NEUROCHEMISTRYMelatonin synthesis arylalkylamine N-acetyltransferases in trout retina and pineal organ are differentBenyassi, Ahmed1,2; Schwartz, Christian1,3; Coon, Steven L.3; Klein, David C.3; Falcón, Jack1,3,4 Author Information 1Laboratoire de Neurobiologie Cellulaire et Neuroendocrinologie, CNRS UMR 6558, Université de Poitiers, 86022 Poitiers-Cedex, France; 2Faculté des Sciences de Fés, Laboratoire de Physiologie Animale, Fés, Morocco; 3Section on Neuroendocrinology, Laboratory of Developmental Neurobiology, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MD 20892-4480, USA 4Corresponding Author and address: Jack Falcón, Section on Neuroendocrinology, Laboratory of Developmental Neurobiology, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MD 20892-4480, USA Received 13 October 1999; accepted 7 november 1999 Acknowledgements: The study was supported by the CNRS, University of Poitiers, and Fondation Langlois (Rennes). We thank Françoise Chevalier, Gaël Epistolin and Nathalie Girard for their technical assistance. NeuroReport 11(2):p 255-258, February 7, 2000. Buy Abstract Serotonin N-acetyltransferase (AANAT) is the first enzyme in the conversion of serotonin to melatonin. Changes in AANAT activity determine the daily rhythm in melatonin secretion. Two AANAT genes have been identified in the pike, pAANAT-1 and pAANAT-2, expressed in the retina and in the pineal, respectively. The genes preferentially expressed in these tissues encode proteins with distinctly different kinetic characteristics. Like the pike, trout retina primarily expresses the AANAT-1 gene and trout pineal primarily expresses the AANAT-2 gene. Here we show that the kinetic characteristics of AANAT in these tissues differ as in pike. These differences include optimal temperature for activity (pineal: 12°C; retina: 25°C) and relative affinity for indoleethylamines compared to phenylethylamines. In addition, retinal AANAT exhibited substrate inhibition, which was not seen with pineal AANAT. The kinetic differences between AANAT-1 and AANAT-2 appear to be defining characteristics of these gene subfamilies, and are not species specific. © 2000 Lippincott Williams & Wilkins, Inc.