Chemistry and Physics of Lipids 164 (2011) 688–695 Contents lists available at ScienceDirect Chemistry and Physics of Lipids jou rn al hom epa ge : www.elsevier.com/locate/chemphyslip Melatonin and structural analogues do not possess antioxidant properties on Fe 2+ -initiated peroxidation of sonicated liposomes made of retinal lipids Natalia Fagali 1 , Angel Catalá ∗,2 Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas, (INIFTA-CCT La Plata-CONICET), Facultad de Ciencias Exactas, Universidad Nacional de La Plata, Casilla de Correo 16, Sucursal 4, 1900 La Plata, Argentina a r t i c l e i n f o Article history: Received 6 May 2011 Received in revised form 14 July 2011 Accepted 15 July 2011 Available online 30 July 2011 Keywords: Lipid peroxidation Retina Liposomes Docosahexaenoic acid Melatonin Indoleamines a b s t r a c t Melatonin and its structural analogues display antioxidant activity in vivo but their activity in model membranes is not very well known. In this study, we have investigated the antioxidant capacity of mela- tonin and structural analogues on Fe 2+ -initiated peroxidation of sonicated liposomes made of retinal lipids. The indoleamines were evaluated against butylated hydroxitoluene (BHT) which was chosen as a reference standard because of its high antioxidant capacity. After the addition of Fe 2+ as initiator of lipid peroxidation, quick production of conjugated dienes was observed. With addition of increasing concentrations of BHT the start of the reaction was delayed and initial reaction rates were lower. How- ever, this reduction was not proportional to the increase in concentration. The start of the reaction and initial reaction rates were not modified in the presence of melatonin and its structural analogues. The formation of TBARS started immediately after the addition of Fe 2+ . The increase in the concentration of BHT avoided the emergence of TBARS. Changes were not observed in the presence of melatonin or structural analogues. Retinal lipids showed a high content of docosahexaenoic (22: 6 4, 7, 10, 13, 16, 19 ) acid, characteristic of this tissue. A little bit of that fatty acid was lost when sonicated liposomes were prepared with these retinal lipids. The polyunsaturated fatty acids (PUFAs) diminished significantly after incubation of liposomes with Fe 2+ during 1 h. BHT preserved PUFAs whereas melatonin and its related indoleamines did not. These data reinforce the hypothesis that melatonin and structural analogues do not possess antioxidant properties per se in this liposomal model system. © 2011 Elsevier Ireland Ltd. All rights reserved. 1. Introduction Reactive oxygen species are continuously formed during cel- lular metabolism and are removed by antioxidants defenses. The reactive oxygen species from endogenous and exogenous sources results in continuous and accumulative oxidative damage to cel- lular components and alters many cellular functions. The most vulnerable molecules to oxidative damage are proteins, lipids and DNA (Kohen and Nyska, 2002; Catalá, 2011a,b). Abbreviations: C18:2 n6, linoleic acid; C20:4 n6, arachidonic acid; C22:6 n3, docosahexaenoic acid; BHT, butylated hydroxitoluene; GC–MS, gas chromatography–mass spectrometry; LOOH, lipid hydroperoxide; MLT, melatonin; 5MTP, 5-methoxytryptamine; 5HOTRP, 5-OH-tryptophan; NAS, N-acetylserotonin; PUFAs, polyunsaturated fatty acids; TBARS, thiobarbituric reactive substances; ROS, reactive oxygen species; SL, sonicated liposomes. ∗ Corresponding author. Tel.: +54 221 425 7430/7291x105; fax: +54 221 425 4642. E-mail address: catala@inifta.unlp.edu.ar (A. Catalá). 1 Fellowship of Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Argentina. 2 Member of Carrera del Investigador Científico, Consejo Nacional de Investiga- ciones Científicas y Técnicas (CONICET), Argentina. In mammalian retina, free radicals and lipoperoxides seem to play important roles in the evolution of different retinopathies including glaucoma, cataractogenesis, diabetic retinopathy, ocular inflammation and retinal degeneration (Ueda et al., 1996; De La Paz and Anderson, 1992). The retina is the neurosensorial tissue of the eye, it is very rich in membranes and therefore, in polyunsatu- rated fatty acids (PUFAs). Because free radicals production induces the lipid peroxyl radical formation, known as secondary free radi- cals, this chain reaction of lipid peroxidation can damage the retina, especially the membranes that play important roles in visual func- tion (Catalá, 2006). Melatonin (N-acetyl-5-methoxytryptamine) is an indoleamine hormone synthesized from tryptophan in pinealocytes, and it acts as an endocrine hormone to influence various circadian activities in many target organs. Two pineal biochemical functions lipoxygena- tion and melatonin synthesis, appear to be synergistically regulated by the status of n-3 essential fatty acids (Catalá, 2010). Melatonin is also synthesized by retinal photoreceptors, but apparently does not normally enter the circulation, so it acts more likely as a paracrine signaling molecule within the eye. Hundred of studies have indicated that melatonin may act as a scavenger of reactive oxygen species (ROS) such as hydroxyl 0009-3084/$ – see front matter © 2011 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.chemphyslip.2011.07.004