Original Contribution THE NEURONAL TOXICITY OF SULFITE PLUS PEROXYNITRITE IS ENHANCED BY GLUTATHIONE DEPLETION: IMPLICATIONS FOR PARKINSON’S DISEASE KARYN-ANN MARSHALL,* MARIANNE REIST,* ,‡ PETER JENNER,* AND BARRY HALLIWELL* ,² *Neurodegenerative Disease Research Centre, Pharmacology Group, King’s College, Manresa Road, London, UK; ² Department of Biochemistry, National University of Singapore, Kent Ridge Crescent, Singapore ; and ‡ currently at ICT, University of Lausanne, Lausanne, Switzerland (Received 4 February 1999; Revised 31 March 1999; Accepted 23 April 1999) Abstract—In Parkinson’s disease (PD) and incidental Lewy body disease glutathione levels in the substantia nigra are decreased by 40 –50%. Both peroxynitrite (ONOO - ) and alterations in the metabolism of sulfur-containing amino acids have been implicated in PD and we have previously shown that sulfite and ONOO - exert synergistic toxicity to a neuronal cell line. This article presents data to show that this synergistic toxicity of sulfite and ONOO - is greatly enhanced by 50% depletion of cellular glutathione levels. The toxicity of sulfite is also slightly enhanced. Neurones with decreased glutathione may be at increased risk from sulfite and especially from the synergistic damaging effects of ONOO - and sulfite. Because sulfite is present normally in the brain as a product of cysteine metabolism, and because increased ONOO - formation has been reported in PD, these events might contribute to neuronal cell death. © 1999 Elsevier Science Inc. Keywords—Sulfite, Peroxynitrite, Parkinson’s disease, Glutathione, Free radicals INTRODUCTION Glutathione (GSH) is an important defense mechanism against oxidative stress in animal cells [1,2]. A moderate decrease in cellular GSH levels does not normally result in cell death, but does render cells more susceptible to a wide range of toxins [1–3]. For example, after treatment with buthionine sulfoximine, an irreversible inhibitor of -glutamylcysteine synthetase (the first enzyme in GSH synthesis [2]), rats showed increased susceptibility to the neurotoxin MPP + [4]. Loss of GSH is thought to be an early event in the pathology of some neurodegenerative disorders. In Parkinson’s disease (PD) and also in the putative preclinical form of PD, incidental Lewy body disease (ILBD), GSH levels are decreased to approxi- mately 50% in the substantia nigra [5,6], and compro- mised GSH levels may predispose the neurones to injury by a range of toxins [1,2,4]. Several neurotoxic agents have been suggested to play a role in PD and other neurodegenerative diseases, in- cluding nitric oxide [7]. Damaging effects of nitric oxide can be exacerbated by its reaction with superoxide rad- icals to yield peroxynitrite [8,9], a potent oxidant, capa- ble of damaging many biologic targets, including thiols, iron-sulfur centers, lipids, proteins, and DNA [7–9]. Evidence is growing to support the involvement of peroxynitrite in neurodegenerative diseases, including PD [7,10 –12]. Sulfite (SO 3 2- ), a much less investigated toxin, is an important intermediate in the metabolic pathway by which sulfur from sulfur-containing amino acids such as cysteine is converted to inorganic sulfate [13]. It is a substrate for the mitochondrial enzyme, sulfite oxidase, which is present in many tissues, although its levels in the brain are reported as low [13–15]. The consequences of a deficiency in this enzyme highlight the potential toxicity of sulfite to the brain: sulfite oxidase deficiency causes severe neurologic dysfunction, the clinical fea- tures of which include severe mental retardation, sei- zures, spastic quadriparesis, and early death [16 –21]. Abnormalities in the metabolism of sulfur-containing amino acids have been reported in several neurodegen- Address correspondence to: Barry Halliwell, National University of Singapore, Department of Biochemistry, Kent Ridge Crescent 119260, Singapore. Tel: +65 874-3247; Fax: +65 779-1453; E-Mail: bchbh@nus.edu.sg. Free Radical Biology & Medicine, Vol. 27, Nos. 5/6, pp. 515–520, 1999 Copyright © 1999 Elsevier Science Inc. Printed in the USA. All rights reserved 0891-5849/99/$–see front matter PII S0891-5849(99)00094-5 515