O-methylation of tea polyphenols catalyzed by human placental cytosolic catechol-O-methyltransferase

In the present study, we evaluated the metabolic O-methylation of several c atechol-containing tea polyphenols by human placental catechol-O-methyltran sferase (COMT). (-)-Epicatechin, (+)-epicatechin, and (-)-epigallocatechin were good substrates for metabolic O-methylation by placental cytosolic COM T (150-500 pmol/mg of protein/min), but (-)-epicatechin gallate and (-)-epi gallocatechin gallate were O-methylated at much lower rates (<50 pmol/mg of protein/min). When (-)-epicatechin was used as substrate, its O-methylatio n by human placental COMT showed dependence on incubation time, cytosolic p rotein concentration, incubation pH, and concentration of S-adenosyl-L-meth ionine (the methyl donor). Analysis of cytosolic COMT from six human term p lacentas showed that the O-methylation of increasing concentrations of (-)- epicatechin or (-)-epigallocatechin follows typical Michaelis-Menten kineti cs, with K-m and V-max values of 2.2 to 8.2 mu M and 132 to 495 pmol/mg of protein/min for (-)-epicatechin and 3.9 to 6.7 mu M and 152 to 310 pmol/mg of protein/min for (-)-epigallocatechin, respectively. Additional analysis revealed that COMT-catalyzed O-methylation of (-)-epicatechin and (-)-epiga llocatechin was strongly inhibited in a concentration-dependent manner by S -adenosyl-L-homocysteine (IC50 = 3.2-5.7 mu M), a demethylated product of S -adenosyl-L-methionine. This inhibition by S-adenosyl-L-homocysteine follow s a mixed (competitive plus noncompetitive) mechanism of enzyme inhibition. In summary, several catechol-containing tea polyphenols are rapidly O-meth ylated by human placental cytosolic COMT. This metabolic O-methylation is s ubject to strong inhibitory regulation by S-adenosyl-L-homocysteine, which is formed in large quantities during the O-methylation of tea polyphenols.