Predominance of 2-hydroxymelatonin over melatonin in plants

The cloning of the gene encoding melatonin 2-hydroxylase (M2H), which is responsible for the synthesis of 2-hydroxymelatonin, has expanded the study of melatonin metabolism in plants. Kinetic analysis of M2H enzymatic activity demonstrated that the catalytic efficiency of M2H is much higher than those of other melatonin biosynthetic enzymes such as serotonin N-acetyltransferase (SNAT) and N-acetylserotonin O-methyltransferase (ASMT), suggesting that melatonin metabolism is rapid in plants. To test this prediction, we selected 24 plant species belonging to 16 families and quantified the levels of melatonin and 2-hydroxymelatonin using liquid chromatography-tandem mass spectrometry (LC-MS/MS). The melatonin levels in most of the species were <1 ng/g fresh weight (FW), while those in leaves from radish and feverfew were 3.5 and 3.3 ng/g FW, respectively. In contrast, the average levels of 2-hydroxymelatonin were much higher at 6.2 ng/g FW. The average ratio of 2-hydroxymelatonin to melatonin in plants was approximately 368:1, indicating that the accumulation of 2-hydroxymelatonin predominates over that of melatonin. These data were consistent with previous results on the kinetics of the corresponding enzymes, as well as with in vivo melatonin conversion data. Among several melatonin metabolites in plants, the most abundant metabolite was found to be 2-hydroxymelatonin (99%) followed by 4-hydroxymelatonin (0.05%), but 6-hydroxymelatonin was not detected in rice seedlings.