Detection of melatonin, its precursors and related enzyme activities in rabbit lens

Melatonin (N -acetyl-5-methoxytryptamine) was detected in extracts of albino rabbit lens using radioimmunoassay. Furthermore, melatonin precursors, tryptophan and serotonin (5-hydroxytryptamine), were found in the lens extracts by high-performance liquid chromatography coupled with fluorometric detection. Also, activities of two melatonin-synthesizing enzymes, serotonin N -acetyltransferase (NAT) and hydroxyindole- O -methyltransferase (HIOMT), were found in the lens. The apparent Michaelis constants (K m) for substrates of NAT in the lens were similar to those reported for the pineal gland, although the apparent K m values for substrates of HIOMT in the lens were 10-fold higher than those in the pineal gland. When the rabbits were entrained to a 14-hr light: 10-hr dark cycle, melatonin levels and NAT activity in the lens showed significant day/night changes with high levels during the dark period, but HIOMT activity did not show these changes. These findings strongly suggest that the rabbit lens may synthesize melatonin from serotonin by the sequential action of NAT and HIOMT, and that the melatonin synthesis may fluctuate in a diurnal and/or circadian manner.