The role of magnesium in the hydrolysis of sodium pyrophosphate by inorganic pyrophosphatase

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Abstract
The prepn. of erythrocyte pyrophosphatase from rat blood is descr. This enzyme requires Mg to catalyze hydrolysis of inorganic pyro-phosphate into orthophosphate. It does not catalyze the hydrolysis of glycerophosphate or thiamine pyrophosphate. There is a similarity between the rate of hydrolysis against magnesium chloride concentration, and of pH depression against magnesium chloride concentration in the pyrophosphate solution. The rate of the hydrolysis of pyrophosphate is directly proportional to enzyme concentration and has a pH opt. at 7.1. The plot of the initial rate of hydrolysis against initial pyrophosphate concn. fits the Michaelis-Menten equation when magnesium chloride concn. is varied with that of the substrate to maintain a constant and nearly opt. ratio. Km was equal to 5.4 x 10-4. It is suggested that the actual substrate for inorganic pyrophosphatase is neither Na4P2O7 nor (HP2O7)3- but the complex anion (MgP2O7)2-.

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