Activation by lithium ions of the inside sodium sites in (Na + + K +)-ATPase

Abstract

1. 1. Purified pig kidney ATPase was incubated in 30–160 mM Tris-HCl with various monovalent cations. 130 mM LiCl stimulated a ouabain-sensitive ATP hydrolysis (about 5% of the maximal (Na + + K +) activity), whereas 160 mM Tris-HCl did not stimulate hydrolysis. Similar results were obtained with human red blood cell broken membranes. 2. 2. In the absence of Na + and with 130 mM LiCl, the ATPase activity as a function of KCl concentration showed an initial slight inhibition (50 μM KCl) followed by an activation (maximal at 0.2 mM KCl) and a further inhibition, which was total at 25 mM KCl. In the absence of LiCl, the rate of hydrolysis was not affected by any of the KCl concentrations investigated. 3. 3. The lithium-activation curve for ATPase activity in the absence of both Na + and K + had sigmoid characteristics. It also showed a marked dependence on the total LiCl + Tris-HCl concentration, being inhibited at high concentrations. This inhibition was more noticeable at low LiCl concentrations. 4. 4. In the absence of Na +, 130 mM Li + showed promoted phosphorylation of ATPase from 1 to 3 mM ATP in the presence of Mg 2+. In enzyme treated with N-ethylmaleimide, the levels of phosphorylation in Li +-containing solutions, amounted to 40% of those in Na +- and up to 7 times of those in K +-containing solutions. 5. 5. The total (Na + + K +)-ATPase activity was markedly inhibited at high buffer concentrations (Tris-HCl, Imidazole-HCl and tetramethylammonium-HEPES gave similar results) in cases when either the concentration of Na + or K + (or both) was below saturation. On the other hand, the maximal (Na + + K +)-ATPase activity was not affected (or very slightly) by the buffer concentration. 6. 6. Under standard conditions (Tris-HCl + NaCl = 160 mM) the Na +-activation curve of Na +-ATPase had a steep rise between 0 and 2.5 mM, a fall between 2.5 and 20 mM and a further increase between 20 and 130 mM. With 30 mM Tris-HCl, the curve rose more steeply, inhibition was noticeable at 2.5 mM Na + and was completed at 5 mM Na +. With Tris-HCl + NaCl = 280 mM, the amount of activation decreased and inhibition at intermediate Na + concentrations was not detected.