Effects of epigallocatechin gallate and quercetin on oxidative damage to cellular DNA

Phenolic phytochemicals are thought to promote optimal health, partly via t heir antioxidant effects in protecting cellular components against free rad icals. The aims of this study were to assess the free radical-scavenging ac tivities of several common phenolic phytochemicals, and then, the effects o f the most potent phenolic phytochemicals on oxidative damage to DNA in cul tured cells. Epigallocatechin gallate (EGCG) scavenged the stable free radi cal, alpha,alpha-diphenyl-beta-picrylhydrazyl (DPPM), most effectively, whi le quercetin was about half as effective. Genistein, daidzein, hesperetin, and naringenin did not scavenge DPPH appreciably. Jurkat T-lymphocytes that were pre-incubated with relatively low concentrations of either EGCG or qu ercetin were less susceptible to DNA damage induced by either a reactive ox ygen species or a reactive nitrogen species, as evaluated by the comet assa y. More specifically, control cells had a comet score of only 17 +/- 5, ind icating minimal DNA damage. Cells challenged with 25 mu M hydrogen peroxide (H2O2) or 100 mu M 3-morpholinosydnonimine (SIN-1, a peroxynitrite generat or) had comet scores of 188 +/- 6 and 125 +/- 12, respectively, indicating extensive DNA damage. The H2O2-induced DNA damage was inhibited with 10 mu M of either EGCG (comet score: 113 +/- 23) or quercetin (comet score: 82 +/ - 7). Similarly, the SIN-1-mediated DNA damage was inhibited with 10 mu M o f either EGCG (comet score: 79 +/- 13) or quercetin (comet score: 72 +/- 17 ). In contrast, noticeable DNA damage was induced in Jurkat T-lymphocytes b y incubating with 10-fold higher concentrations (i.e., 100 mu M) of either ECCG (comet score: 56 +/- 17) or quercetin (comet score: 64 +/- 13) by them selves. Collectively, these data suggest that low concentrations of EGCG an d quercetin scavenged free radicals, thereby inhibiting oxidative damage to cellular DNA. But, high concentrations of either EGCG or quercetin alone i nduced cellular DNA damage. (C) 2000 Published by Elsevier Science B.V. All rights reserved.