Methyltetrahydrofolic acid mediates N- and O-methylation of biogenic amines.

A variety of mammalian and avian tissues N- and O-methylate indoleamines and phenylethylamines, with methyltetrahydrofolic acid as the methyl donor. Because it is considerably more efficient than S-adenosylmethionine, methyltetrahydrofolic acid may be the natural methyl donor in this reaction. With methyltetrahydrofolic acid, serotonin is O-methylated to 5-methoxytryptamine, a novel indoleamine in mammalian brain.