(-)-Epigallocatechin gallate (EGCG), the most abundant and most biologically active compound in tea, has been proposed to have many beneficial health effects. The metabolic fate of EGCG, however, is not well understood. In the present study, we found that EGCG can be oxidized by peroxidase and hydrogen peroxide and then reacted with cysteine or glutathione to form conjugates. The structures of the cysteine and glutathione conjugates of EGCG were identified using 2D NMR and MS. Two thiol conjugates of EGCG (2′-cysteinyl EGCG and 2″-cysteinyl EGCG) were identified by ESI-LC-MS/MS analysis from the urine samples of mice administered 200 or 400 mg/kg EGCG, i.p. These conjugates were not found in urine samples of mice after receiving EGCG at 50 mg/kg i.p., or 2000 mg/kg i.g., or in human urine following consumption of 3 g of decaffeinated green tea solids (containing 333 mg EGCG). At high doses, EGCG is believed to be oxidized to form EGCG quirione, which can react with glutathione to form the thiol conjugates. These results suggest that detectable amounts of thiol conjugates of EGCG are formed only after rather high doses of EGCG are given to the mice.
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