Abstract

In the present study, we evaluated the metabolicO-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 ofS-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 andV_max values of 2.2 to 8.2 μM and 132 to 495 pmol/mg of protein/min for (−)-epicatechin and 3.9 to 6.7 μM and 152 to 310 pmol/mg of protein/min for (−)-epigallocatechin, respectively. Additional analysis revealed that COMT-catalyzedO-methylation of (−)-epicatechin and (−)-epigallocatechin was strongly inhibited in a concentration-dependent manner byS-adenosyl-l-homocysteine (IC₅₀ = 3.2–5.7 μM), a demethylated product ofS-adenosyl-l-methionine. This inhibition byS-adenosyl-l-homocysteine follows a mixed (competitive plus noncompetitive) mechanism of enzyme inhibition. In summary, several catechol-containing tea polyphenols are rapidlyO-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 theO-methylation of tea polyphenols.