Hydroxytryptamine acetyltransferase

Serotonin is an indolamine neurotransmitter, derived from the amino acid L-tryptophan. Tryptophan is converted to 5-hydroxytryptophan (5-HTP) by tryptophan hydroxylase. 5-HTP is converted to 5-hydroxytryptamine (serotonin, 5-HT) by aromatic amino acid decarboxylase. In the pineal gland, 5-HT may be further converted to /V-acetyl serotonin by 5-HT /V-acetyltransferase and then to melatonin by 5-hyroxyindole-O-methyltransferase. 5-HT is catabolized by monoamine oxidase, and the primary end metabolite is 5-hydroxyindoleacetic acid (5-HIAA). 

N-Acetyltransferase uses acetyl-CoA to acetylate the amino moiety of arylal-kylamines. In mammalian pineal gland, this enzyme catalyzes the production of N-acetyl-5-hydroxytryptamine, which is the precursor of melatonin. It is also involved in the inactivation of monoaminergic neurotransmitters in insects. 

M is synthesized from serotonin by acetylation to A-acetyl rotonin, followed by methylation (catalysed by acetylserotonin methyltransferase, EC 2.1.1.4) to M.In the pineal gland M. biosynthesisis is controlled by epinephrine, which stimulates adenylate t clase (EC 4.6.1.1), which in turn stimulates protein kinase. The first three steps of M. synthesis (L-tryptophan S-hydroxytryptophan -> serotonin - A-acetyl-5-hydroxytryptamine) are promoted in this way, but the methyltransferase is not affected. However, the rate-limiting step is catalysed by serotonin A-acetyltransferase. M. is inactivated and excreted as 6-hydroxymelatonin, or as 5-methoxyindole-acetic acid. 

The basal ganglia also contain relatively large amounts of 5-hydroxytryptamine, substance P and acetylcholine. The caudate nucleus has a higher acetylcholine and choline acetyltransferase activity than has any... 

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