PARTIAL CHARACTERIZATION OF SEROTONIN N-ACETYLTRANSFERASES FROM NORTHERN PIKE (ESOX-LUCIUS, L) PINEAL ORGAN AND RETINA - EFFECTS OF TEMPERATURE

In vertebrates; the nocturnal rise in pineal organ and retinal melaton in synthesis results from the increase in the activity of the serotoni n N-acetyltransferase (NAT), a cAMP-dependent enzyme. In the Gs pineal organ in culture, light and temperature act in a similar manner on cA MP content and NAT activity, It is not known whether the effects of te mperature are mediated through cAMP or through modifications of NAT ki netics. The present study was designed: (1) to find out whether NAT ac tivity from pineal organ homogenates is similar to NAT activity from p ineal organs in culture, with regard to variations in temperature, and (2) to compare NAT activity from the pineal organ and the retina, Pin eal organ and retinal NAT activity increased linearly with protein con centrations, Higher activities were obtained with 0.2 mol/l of phospha te buffer, pH 6. Higher molarity or a higher PH induced a decrease in retinal and pineal organ NAT activity: retinal NAT was more sensitive than pineal organ NAT to changes in molarity, whereas the opposite hel d true as far as pH was concerned. Pineal organ and retinal NAT obeyed ce Michaelis-Menten equation with respect to increasing concentration s of acetyl-coenzyme A, With increasing concentrations of tryptamine: (1) pineal organ NAT activity increased in a manner suggesting positiv e co-operativity, (2) retinal NAT displayed, after an initial increase , inhibition by substrate The kinetics of the reactions were temperatu re dependent. Maximal activities were reached at 18/20 degrees C in th e pineal organ and al 37 degrees C in the retina. The present study is the first to describe the optimum conditions fur the assay of NAT act ivity in homogenates from the retina of fish and from the pineal organ of poikilotherms, and also the first to compare some characteristics of MAT activity from these two analogous organs;Our results suggest th at the effects of temperature on melatonin production are mediated, at least in part, through modifications of NAT kinetics. Future studies will aim to clarify whether the activities measured in the pineal orga n and retinal homogenates reflect the presence of one or of several en zymes.