N-Acetylserotonin O-methyltransferase also known as ASMT is an enzyme that catalyzes the final reaction in melatonin biosynthesis, converting Normelatonin to melatonin. This reaction is embedded in the more general tryptophan metabolism pathway. The enzyme also catalyzes a second reaction in tryptophan metabolism: the conversion of 5-hydroxy-indoleacetate to 5-methoxy-indoleacetate.