These tools will no longer be maintained as of December 31, 2024. Archived website can be found here. PubMed4Hh GitHub repository can be found here. Contact NLM Customer Service if you have questions.


Pubmed for Handhelds

PUBMED FOR HANDHELDS

Search MEDLINE/PubMed

  • Title: O-Methylation of tea polyphenols catalyzed by human placental cytosolic catechol-O-methyltransferase.
    Author: Zhu BT, Patel UK, Cai MX, Conney AH.
    Journal: Drug Metab Dispos; 2000 Sep; 28(9):1024-30. PubMed ID: 10950844.
    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.
    [Abstract] [Full Text] [Related] [New Search]