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J. Biol. Chem., Vol. 281, Issue 47, 35785-35793, November 24, 2006

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A Functional Transsulfuration Pathway in the Brain Links to Glutathione Homeostasis^*

Victor Vitvitsky, Mark Thomas^¶||, Anuja Ghorpade^¶||, Howard E. Gendelman^¶||, and Ruma Banerjee¹

From the Redox Biology Center and Department of Biochemistry, University of Nebraska-Lincoln, Lincoln, Nebraska 68588, and the ^¶Center for Neurovirology and Neurodegenerative Disorders and the ^||Department of Pharmacology and Experimental Neuroscience, University of Nebraska Medical Center, Omaha, Nebraska 68198

Oxidative stress and diminished glutathione pools play critical roles in the pathogenesis of neurodegenerative diseases, including Alzheimer and Parkinson disease. Synthesis of glutathione, the most abundant mammalian antioxidant, is regulated at the substrate level by cysteine, which is synthesized from homocysteine via the transsulfuration pathway. Elevated homocysteine and diminished glutathione levels, seen in Alzheimer and Parkinson disease patients suggest impairments in the transsulfuration pathway that connects these metabolites. However, the very existence of this metabolic pathway in the brain is a subject of controversy. The product of the first of two enzymes in this pathway, cystathionine, is present at higher levels in brain as compared with other organs. This, together with the reported absence of the second enzyme, -cystathionase, has led to the suggestion that the transsulfuration pathway is incomplete in the brain. In this study, we incubated mouse and human neurons and astrocytes and murine brain slices in medium with [³⁵S]methionine and detected radiolabel incorporation into glutathione. This label transfer was sensitive to inhibition of -cystathionase. In adult brain slices, 40% of the glutathione was depleted within 10 h following -cystathionase inhibition. In cultured human astrocytes, flux through the transsulfuration pathway increased under oxidative stress conditions, and blockade of this pathway led to reduced cell viability under oxidizing conditions. This study establishes the presence of an intact transsulfuration pathway and demonstrates its contribution to glutathione-dependent redox-buffering capacity under ex vivo conditions in brain cells and slices.

Received for publication, March 17, 2006 , and in revised form, September 20, 2006.

^* This work was supported by National Institutes of Health Grants DK64959, P20 RR17675, and 2 R37 NS36126 and by the Jonty Foundation. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

¹ To whom correspondence should be addressed. Tel.: 402-472-2941; Fax: 402-472-4961; E-mail: rbanerjee1{at}unl.edu.

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