Acceleration of the rate of fluorescence decrease by high concentrations of ATP under the condition of accumulation of ADP-sensitive phosphoenzyme in Na+,K+-ATPase

Addition of up to 300 microM ATP in the presence of 2 M NaCl with MgCl2 to pig kidney Na+,K+-ATPase treated with N-p-(2-benzimidazolyl)phenyl]maleimide seemed to be insufficient to saturate the rate of the fluorescence decrease. However, both the extent of the decrease and the amount of phosphoenzyme at a steady state were saturated below 20 microM ATP. Addition of Mg2+ with Na+ to the enzyme preincubated with 20 to 600 microM ATP gave nearly the same rate constant, which was below 50% of that obtained by adding 300 microM ATP to the Na+-form enzyme in the presence of Mg2+. High concentrations of ATP affected neither the rate of light-scattering change (Taniguchi, K. et al. (1986) J. Biol. Chem. 261, 3272-3281) after ADP-sensitive phosphoenzyme formation (E1P) nor that of the breakdown of E1P. A stoichiometric amount of 32P]Pi was liberated from 32P]E1P. The data suggested that ATP did not bind to E1P in such a way as to increase the extent of phosphorylation further or to accelerate dephosphorylation. The data also suggested that the reason for the large difference in the apparent affinity of ATP as evaluated from the rate and the extent of fluorescence change is the large dissociation constant for ATP of a Michaelis complex.