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Vol. 31, Issue 5, 572-579, May 2003
)-Epigallocatechin
Gallate
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.
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