Isolation and characterization of the components of the sodium pump

Stimulation of the sodium pump by azide and high internal sodium: changes in the number of pumping sites and turnover rate

1. The effects of 5 mM azide on [3H]ouabain uptake and 22Na efflux were determined. Both glycoside uptake and 22Na efflux were enhanced by azide. 2. Azide stimulated the Na pump in muscles whose pumping sites had been inhibited by ouabain and then transferred to a glycoside-free solution. This stimulation was observed before detecting any recovery of the initial pumping activity. 3. When both the resting and the azide-stimulated 22Na efflux had been blocked by ouabain, an additional exposure to azide, in a ouabain-free solution, had no further effects on 22Na efflux. 4. It is concluded that the increase in Na pumping caused by azide is due in part to an increase in the number of pumping sites. 5. [3H]ouabain binding was measured in muscles with different intracellular alkali cation concentrations. Variations in [Na]i from 15 up to 50 mM did not significantly affect the amount of glycoside bound. A substantial increase in binding occurred when [Na]i reached 70 mM. 6. It is proposed that the increase in Na extrusion that occurs during the recovery of Na loaded muscles mostly results from an increased turnover rate of the pump rather than from an increase in number of pumping sites.

Purification and characterization of (Na+, K+)-ATPase. V. Conformational changes in the enzyme Transitions between the Na-form and the K-form studied with tryptic digestion as a tool

1. Purified (Na+, K+)-ATPase consisting of membrane fragments was digested with trypsin. The time course of enzyme inactivation was related to the electrophoretic pattern of native and cleaved proteins remaining in the membrane. 2. Differences in both the inactivation kinetics and the cleavage of the large chain (mol. wt 98 000) allow distinction of two patterns of tryptic digestion of (Na+, K+)-ATPase seen with Na+ or K+ in the medium. 3. With K+, the inactivation of (Na+, K+)-ATPase is linear with time in semilogarithmic plots and the activity is lost in parallel with cleavage of the large chain to fragments with molecular weights 58 000 and 48 000. 4. With Na+, the inactivation curves are biphasic. In the initial phase of rapid inactivation, 50% of the activity is lost with minor changes in the composition of the large chain. In the final phase, the large chain is cleaved at a low rate to a fragment with a molecular weight of 78 000. 5. It is concluded that the regions of the large chain exposed in the presence of K+ are distinct from the regions exposed in presence of Na+ and that two conformations of (Na+, K+)-ATPase can be sensed with trypsin, a (t)K-form and a (t)Na-form. 6. Reaction of the (t)K-form with ATP cause transition to the (t)Na-form. Relatively high concentrations of ATP are required and Mg2+ is not necessary. Phosphorylation of (Na+, K+)-ATPase is accompanied by transition from the (t)Na-form to the (t)K-form. Previous kinetic data suggest that these conformational changes are accompanied by shifts in the affinities of the enzyme for Na+ and K+.

The (Na++K+) activated enzyme system and its relationship to transport of sodium and potassium

It seems to be the membrane bound (Na++K+)-activated enzyme system which transforms the energy from a hydrolysis of ATP into a vectorial movement of sodium out and potassium into the cell against electrochemical gradients, i.e. this systems seems to be the transport system for sodium and potassium (see, for example, review by Skou, 1972; Hokin & Dahl, 1972).