title: Rapid conversion of tea catechins to monomethylated products by rat liver cytosolic catechol-O-methyltransferase.
abstract: 1. The metabolic O-methylation of several catechol-containing tea polyphenols by rat liver cytosolic catechol-O-methyltransferase (COMT) has been studied. 2. When (-)-epicatechin was used as substrate, its O-merthylation showed dependence on incubation time, cytosolic protein concentration, incubation pH and concentration of S-adenosyl-L-methionine. The O-methylation of increasing concentrations of (-)-epicatechin followed typical Michaelis-Menten kinetics, and the apparent Km and Vmax were 51 microM and 2882 pmol mg protein(-1) min(-1), respectively, at pH 7.4, and were 17 microM and 2093 pmol mg protein(-1) min(-1), respectively, at pH 10.0. 3. Under optimized conditions for in vitro O-methylation, (-)-epicatechin, (+)-epicatechin and (-)-epigallocatechin were rapidly O-methylated by rat liver cytosol. In comparison, (-)-epicatechin gallate and (-)-epigallocatechin gallate vere O-methylated at significantly lower rates under the same reaction conditions. catalysed O-methylation of (-)-epicatechin and (-)-epigallocatechin was inhibited in a concentration-dependent manner by S-adenosyl-L-homocysteine, a demethylated product of S-adenosyl-L-methionine. The IC50 was approximately 10 microM. 5. In summary, the results showed that several catechol-containing tea polyphenols were rapidly O-methylated by rat liver cytosolic COMT. These observations raise the possibility that some of the biological effects of tea polyphenols may be exerted by their O-methylated products or may result from their potential inhibition of the COMT-catalysed O-methylation of endogenous catecholamines and catechol oestrogens.
authors.affiliation: Department of Basic Pharmaceutical Sciences, College of Pharmacy, University of South Carolina, Columbia 29208, USA. BTZhu@cop.sc.edu
authors.forename: B T
authors.initials: BT
authors.lastname: Zhu
authors.forename: U K
authors.initials: UK
authors.lastname: Patel
authors.forename: M X
authors.initials: MX
authors.lastname: Cai
authors.forename: A J
authors.initials: AJ
authors.lastname: Lee
authors.forename: A H
authors.initials: AH
authors.lastname: Conney
mesh_terms: D000818:Animals; D002392:Catechin / Q000031:analogs & derivatives* / Q000378:metabolism*; D002394:Catechol O-Methyltransferase / Q000187:drug effects / Q000378:metabolism*; D003600:Cytosol / Q000187:drug effects / Q000378:metabolism*; D005260:Female; D005419:Flavonoids; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008099:Liver / Q000187:drug effects / Q000378:metabolism*; D013058:Mass Spectrometry / Q000379:methods; D008745:Methylation; D010636:Phenols / Q000378:metabolism; D011108:Polymers / Q000378:metabolism; D051381:Rats; D017207:Rats, Sprague-Dawley; D012435:S-Adenosylhomocysteine / Q000494:pharmacology; D013662:Tea / Q000737:chemistry*
chemical_list: D005419:Flavonoids; D010636:Phenols; D011108:Polymers; D013662:Tea; D002392:Catechin; D012435:S-Adenosylhomocysteine; C045651:epigallocatechin gallate; D002394:Catechol O-Methyltransferase; C057580:gallocatechol