Molecular mechanisms of beneficial effects of lipoic acid in copper intoxicated rats assessment by FTIR and ESI-MS Ruz ˇica S. Nikolic ´ a,⇑ , Nenad S. Krstic ´ a , Goran M. Nikolic ´ b , Gordana M. Kocic ´ c , Milorad D. Cakic ´ d , Darko H. An - delkovic ´ a a University of Niš, Faculty of Science and Mathematics, Department of Chemistry, Višegradska 33, 18000 Niš, Serbia b University of Niš, Faculty of Medicine, Department of Chemistry, Bulevar dr Zorana Ðin - dic ´a 81, 18000 Niš, Serbia c University of Niš, Faculty of Medicine, Department of Biochemistry, Bulevar dr Zorana Ðin - dic ´a 81, 18000 Niš, Serbia d University of Niš, Faculty of Technology, Bulevar oslobo - denja 124, 16000 Leskovac, Serbia article info Article history: Received 31 December 2013 Accepted 15 April 2014 Available online xxxx Keywords: Lipoic acid Copper toxicity Malondialdehyde FTIR ESI-MS abstract The effect of lipoic acid (LA) supplementation to Cu(II) ion intoxicated rats was investigated by measuring the level of malondialdehyde (MDA), a common marker for oxidative stress, in liver and kidney tissues. Significant decrease of MDA was achieved by LA administration to rats one day after Cu(II) ion intoxica- tion thus proving that LA can have beneficial effects in the case of heavy metals intoxication. FTIR spec- troscopy and ESI-MS were used to investigate molecular mechanisms of Cu(II) ion–ligand interactions in model systems containing LA or its reduced form DHLA at milimolar and micromolar levels, respectively. FTIR spectra revealed that LA behaves as a monodentate ligand while DHLA behaves as a bidentate ligand in interaction with Cu(II) ion. ESI-MS revealed that there was direct interaction of LA with Cu(II) ions even at the micromolar level. Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction Copper is a biogenic element found in numerous enzymic sys- tems [1–3] and also has an important role in suppressing inflam- matory processes [4]. Human body contains about 80–120 mg of copper under normal physiological conditions, and normal concen- tration range in blood plasma is 70–150 lg/L [5]. Changes of cop- per level outside the normal ranges is connected with many diseases like aceruloplasminemia, Wilson’s disease, the Menkes disease, Alzheimer’s disease, and others [3]. Increased intake of copper may cause its accumulation in some tissues and organs and chronic intoxication may cause disorder of some metabolic pathways, reduction of free radical species and even apoptosis [4]. It is well known that Cu(II) ion (d 9 configuration) can give coor- dination compounds of various coordination and geometry through O, N, and S-donor atoms of different biomolecules and small molecules [2]. This fact, together with its redox activity, is responsible for various roles copper can play in living organisms and possible interactions with many antioxidant nutrients, includ- ing lipoic acid, are of special importance [5,6]. a-Lipoic acid ((R)-5-(1,2-dithiolan-3-yl)pentanoic acid) is an organic compound occurring naturally in small quantities in plants and animals as well as in humans [7]. Its content in human serum is about 16 mg/L [8]. Lipoic acid contains two sulfhydryl groups which may be in either oxidized or reduced state (Fig. 1). Reduced form is called dihydrolipoic acid (DHLA) while oxidized form is usually referred as lipoic acid (LA) [9]. LA can be found in various foods covalently bound to lysine in proteins (lipoyllysine). Although it was found in foodstuffs of both plant and animal origin, quantitative data about the content of LA or lipoyllysine are limited. Animal organs rich in lipoyllysine (1– 3 lg/g) are kidneys, heart, and liver. Plant species containing lip- oyllysine are usually the ones rich in chlorophyll, like spinach and broccoli [10]. LA taken by food is activated with ATP or GTP by lipoate-activating enzyme and then transferred to LA-depen- dent enzymes by lipoyl transferase [11]. Oral administration of high doses of LA (50 mg and more) causes significant, but tempo- rary, increase of free LA in plasma and cells. Pharmacokinetic stud- ies in humans showed that about 30–40% of orally administrated LA was absorbed [12]. LA is rapidly reduced to DHLA in cells and in vitro studies showed its quick elimination [13]. Endogenous synthesized LA is covalently bound to specific pro- teins acting as cofactors in some important enzymic complexes. Beside its physiological role in protein-bound form there is http://dx.doi.org/10.1016/j.poly.2014.04.033 0277-5387/Ó 2014 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. Tel.: +381 637618300. E-mail address: ruzicanf@yahoo.com (R.S. Nikolic ´). Polyhedron xxx (2014) xxx–xxx Contents lists available at ScienceDirect Polyhedron journal homepage: www.elsevier.com/locate/poly Please cite this article in press as: R.S. Nikolic ´ et al., Polyhedron (2014), http://dx.doi.org/10.1016/j.poly.2014.04.033