title: O-Methylation of tea polyphenols catalyzed by human placental cytosolic catechol-O-methyltransferase.
abstract: In the present study, we evaluated the metabolic O-methylation of several catechol-containing tea polyphenols by human placental catechol-O-methyltransferase (COMT). (-)-Epicatechin, (+)-epicatechin, and (-)-epigallocatechin were good substrates for metabolic O-methylation by placental cytosolic COMT (150-500 pmol/mg of protein/min), but (-)-epicatechin gallate and (-)-epigallocatechin gallate were O-methylated at much lower rates (<50 pmol/mg of protein/min). When (-)-epicatechin was used as substrate, its O-methylation by human placental COMT showed dependence on incubation time, cytosolic protein concentration, incubation pH, and concentration of S-adenosyl-L-methionine (the methyl donor). Analysis of cytosolic COMT from six human term placentas showed that the O-methylation of increasing concentrations of (-)-epicatechin or (-)-epigallocatechin follows typical Michaelis-Menten kinetics, with K(m) and V(max) values of 2.2 to 8.2 microM and 132 to 495 pmol/mg of protein/min for (-)-epicatechin and 3.9 to 6.7 microM and 152 to 310 pmol/mg of protein/min for (-)-epigallocatechin, respectively. Additional analysis revealed that COMT-catalyzed O-methylation of (-)-epicatechin and (-)-epigallocatechin was strongly inhibited in a concentration-dependent manner by S-adenosyl-L-homocysteine (IC(50) = 3.2-5.7 microM), a demethylated product of S-adenosyl-L-methionine. This inhibition by S-adenosyl-L-homocysteine follows a mixed (competitive plus noncompetitive) mechanism of enzyme inhibition. In summary, several catechol-containing tea polyphenols are rapidly O-methylated by human placental cytosolic COMT. This metabolic O-methylation is subject to strong inhibitory regulation by S-adenosyl-L-homocysteine, which is formed in large quantities during the O-methylation of tea polyphenols.
authors.affiliation: Department of Basic Pharmaceutical Sciences, College of Pharmacy, University of South Carolina, Columbia 29208, USA. btzhu@pharm.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 H
authors.initials: AH
authors.lastname: Conney
mesh_terms: D002392:Catechin / Q000378:metabolism; D002394:Catechol O-Methyltransferase / Q000378:metabolism*; D002851:Chromatography, High Pressure Liquid; D003600:Cytosol / Q000201:enzymology; D004305:Dose-Response Relationship, Drug; D005260:Female; D005419:Flavonoids; D006801:Humans; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D013058:Mass Spectrometry; D008745:Methylation / Q000187:drug effects; D010636:Phenols / Q000378:metabolism*; D010920:Placenta / Q000201:enzymology; D011108:Polymers / Q000378:metabolism*; D011247:Pregnancy; D012435:S-Adenosylhomocysteine / Q000494:pharmacology; D013662:Tea / Q000737:chemistry
chemical_list: D005419:Flavonoids; D010636:Phenols; D011108:Polymers; D013662:Tea; D002392:Catechin; D012435:S-Adenosylhomocysteine; D002394:Catechol O-Methyltransferase