Biochem Pharmacol. 2005 May 15;69(10):1523-31.
Chen D, Wang CY, Lambert JD, Ai N, Welsh WJ, Yang CS.
Department of Chemical Biology, Ernest Mario School of Pharmacy, Rutgers University, 164 Frelinghuysen Road, Piscataway, NJ 08854, USA.
(-)-Epigallocatechin-3-gallate (EGCG) is the major polyphenol present in green tea. We previously demonstrated that EGCG was both a substrate and potent inhibitor of human liver cytosolic catechol-O-methyltransferease (COMT). We now report the structure-activity relationship for the inhibition of COMT-catalyzed O-methylation of catecholestrogens in human liver cytosol by tea catechins and some of their metabolites.
The most potent inhibitors were catechins with a galloyl-type D-ring, including EGCG (IC(50)=0.07muM), 4′-O-methyl-EGCG (IC(50)=0.10muM), 4′,4′-di-O-methyl-EGCG (4′,4′-DiMeEGCG) (IC(50)=0.15muM), and (-)-epicatechin-3-gallate (ECG) (IC(50)=0.20muM).
Catechins without the D-ring showed two to three orders of magnitude less inhibitory potency. Enzyme kinetic analyses revealed that EGCG behaved as a mixed inhibitor, whereas 4′,4′-di-O-methyl-EGCG exhibited competitive kinetics for the S-adenosylmethionine (SAM), and noncompetitive kinetics for the catechol binding site.
These compounds may represent a new type of COMT inhibitor. In silico molecular-modeling studies using a homology model of human COMT were conducted to aid in the understanding the catalytic and inhibitory mechanisms. Either D-ring or B-ring of EGCG could be accommodated to the substrate binding pocket of human COMT.
However, the close proximity (2.6A) of 4′-OH to the critical residue Lys144, the higher acidity of the hydroxyl groups of the D-ring, and the hydrophobic interactions between the D-ring and residues in the binding pocket greatly facilitated the interaction of the D-ring with the enzyme, and resulted in increased inhibitory potency. These results provide mechanistic insight into the inhibition of COMT by commonly consumed tea catechins.
PMID: 15857617 [PubMed – in process]