Effect of intravesicular monovalent cations on the steady state of the phosphoenzyme of adenosine triphosphatase dependent on sodium and potassium ions in inside-out plasma-membrane vesicles.

Phosphorylation of the ATPase dependent on Na+ and K+ is promoted through the synergistic action of cations on both sides of the membrane. This phenomenon has been observed in plasma membrane vesicles isolated from sheep-kidney outer medulla which accept ATP from the outside surface (inside-out) and which are tight for sodium ions. In these inside-out vesicles phosphorylating capacity is low even in the presence of 100 mM extravesicular sodium chloride as is turnover of the enzyme. The level of the phosphoenzyme and the transient release of inorganic phosphate from the phosphoenzyme increases several-fold when sodium chloride is allowed to equilibrate over the membrane, 25 mM intravesicular NaCl is necessary to obtain the half-maximum level of the phosphoenzyme. This result shows that intravesicular (= extracellular) low affinity sites are involved in the phosphorylation. Intravesicular potassium ions modify the activating action of Na+ on the phosphorylation by increasing the steady state of the phosphoenzyme at low intravesicular sodium ion concentrations. This suggests that Na+ and K+ compete with each other for the intravesicular cation-binding site.