavenging of hydrogen peroxide and inhibition of ultraviolet light-induced oxidative DNA damage by aqueous extracts from green and black teas

Abstract

Aqueous extracts of green and black teas have been shown to inhibit a variety of experimentally induced animal tumors, particularly ultraviolet (UV) B light-induced skin carcinogenesis. In the present study, we compared the effects of different extractable fractions of green and black teas on scavenging hydrogen peroxide (H 2O 2), and UV irradiation-induced formation of 8-hydroxy 2′-deoxyguanosine (8-OHdG) in vitro. Green and black teas have been extracted by serial chloroform, ethyl acetate and n-butanol, and divided into four subfractions designated as GT1-4 for green tea and BT1-4 for black tea, respectively. The total extracts from green and black teas exhibited a potent scavenging capacity of exogenous H 2O 2 in a dose-dependent manner. It appeared that the total extracts from black tea scavenged H 2O 2 more potently than those from green tea. When tested individually, the potency of scavenging H 2O 2 by green tea subfractions was: GT2 > GT3 > GT1 > GT4, whereas the order of efficacy for black tea was: BT2 > BT3 > BT4 > BT1. In addition, we demonstrated that total fractions of green and black teas substantially inhibited the induction of 8-OHdG in calf thymus by all three portions of UV spectrum (UVA, B and C). Consistent with the capacity of scavenging H 2O 2, the subfractions from black tea showed a greater inhibition of UV-induced 8-OHdG than those from green tea. At low concentrations, the order of potency of quenching of 8-OHdG by green tea subfractions was: GT2 > GT3 > GT4 > GT1 and the efficacy of all subfractions became similar at high concentrations. All subfractions of the black tea except BT1 strongly inhibited UV-induced 8-OHdG and the order of potency was: BT2 > BT3 > BT4 > BT1. Addition of (−)-epigallocatechin gallate (EGCG), an ingredient of green tea extract, to low concentration of green and black tea extracts substantially enhanced the scavenging of H 2O 2 and quenching of 8-OHdG, suggesting the important role of EGCG in the antioxidant activities of tea extracts. The potent scavenging of oxygen species and blocking of UV-induced oxidative DNA damage may, at least in part, explain the mechanism(s) by which green/black teas inhibit photocarcinogenesis.