The Melatonin reference article from the English Wikipedia on 24-Apr-2004
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Melatonin

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Melatonin, or 5-methoxy-N-acetyltryptamine, is a hormone produced by the pineal gland, located in the center of the brain. It is a derivative of the amino acid tryptophan.

Melatonin helps regulate sleep-wake or circadian rhythms. Normally, production of melatonin by the pineal is stimulated by darkness and inhibited by light.

In recent times, melatonin has become available as a drug and a supplement. It appears to have some use against insomnia and jet lag.

To synthesize melatonin, serotonin is converted to N-acetyl serotonin by the enzyme 5-HT N-acetyltransferase. N-acetyl serotonin is then converted to melatonin by the enzyme 5-hydroxyindole-O-methyltransferase. See serotonin for details on how tryptophan is converted to serotonin.

Commercial melatonin may be extracted from bovine pineal gland. There may be a risk of contracting mad cow disease. Ref