On the antioxidant properties of kynurenic acid: Free radical scavenging activity and inhibition of oxidative stress R. Lugo-Huitrón a , T. Blanco-Ayala a , P. Ugalde-Muñiz a , P. Carrillo-Mora a , J. Pedraza-Chaverrí b , D. Silva-Adaya a , P.D. Maldonado c , I. Torres d , E. Pinzón d , E. Ortiz-Islas e , T. López e , E. García f , B. Pineda f , M. Torres-Ramos a , A. Santamaría a , V. Pérez-De La Cruz a, ⁎ a Laboratorio de Aminoácidos Excitadores, Instituto Nacional de Neurología y Neurocirugía Manuel Velasco Suárez, S.S.A., Mexico D.F. 14269, Mexico b Departamento de Biología, Facultad de Química, Universidad Nacional Autónoma de México, Mexico D.F. 04510, Mexico c Laboratorio de Patología Vascular Cerebral, Instituto Nacional de Neurología y Neurocirugía Manuel Velasco Suárez, Mexico, DF 14269, Mexico d Unidad del Bioterio, Facultad de Medicina, Universidad Nacional Autónoma de México, Mexico D.F. 04510, Mexico e Laboratorio de Nanotecnología, Instituto Nacional de Neurología y Neurocirugía Manuel Velasco Suárez, S.S.A., Mexico D.F. 14269, Mexico f Laboratorio de Neuroinmunología, Instituto Nacional de Neurología y Neurocirugía Manuel Velasco Suárez, S.S.A., Mexico D.F. 14269, Mexico abstract article info Article history: Received 23 May 2011 Received in revised form 21 June 2011 Accepted 3 July 2011 Available online 13 July 2011 Keywords: Kynurenic acid Oxidative stress Antioxidant defense ROS formation Lipid peroxidation Iron sulfate Kynurenic acid (KYNA) is an endogenous metabolite of the kynurenine pathway for tryptophan degradation and an antagonist of both N-methyl-D-aspartate (NMDA) and alpha-7 nicotinic acetylcholine (α7nACh) receptors. KYNA has also been shown to scavenge hydroxyl radicals (•OH) under controlled conditions of free radical production. In this work we evaluated the ability of KYNA to scavenge superoxide anion (O 2 • - ) and peroxynitrite (ONOO - ). The scavenging ability of KYNA (expressed as IC 50 values) was as follows: •OH=O 2 • - N ONOO - . In parallel, the antiperoxidative and scavenging capacities of KYNA (0–150 μM) were tested in cerebellum and forebrain homogenates exposed to 5 μM FeSO 4 and 2.5 mM 3-nitropropionic acid (3- NPA). Both FeSO 4 and 3-NPA increased lipid peroxidation (LP) and ROS formation in a significant manner in these preparations, whereas KYNA significantly reduced these markers. Reactive oxygen species (ROS) formation were determined in the presence of FeSO 4 and/or KYNA (0–100 μM), both at intra and extracellular levels. An increase in ROS formation was induced by FeSO 4 in forebrain and cerebellum in a time-dependent manner, and KYNA reduced this effect in a concentration-dependent manner. To further know whether the effect of KYNA on oxidative stress is independent of NMDA and nicotinic receptors, we also tested KYNA (0– 100 μM) in a biological preparation free of these receptors – defolliculated Xenopus laevis oocytes – incubated with FeSO 4 for 1 h. A 3-fold increase in LP and a 2-fold increase in ROS formation were seen after exposure to FeSO 4 , whereas KYNA attenuated these effects in a concentration-dependent manner. In addition, the in vivo formation of •OH evoked by an acute infusion of FeSO 4 (100 μM) in the rat striatum was estimated by microdialysis and challenged by a topic infusion of KYNA (1 μM). FeSO 4 increased the striatal •OH production, while KYNA mitigated this effect. Altogether, these data strongly suggest that KYNA, in addition to be a well- known antagonist acting on nicotinic and NMDA receptors, can be considered as a potential endogenous antioxidant. © 2011 Elsevier Inc. All rights reserved. 1. Introduction The kynurenine pathway (KP) of tryptophan degradation contains neuroactive and/or redox active metabolites, namely kynurenic acid (KYNA), 3-hydroxykynurenine (3-HK) and quinolinic acid (QUIN). These compounds are produced in the mammalian brain in nanomolar concentrations and have been related with physiological processes. In contrast, metabolic alterations of KP have been involved in the physiopathology of different neurological and psychiatric disorders, including Huntington's disease, Alzheimer's disease and schizophrenia (Erhardt et al., 2004; Reinhard, 2004; Schwarcz and Pellicciari, 2002; Stone and Darlington, 2002). In particular, KYNA, a neuroactive metabolite at the KP, is synthesized by irreversible transamination of L-kynurenine (L-KYN), and this reaction is catalyzed by kynurenine aminotransferases I and II (KAT I, KAT II) in the brain and peripheral tissues. Moreover, KYNA has been typically described as an inhibitory compound in the central nervous system (CNS) since, at physiological concentrations, this metabolite selectively acts as an antagonist of N-methyl-D-aspartate receptors (NMDAr) by blocking the co-agonist site for glycine (Kemp et al., 1988; Kessler et al., 1989), as well as a non-competitive inhibitor Neurotoxicology and Teratology 33 (2011) 538–547 ⁎ Corresponding author at: Laboratorio de Aminoácidos Excitadores, Instituto Nacional de Neurología y Neurocirugía Manuel Velasco Suárez, Insurgentes Sur 3877, Mexico D.F. 14269, Mexico. Tel.: + 52 55 5606 3822x2013. E-mail address: veped@yahoo.com.mx (V.P.-D. La Cruz). 0892-0362/$ – see front matter © 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.ntt.2011.07.002 Contents lists available at ScienceDirect Neurotoxicology and Teratology journal homepage: www.elsevier.com/locate/neutera