Detection of melatonin and serotonin N-acetyltransferase and hydroxyindole-O-methyltransferase activities in rat ovary.

Melatonin (N-acetyl-5-methoxytryptamine) and the activities of two melatonin-synthesizing enzymes, serotonin N-acetyltransferase (acetyl coenzyme A: arylalkylamine N-acetyltransferase EC 2.3.1.87; NAT) and hydroxyindole-O-methyltransferase (S-adenosyl-L-methionine: N-acetylserotonin-O-methyltransferase EC 2.1.1.4; HIOMT), were assayed in extracts of ovaries obtained from virgin Wistar-derived rats (7-9 week-old) during the light period of a 12 h light/12 h dark cycle. Melatonin was detected in the rat ovary using reverse-phase high-performance liquid chromatography (HPLC) coupled with fluorometric detection and radioimmunoassay (RIA). In addition, NAT and HIOMT activities were found in rat ovary. The apparent Michaelis constants (Km) for the substrates of NAT and HIOMT in the rat ovary were similar to those reported for the pineal gland and retina. These data suggest that the rat ovary, like the pineal gland and the retina, may synthesize melatonin from serotonin by the sequential action of NAT and HIOMT.