Within the pinealocyte, tryptophan, an amino acid taken up from the blood, is metabolized to a variety of potential hormones (Fig. 10-2).2 The compound that has been most thoroughly investigated is N-acetyl-5-methoxytryptamine, or melatonin. Tryptophan is initially metabolized to serotonin (5-hydroxytryptamine); the concentration of this monoamine in the pineal gland exceeds that in any other organ. Especially
during the daily dark period, serotonin is N-acetylated by the enzyme serotonin N-acetyltransferase to form N-acetylserotonin.3 The activity of N-acetyltransferase increases many-fold during darkness (Fig. 10-3). N-acetylserotonin is converted to melatonin by the action of hydroxyindole-O-methyltransferase. Once formed, melatonin is rapidly secreted into the blood vascular system and, as a consequence, plasma melatonin levels typically are highest at night, when pineal melatonin production is greatest.