A mechanism of sulfite neurotoxicity: direct inhibition of glutamate dehydrogenase.
Exposure of Neuro-2a and PC12 cells to micromolar concentrations of sulfite caused an increase in reactive oxygen species and a decrease in ATP. Likewise, the biosynthesis of ATP in intact rat brain mitochondria from the oxidation of glutamate was inhibited by micromolar sulfite. Glutamate-driven respiration increased the mitochondrial membrane potential (MMP), and this was abolished by sulfite but the MMP generated by oxidation of malate and succinate was not affected. The increased rate of production of NADH from exogenous NAD+ and glutamate added to rat brain mitochondrial extracts was inhibited by sulfite, and mitochondria preincubated with sulfite failed to reduce NAD+. Glutamate dehydrogenase (GDH) in rat brain mitochondrial extract was inhibited dose-dependently by sulfite as was the activity of a purified enzyme. An increase in the Km (glutamate) and a decrease in Vmax resulting in an attenuation in Vmax/Km (glutamate) at 100 microm sulfite suggest a mixed type of inhibition. However, uncompetitive inhibition was noted with decreases in both Km (NAD+) and Vmax, whereas Vmax/Km (NAD+) remained relatively constant. We propose that GDH is one target of action of sulfite, leading to a decrease in alpha-ketoglutarate and a diminished flux through the tricarboxylic acid cycle accompanied by a decrease in NADH through the mitochondrial electron transport chain, a decreased MMP, and a decrease in ATP synthesis. Because glutamate is a major metabolite in the brain, inhibition of GDH by sulfite could contribute to the severe phenotype of sulfite oxidase deficiency in human infants.
Zhang
Xin
X
Department of Biochemistry, Faculty of Medicine, National University of Singapore, 8 Medical Drive, Singapore 117597, Singapore.
Vincent
Annette Shoba
AS
Halliwell
Barry
B
Wong
Kim Ping
KP
D000255:Adenosine Triphosphate / Q000737:chemistry; D000818:Animals; D001667:Binding, Competitive; D001921:Brain / Q000378:metabolism; D002460:Cell Line; D003497:Cyclic N-Oxides / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D005969:Glutamate Dehydrogenase / Q000037:antagonists & inhibitors*; D018698:Glutamic Acid / Q000737:chemistry; D006801:Humans; D007700:Kinetics; D008293:Malates / Q000737:chemistry; D008564:Membrane Potentials; D018613:Microscopy, Confocal; D008928:Mitochondria / Q000378:metabolism; D009243:NAD / Q000737:chemistry; D009474:Neurons / Q000187:drug effects* / Q000378:metabolism; D010100:Oxygen / Q000378:metabolism; D016716:PC12 Cells; D010641:Phenotype; D051381:Rats; D017382:Reactive Oxygen Species; D013113:Spin Labels; D019802:Succinic Acid / Q000737:chemistry; D013447:Sulfites / Q000737:chemistry / Q000633:toxicity*; D013997:Time Factors
D003497:Cyclic N-Oxides; D008293:Malates; D017382:Reactive Oxygen Species; D013113:Spin Labels; D013447:Sulfites; D009243:NAD; C098505:5-diethoxyphosphoryl-5-methyl-1-pyrroline N-oxide; D018698:Glutamic Acid; C030298:malic acid; D000255:Adenosine Triphosphate; D019802:Succinic Acid; D005969:Glutamate Dehydrogenase; D010100:Oxygen