Enzymology of Methylation of Tea Catechins and Inhibition of Catechol-O-methyltransferase by (
)-Epigallocatechin Gallate
Hong Lu, Xiaofeng Meng, and Chung S. Yang
Department of Chemical Biology, Ernest Mario School of Pharmacy, Rutgers, the State University of New Jersey, Piscataway, New Jersey
(
)-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 Km and Vmax values for EGC methylation were higher than EGCG; for example, with human liver cytosol, the Km were 4.0 versus 0.16 µM and Vmax 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 the D-ring of EGCG greatly inhibited the methylation on the same ring, but glucuronidation on the A-ring of EGCG or EGC did not affect their methylation. Using EGC and 3,4-dihydroxy-L-phenylalanine as substrates, EGCG, 4"-O-methyl-EGCG, and 4',4"-di-O-methyl-EGCG were all potent inhibitors (IC50 ~0.2 µM) of COMT. The COMT-inhibiting activity of ()-EGCG-3'-O-glucuronide was similar to EGCG, but ()-EGCG-4"-O-glucuronide was less potent. The present work provides basic information on the methylation of EGCG and suggests that EGCG may inhibit COMT-catalyzed methylation of endogenous and exogenous compounds.
Drug Metabolism and Disposition, Vol. 31, Issue 5, 572-579, May 2003
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