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The mammalian enzymes responsible for reduction of the environmentally prevalent arsenate (AsV) to the much more toxic arsenite (AsIII) are unknown. In the previous paper (Nemeti and Gregus, 2005), we proposed that glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and/or phosphoglycerate kinase (PGK) may catalyze reduction of AsV in human red blood cells (RBC), hemolysate, or rat liver cytosol. In testing this hypothesis, we show here that, if supplied with glutathione (GSH), NAD, and glycolytic substrate, the mixture of purified GAPDH and PGK indeed catalyzes the reduction of AsV. Further analysis revealed that GAPDH is endowed with AsV reductase activity, whereas PGK serves as an auxiliary enzyme, when 3-phosphoglycerate is the glycolytic substrate. The GAPDH-catalyzed AsV reduction required GSH, NAD, and glyceraldehyde-3-phosphate. ADP and ATP moderately, whereas NADH strongly inhibited the AsV reductase activity of the enzyme even in the presence of NAD. Koningic acid (KA), a specific and irreversible inhibitor of GAPDH, inhibited both the classical enzymatic and the AsV-reducing activities of the enzyme in a concentration-dependent fashion. To assess the contribution of GAPDH to the reduction of AsV carried out by hemolysate, rat liver cytosol, or intact erythrocytes, we determined the concentration-dependent effect of KA on AsV reduction by these cells and extracts. Inactivation of GAPDH by KA abolished AsV reduction in intact RBC as well as in the hemolysate and the liver cytosol, when GAPDH in the latter extracts was abundantly supplied with exogenous NAD and glycolytic substrate. However, despite complete inactivation of GAPDH by KA, the hepatic cytosol exhibited significant residual AsV-reducing activity in the absence of exogenous NAD and glycolytic substrate, suggesting that besides GAPDH, other cytosolic enzyme(s) may contribute to AsV reduction in the liver. In conclusion, the key glycolytic enzyme GAPDH can fortuitously catalyze the reduction of AsV to AsIII, if GSH, NAD, and glycolytic substrate are available. AsV reduction may take place during, or as a consequence of, the arsenolytic cleavage of the thioester bond formed between the enzyme's Cys149 and the 3-phosphoglyceroyl moiety of the substrate. Although GAPDH is exclusively responsible for reduction of AsV in human erythrocytes, its role in AsV reduction in vivo remains to be determined.

The Glycolytic Enzyme Glyceraldehyde-3-Phosphate Dehydrogenase Works as an Arsenate Reductase in Human Red Blood Cells and Rat Liver Cytosol