Na+- and K+-dependent oligomeric interconversion among αβ-protomers, diprotomers and higher oligomers in solubilized Na+/K+-ATPase

Protein fractions of a higher-oligomer (H), (alpha beta)(2)-diprotomer (D) and alpha beta-protomer (P) were separated from dog kidney Na+/K+-ATPase solubilized in the presence of NaCl and KCl. Na+/K+-dependent interconversion of the oligomers was analysed using HPLC at 0 degrees C. With increasing KCl concentrations, the content or amount of D increased from 27.6 to 54.3% of total protein, i.e. Delta C-max = 26.7%. Delta C-max for the sum of D and H was equivalent to the absolute value of Delta C-max for P, regardless of the anion present, indicating that K+ induced the conversion of P into D and/or H, and Na+ had the opposite effect. When enzymes that had been denatured to varying degrees by aging were solubilized, Delta C-max increased linearly with the remaining ATPase activity. The magnitude of the interconversion could be explained based on an equilibrium of D reversible arrow 2P, assuming 50-fold difference in the K-d between KCl and NaCl, and coexistence of unconvertible oligomers, which comprised as much as 39% of the eluted protein. Oligomeric interconversion, determined as a function of the KCl or NaCl concentration, showed K0.5S of 64.8 mu M and. 6.50 mM for KCl and NaCl, respectively, implying that oligomeric interconversion was coupled with Na+/K+-binding to their active transport sites.