NORDIHYDROGUAIARETIC ACID (NDGA) IS BOTH A SUBSTRATE AND AN INHIBITOR OF CATECHOL-O-METYLTRANSFERASE (COMT)

Abstract

Nordihydroguaiaretic acid (NDGA) is the major lignan metabolite of Larrea nitida (LN), which belongs to the Zygophyllaceae and is widely used in South America to treat various diseases. The antioxidant and phytoestrogenic activity of NDGA has been reported in in vitro study. The present study investigated the novel biological activity of NDGA and its metabolite, 3-methyl NDGA to target catechol O-methyl transferase (COMT). COMT is one of metabolizing enzymes that catalyze the O-methylation of both endogenous and exogenous catechols such as dopamine, epinephrine, and 4-hydroxyl equilenin. We found that NDGA is both a substrate and an inhibitor of COMT. HPLC, LC-MS/MS, NMR analyses of the reaction mixtures of human recombinant COMT, NDGA, and S-adenosyl methionine (SAM) demonstrated formation of two methoxylated metabolites, 3-methyl NDGA (3-MeO-NDGA) and 4-methyl NDGA. Km value for the COMT-catalyzed conversion of NDGA was found to be 2.6 μM. In addition, NDGA inhibited the COMT-catalyzed methylation of dihydroxybenzoicacid (DHBA) with IC50 of 16.8 μM. The NDGA-mediated COMT inhibition included the product inhibition based on the concentration-activity curve, thus, we further investigated the inhibitory effect of 3-MeO-NDGA on COMT-catalyzed DHBA metabolism. 3-MeO-NDGA inhibited the DHBA metabolism in a reversible mode with a IC50 value of 31.9 μM. Our data suggest that NDGA is both a substrate for the COMT and an inhibitor of COMT activity and 3-MeO-NDGA is also a COMT inhibitor. Considering that the COMT inhibition plays a critical theraputical role of sustained metabolism of neurotransmitter such as dopamine in neurodegerative diseases, our finding is important to help develop the botanical therapeutics using NDGA or the extracts of Larrea nitida.