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(−)-Epigallocatechin gallate (EGCG) and (−)-epigallocatechin (EGC) are major green tea catechins with antioxidant and anticancer activities. In this study, we characterized the glucuronidation of EGCG and EGC in human, mouse, and rat microsomes and by nine different human UGT 1A and 2B isozymes expressed in insect cells. Six EGCG and EGC glucuronides were biosynthesized, and their structures were identified for the first time. (−)-EGCG-4"-O-glucuronide was the major EGCG glucuronide formed in all incubations. The catalytic efficiency (V _max/K _m) for (−)-EGCG-4"-O-glucuronide formation followed the order: mouse intestine > mouse liver > human liver > rat liver ≫ rat small intestine. The UGT-catalyzed glucuronidation of EGC was much lower than that of EGCG. TheV _max/K _m for (−)-EGC-3′-O-glucuronide followed the following order: mouse liver > human liver > rat liver > rat and mouse small intestine …