The effect of Mg 2+ and chelating agents on intermediary steps of the reaction of Na +, K +-activated ATPase

Abstract

1. (1)|It has been investigated how varying concentrations of free magnesium with and without EDTA influence the properties of the phospho-enzyme formed in the presence of sodium by the (Na + + K +)-activated enzyme system. 2. (2)|The phospho-enzyme formed in the presence of sodium and a high concentration of free magnesium has the same rate of (a) spontaneous dephosphorylation, (b) dephosphorylation after addition of potassium, and (c) dephosphorylation after addition of ADP, as a phospho-enzyme formed in the presence of sodium and a low concentration of magnesium. 3. (3)|With sodium and a given concentration of free magnesium, high or low, EDTA present during formation of the phospho-enzyme leads to a decrease in the rate of (a) spontaneous dephosphorylation, and (b) dephosphorylation after addition of potassium to the phospho-enzyme. 4. (4)|The rate of dephosphorylation after addition of ADP to phosphoenzyme formed without and with EDTA is the same. But as the rate of spontaneous dephosphorylation is lower with EDTA than without, ADP gives a higher increase in the rate of dephosphorylation of phospho-enzyme formed with EDTA than without. 5. (5)|The experiments thus show that the reported different sensitivity towards potassium and ADP of phospho-enzyme formed in the presence of a low and high concentration of free magnesium, respectively, is due to the EDTA used to decrease the free magnesium concentration and not to the decrease in the free magnesium as such.