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