High-affinity 86Rb-binding and structural changes in the alpha-subunit of Na+,K+-ATPase as detected by tryptic digestion and fluorescence analysis

Molecular weights of alpha beta-protomeric and oligomeric units of soluble (Na+, K+)-ATPase determined by low-angle laser light scattering after high-performance gel chromatography

The (Na+, K+)-ATPase of canine renal outer medulla was solubilized with a nonionic surfactant, octaethylene glycol n-dodecyl ether (C12E8), in the presence of 0.2 M sodium ion. The solubilized ATPase retained 74% of the enzymatic activity expressed before solubilization. Molecular species of the solubilized ATPase were analyzed by high-performance chromatography through a TSK-GEL G3000SW column in the presence of 1 mg/ml C12E8 at 23 degrees C. The eluate was monitored by one or two monitors chosen from the following: an ultraviolet absorption monitor, a precision differential refractometer and a low-angle laser light scattering photometer. The three kinds of elution pattern thus obtained can best be interpreted by assuming the presence of at least four kinds of protein component with molecular weights 1 740 000 +/- 230 000, 836 000 +/- 82 000, 286 000 +/- 30 000 and 123 000 +/- 8 000, respectively. Among them, those with the last two molecular weight were the major components. The amounts of the first three components were found to increase with time during the incubation before application to the column at the expense of that of the last one. The amounts of the last two were 18 and 73%, respectively, when measured immediately after the solubilization. A stoichiometric composition of 1:1 molar ratio for the alpha and beta polypeptide chains was obtained for the two major components as well as for the intact ATPase by high-performance gel chromatography in the presence of sodium dodecyl sulfate using the same column as above. The (Na+, K+)-ATPase was, thus, indicated to be solubilized with C12E8 to give the alpha beta-protomer and its dimer as the main components.

A model for the reaction pathways of the K+-dependent phosphatase activity of the (Na+ + K+)-dependent ATPase

(Na+ + K+)-dependent ATPase preparations from rat brain, dog kidney, and human red blood cells also catalyze a K+ -dependent phosphatase reaction. K+ activation and Na+ inhibition of this reaction are described quantitatively by a model featuring isomerization between E1 and E2 enzyme conformations with activity proportional to E2K concentration: (formula; see text) Differences between the three preparations in K0.5 for K+ activation can then be accounted for by differences in equilibria between E1K and E2K with dissociation constants identical. Similarly, reductions in K0.5 produced by dimethyl sulfoxide are attributable to shifts in equilibria toward E2 conformations. Na+ stimulation of K+ -dependent phosphatase activity of brain and red blood cell preparations, demonstrable with KCl under 1 mM, can be accounted for by including a supplementary pathway proportional to E1Na but dependent also on K+ activation through high-affinity sites. With inside-out red blood cell vesicles, K+ activation in the absence of Na+ is mediated through sites oriented toward the cytoplasm, while in the presence of Na+ high-affinity K+ -sites are oriented extracellularly, as are those of the (Na+ + K+)-dependent ATPase reaction. Dimethyl sulfoxide accentuated Na+ -stimulated K+ -dependent phosphatase activity in all three preparations, attributable to shifts from the E1P to E2P conformation, with the latter bearing the high-affinity, extracellularly oriented K+ -sites of the Na+ -stimulated pathway.

Soluble and enzymatically stable (Na+ + K+)-ATPase from mammalian kidney consisting predominantly of protomer alpha beta-units. Preparation, assay and reconstitution of active Na+, K+ transport

Soluble (Na+ + K+)-ATPase consisting predominantly of alpha beta-units with Mr below 170 000 was prepared by incubating pure membrane-bound (Na+ + K+)-ATPase (35-48 mumol Pi/min per mg protein) from the outer renal medulla with the non-ionic detergent dodecyloctaethyleneglycol monoether (C12E8). (Na+ + K+)-ATPase and potassium phosphatase remained fully active in the detergent solution at C12E8/protein ratios of 2.5-3, at which 50-70% of the membrane protein was solubilized. The soluble protomeric (Na+ + K+)-ATPase was reconstituted to Na+, K+ pumps in phospholipid vesicles by the freeze-thaw sonication procedure. Protein solubilization was complete at C12E8/protein ratios of 5-6, at the expense of partial inactivation, but (Na+ + K+)-ATPase and potassium phosphatase could be reactivated after binding of C12E8 to Bio-Beads SM2. At C12E8/protein ratios higher than 6 the activities were irreversibly lost. Inactivation could be explained by delipidation. It was not due to subunit dissociation since only small changes in sedimentation velocities were seen when the C12E8/protein ratio was increased from 2.9 to 46. As determined immediately after solubilization, S20,w was 7.4 S for the fully active (Na+ + K+)-ATPase, 7.3 S for the partially active particle, and 6.5 S for the inactive particle at high C12E8/protein ratios. The maximum molecular masses determined by analytical ultracentrifugation were 141 000-170 000 dalton for these protein particles. Secondary aggregation occurred during column chromatography, with formation of enzymatically active (alpha beta)2-dimers or (alpha beta)3-trimers with S20,w = 10-12 S and apparent molecular masses in the range 273 000-386 000 daltons. This may reflect non-specific time-dependent aggregation of the detergent micelles.