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Oxidative Medicine and Cellular Longevity
Volume 2016 (2016), Article ID 6143753, 11 pages
http://dx.doi.org/10.1155/2016/6143753
Research Article

Alternatively Spliced Methionine Synthase in SH-SY5Y Neuroblastoma Cells: Cobalamin and GSH Dependence and Inhibitory Effects of Neurotoxic Metals and Thimerosal

1Department of Food Science and Nutrition, College of Agricultural and Marine Sciences, Sultan Qaboos University, 123 Al-Khoud, Oman
2Natural Sciences Department, Regis College, Weston, MA 02493, USA
3Department of Neurology, Boston Children’s Hospital, Boston, MA 02215, USA
4Department of Pharmaceutical Sciences, MCPHS University, Manchester, NH 03101, USA
5Department of Medicine, New York University Medical School, New York, NY 10016, USA
6Department of Pharmaceutical Sciences, Northeastern University, Boston, MA 02115, USA
7Department of Pharmaceutical Sciences, Nova Southeastern University, Fort Lauderdale, FL 33328, USA

Received 6 September 2015; Revised 28 December 2015; Accepted 10 January 2016

Academic Editor: Antonio Ayala

Copyright © 2016 Mostafa Waly et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

The folate and cobalamin (Cbl-) dependent enzyme methionine synthase (MS) is highly sensitive to oxidation and its activity affects all methylation reactions. Recent studies have revealed alternative splicing of MS mRNA in human brain and patient-derived fibroblasts. Here we show that MS mRNA in SH-SY5Y human neuroblastoma cells is alternatively spliced, resulting in three primary protein species, thus providing a useful model to examine cofactor dependence of these variant enzymes. MS activity was dependent upon methylcobalamin (MeCbl) or the combination of hydroxocobalamin (OHCbl) and S-adenosylmethionine (SAM). OHCbl-based activity was eliminated by depletion of the antioxidant glutathione (GSH) but could be rescued by provision of either glutathionylcobalamin (GSCbl) or MeCbl. Pretreatment of cells with lead, arsenic, aluminum, mercury, or the ethylmercury-containing preservative thimerosal lowered GSH levels and inhibited MS activity in association with decreased uptake of cysteine, which is rate-limiting for GSH synthesis. Thimerosal treatment decreased cellular levels of GSCbl and MeCbl. These findings indicate that the alternatively spliced form of MS expressed in SH-SY5Y human neuronal cells is sensitive to inhibition by thimerosal and neurotoxic metals, and lower GSH levels contribute to their inhibitory action.