A lignan O-methyltransferase catalyzing the regioselective methylation of matairesinol in Carthamus tinctorius

Abstract

Lignans are a group of plant phenolic compounds with various biological activities, including antitumor and antioxidant properties. O-Methylation is a critical step in biosynthesis of these compounds. However, little is known about the O-methyltransferase (OMT) enzymes that catalyze lignan O-methylation. We discovered a highly regioselective OMT activity in safflower (Carthamus tinctorius) seeds that catalyzed the methylation of matairesinol, a dibenzylbutyrolactone lignan, into 4′-O-methylmatairesinol (arctigenin) but not 4-O-methylmatairesinol (isoarctigenin). By examining such OMT activity in correlation with OMT transcript abundances during seed development, we cloned a few putative OMT cDNAs and produced their recombinant proteins in Escherichia coli. Among them, one protein exhibited O-methylation activity for matairesinol with the regioselectivity identical to that of the plant protein, and was named C. tinctorius matairesinol OMT (CtMROMT). CtMROMT did not show any detectable OMT activities towards phenylpropanoid monomers under the reaction conditions tested, while it methylated flavonoid apigenin efficiently into 4′-O-methylapigenin (acacetin). However, quantitative real-time polymerase chain reaction analysis demonstrated that expression of the CtMROMT gene was synchronized with the CtMROMT activity profile and arctigenin accumulation in the plant. These results demonstrated that CtMROMT is a novel plant OMT for lignan methylation.