An enzyme that catalyzes the transfer of a methyl group from S-adenosylmethionine to N-acetylserotonin to form N-acetyl-5-methoxytryptamine (MELATONIN).

Synonyms: Hydroxyindole O-Methyltransferase, S-Adenosyl-L-methionine:N-acetylserotonin O-methyltransferase, EC 2.1.1.4, HIOMT, Hydroxyindole O Methyltransferase, Acetylserotonin Methyltransferase, O-Methyltransferase, Hydroxyindole, Oxyindol O Methyltransferase, Methyltransferase, Acetylserotonin, Acetylserotonin O Methyltransferase, Oxyindol-O-Methyltransferase, O-Methyltransferase, Acetylserotonin, Acetylserotonin O-Methyltransferase