Melatonin, its precursors, and synthesizing enzyme activities in the human ovary

The presence of melatonin (N-acetyl-5-methoxytryptamine) and its precursors, serotonin (5-hydroxytryptamine) and N-acetylserotonin, was demonstrated in extracts of human ovary using reverse-phase high-performance liquid chromatography coupled with fluorometric detection. In addition, activities of two melatonin-synthesizing enzymes, arylalkylamine N-acetyltransferase (NAT) and hydroxyindole-O-methyltransferase (HIOMT), were found in human ovary homogenates. The apparent Michaelis constants for the substrates of NAT and HIOMT in the human ovary were similar to those reported for the pineal glands of humans and other mammals. These findings strongly suggest that the human ovary, like the pineal gland, may synthesize melatonin from serotonin by the sequential action of NAT and HIOMT.