Ž . European Journal of Pharmacology 428 2001 169–175 www.elsevier.comrlocaterejphar N-acetylserotonin suppresses hepatic microsomal membrane rigidity associated with lipid peroxidation Joaquın J. Garcıa a,b , Russel J. Reiter a, ) , Malgorzata Karbownik a , Juan R. Calvo a , ´ ´ Genaro G. Ortiz a , Dun-Xian Tan a , Enrique Martınez-Balların b , Darıo Acuna-Castroviejo a,c ´ ´ ´ ˜ a Department of Cellular and Structural Biology, Mail code 7762, UniÕersity of Texas Health Science Center, San Antonio, 7703 Floyd Curl DriÕe, San Antonio TX, 78229-3900 USA b Department of Pharmacology and Physiology, UniÕersity of Zaragoza, Spain c Department of Physiology, UniÕersity of Granada, Spain Received 10 May 2001; received in revised form 13 August 2001; accepted 17 August 2001 Abstract N-acetylserotonin, the immediate precursor of melatonin in the tryptophan metabolic pathway in the pineal gland, has been reported to be an antioxidant. The aim of this work was to test the effect of N-acetylserotonin in stabilizing biological membranes against oxidative Ž . stress. Hepatic microsomal membranes from male adult rats were incubated with N-acetylserotonin 0.001–3 mM before inducing lipid peroxidation using FeCl , ADP and NADPH. Control experiments were done by incubating microsomal membranes with N-acetylserotonin 3 in the absence of lipid peroxidation-inducing drugs. Membrane fluidity was assessed by fluorescence spectroscopy and malonaldehyde plus 4-hydroxyalkenals concentrations were measured to estimate the degree of lipid peroxidation. Free radicals induced by the combination of FeCl qADP qNADPH produced a significant decrease in the microsomal membrane fluidity, which was associated 3 with an increase in the malonaldehyde plus 4-hydroxyalkenals levels. These changes were suppressed in a concentration-dependent Ž manner when N-acetylserotonin was added in the incubation buffer. In the absence of lipid peroxidation, N-acetylserotonin 0.001–3 . mM did not change membrane fluidity nor malonaldehyde plus 4-hydroxyalkenals levels. These results suggest that the protective role of N-acetylserotonin in preserving optimal levels of fluidity of the biological membranes may be related to its ability to reduce lipid peroxidation. q 2001 Elsevier Science B.V. All rights reserved. Keywords: N-acetylserotonin; Lipid peroxidation; Membrane fluidity; Microsome 1. Introduction Ž . Since the demonstration by Tan et al. 1993 of the antioxidant ability of melatonin, several reports claim that other pineal products may also exhibit antioxidant effects Ž . Poeggeler et al., 1996; Pless et al., 1999; Qi et al., 2000 . N-acetylserotonin is an indoleamine isolated from the pineal gland as well as from several extrapineal tissues such as Ž . the retina, digestive tract and ovary Itoh et al., 1997 . The concentration of N-acetylserotonin in mammalian plasma Ž . is in the nanomolar range Younglai et al., 1986 . N-acetylserotonin is the immediate precursor of mela- tonin in the metabolism of tryptophan in the pineal gland. Chemically, N-acetylserotonin only differs from melatonin in the substitution of a hydroxy group for the methoxy ) Corresponding author. Tel.: q 1-210-567-3860; fax: q 1-210-567- 6948. Ž . E-mail address: Reiter@uthscsa.edu R.J. Reiter . Ž . group in position 5 of the indole ring Fig. 1 . Both indoleamines show light-controlled daily rhythms in their synthesis with peak production occurring at night and a nadir during the photophase. These rhythms are a conse- quence of the increased activity of serotonin N-acetyltrans- Ž . Ž ferase E.C. 2.3.1.5 at night Pang et al., 1984; Ho et al., 1985; Namboodiri et al., 1985; Mıguez et al., 1996; ´ . Viswanathan et al., 1998 . As occurs with melatonin, the Ž pineal content of N-acetylserotonin declines with age Pang . et al., 1984; Mıguez et al., 1998 and both indoleamines ´ have protective effects against toxin-induced damage in Ž several organs Bachurin et al., 1999; Lopez-Gonzalez et ´ ´ al., 2000; Ohta et al., 2000; Montilla et al., 2000; Martın et ´ . al., 2000; Calvo et al., 2001 . The lipid dynamic of the biological membranes modu- lates essential cell functions including cell growth, solute transport, signal transduction and membrane-associated en- Ž zyme activities Levi et al., 1990; Sunshine and McNamee, 1998; Boyan et al., 1999; Prasad et al., 1999; Emmerson et 0014-2999r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. Ž . PII: S0014-2999 01 01342-5