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(−)-Epigallocatechin gallate (EGCG) and (−)-epigallocatechin (EGC) are the major polyphenolic constituents in green tea. In this study, we characterized the enzymology of cytosolic catechol-O-methyltransferase (COMT)-catalyzed methylation of EGCG and EGC in humans, mice, and rats. At 1 μM, EGCG was readily methylated by liver cytosolic COMT to 4"-O-methyl-EGCG and then to 4′,4"-di-O-methyl-EGCG; EGC was methylated to 4′-O-methyl-EGC. The K _m andV _max values for EGC methylation were higher than EGCG; for example, with human liver cytosol, theK _m were 4.0 versus 0.16 μM andV _max were 1.28 versus 0.16 nmol/mg/min. Rat liver cytosol had higher COMT activity than that of humans or mice. The small intestine had lower specific activity than the liver in the methylation of EGCG and EGC. Glucuronidation on the B-ring or thed-ring of EGCG greatly inhibited the methylation on the same ring, but …