Characterization of serotonin N-acetyltransferase activity in the retina of the Mongolian gerbil, Meriones unguiculates.

In order to better understand the nature of melatonin synthesis in the mammalian retina, characterization of the enzyme serotonin N-acetyltransferase (NAT; EC 2.3.1.87) was undertaken in the Mongolian gerbil. Retinal and pineal homogenates were assayed for NAT activity by means of the radiometric method of Deguchi and Axelrod (1972). The activities of the retinal and pineal enzymes were similar with regard to pH optima and reaction rate. However, enzyme product formation by retinal NAT under varying substrate concentrations (including no substrate) remained virtually constant, hence preventing the determination of retinal NAT kinetics. In contrast, the pineal enzyme was determined to have a maximal velocity of 1 pmol/min and an apparent Michaelis constant of 100.7 microM, which agrees well with earlier reports in the rat. A daily rhythm of retinal NAT activity was observed with low levels during the daytime, early nighttime and very late nighttime hours. Peak levels were observed at 01.00 h in darkness. Acute light exposure reduced nocturnal NAT activity significantly within 10 min. This is the first demonstration of such an effect on retinal NAT activity in a mammalian species.