Regulation of (Na++K+)-ATPase by inorganic phosphate: pH dependence and physiological implications

Inhibition of Na++K+-dependent ATPase activity by Pi was maximal in the pH range of 6.1-7, but decreased with increasing pH in the range of 7-8.5. Ki of Pi was 2.8 mM at pH 7.1, and 12 mM at pH 7.8. K+-dependent phosphorylation of the enzyme by Pi, which is thought to be responsible for inhibition of ATPase activity, also decreased with increasing pH. The data suggest that (a) previously observed requirement of high Pi concentrations for inhibition of ATPase activity and associated pump fluxes may have been due to high pH of the assays; (b) at normal values of intracellular pH the pump may be partially inhibited by intracellular Pi; and (c) this effect of Pi may be amplified or dampened with alterations in intracellular pH and ATP/Pi ratio.

Interaction of Ca2+ with (Na+ + K+)-ATPase: properties of the Ca2+-stimulated phosphatase activity

Ca2+ inhibited the Mg2+-dependent and K+-stimulated p-nitrophenylphosphatase activity of a highly purified preparation of dog kidney (Na+ + K+)-ATPase. In the absence of K+, however, a Mg2+-dependent and Ca2+-stimulated phosphatase was observed, the maximal velocity of which, at pH 7.2, was about 20% of that of the K+-stimulated phosphatase. The Ca2+-stimulated phosphatase, like the K+-stimulated activity, was inhibited by either ouabain or Na+ or ATP. Ouabain sensitivity was decreased with increase in Ca2+, but the K0.5 values of the inhibitory effects of Na+ and ATP were independent of Ca2+ concentration. Optimal pH was 7.0 for Ca2+-stimulated activity, and 7.8-8.2 for the K+-stimulated activity. The ratio of the two activities was the same in several enzyme preparations in different states of purity. The data indicate that (a) Ca2+-stimulated phosphatase is catalyzed by (Na+ + K+)-ATPase; (b) there is a site of Ca2+ action different from the site at which Ca2+ inhibits in competition with Mg2+; and (c) Ca2+ stimulation can not be explained easily by the action of Ca2+ at either the Na+ site or the K+ site.

Reaction of (Na+ + K+)-dependent adenosine triphosphatase with inorganic phosphate. Regulation by Na+, K+, and nucleotides

Effects of Na+, K+, and nucleotides on Mg2+-dependent phosphorylation of (Na+ + K+)-dependent adenosine triphosphatase by Pi were studied under equilibrium conditions. Na+ was a linear competitive inhibitor with respect to Mg2+ and a mixed inhibitor with respect to Pi. K+ was a partial inhibitor; it interacted with positive cooperativity and induced negative cooperativities in the interactions of Mg2+ and Pi with the enzyme. Adenyl-5'-yl (beta, gamma-methylene)diphosphonate, a nonhydrolyzable analog of ATP, interacted with negative cooperativity to inhibit phosphorylation in competition with Pi. ATP was also a competitive inhibitor. Na+ and K+ acted antagonistically, Na+ and nucleotides inhibited synergistically, and K+ and nucleotides were mutually exclusive. In the presence of ouabain, when nucleotides were excluded from the site inhibiting phosphorylation, a low affinity regulatory site for nucleotides became apparent, the occupation of which reduced the rate of dephosphorylation and the initial rate of phosphorylation of the enzyme without affecting the equilibrium constant of the reaction of Pi with the ouabain-complexed enzyme. The regulatory site was also detected in the absence of ouabain. The data suggest that catalytic and transport functions of the oligomeric enzyme may be regulated by homotropic and heterotropic site-site interactions, ligand-induced slow isomerizations, and distinct catalytic and regulatory sites for ATP.

Subunit associations of (Na+ + K+)-dependent adenosine triphosphatase. Chemical cross-linking studies

Cross-linking reactions of the alpha- and beta-subunits of the purified membrane-bound enzyme with several reagents were studied. In the presence of 1,5-difluoro-2,4-dinitrobenzene, formation of a cross-linked alpha, beta-dimer was affected specifically by K+ + ATP or enzyme phosphorylation. The same conditions affected the formation of cross-linked alpha, alpha-dimer in the presence of 4,4'-difluoro-3,3'-dinitrodiphenyl sulfone or o-phenanthroline-Cu2+. Since noncovalent alpha, beta-association has been established, the data suggest K+ + ATP-induced or phosphorylation-induced changes in alpha, beta-domain and alpha, alpha-domain of an oligomer of alpha, beta-dimer. When the formation of cross-linked alpha, beta-dimer or alpha, alpha-dimer was induced by phosphorylation, only half of the subunits were cross-linked, suggesting the existence of a cooperative tetramer of alpha, beta-dimer. When microsomes or red cell membranes were exposed to 32Pi under phosphorylation-induced cross-linking conditions, the only products were alpha, beta-dimer and alpha, alpha-dimer, indicating the existence of an oligomer of alpha, beta-dimer in crude membranes. Subunits of the enzyme solubilized with octaethylene glycol dodecyl ether, by methods that have been suggested to yield unassociated alpha, beta-dimers, underwent spontaneous cross-linking that was not affected by enzyme dilution. Since the largest product was alpha 2 beta 2, the solubilized enzyme is at least a dimer of alpha, beta-dimer. The findings establish that the membrane-bound enzyme is an oligomer of alpha, beta-dimer. Whether or not a single alpha, beta-dimer is capable of catalytic and transport functions has not been determined.

(Na+ + K+)-dependent adenosine triphosphatase. Regulation of inorganic phosphate, magnesium ion, and calcium ion interactions with the enzyme by ouabain

1. (Na+ + K+)-dependent adenosine triphosphatase was phosphorylated on the alpha-subunit by Pi in the presence of Mg2+. Phosphorylation was stimulated by ouabain. The interactions of Pi, Mg2+, and ouabain with the enzyme could be explained by a random terreactant scheme in which the binding of each ligand to the enzyme increased the affinities for the other two. Dissociation constants of all steps of this scheme were estimated. 2. In the presence of Pi and ouabain and without added Mg2+, the phosphoenzyme was formed. Because this could be prevented by ethylenediaminetetraacetic acid, but not ethylene glycol bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid, phosphoenzyme formation under these conditions was probably dependent on traces of endogenous Mg2+. The ability of this Mg2+ to support phosphorylation could be explained by the large increase in the enzyme's affinity for Mg2+ by ouabain. 3. In the absence of ouabain, Ca2+ did not support phosphorylation and inhibited Mg2+-dependent phosphorylation. At lower concentrations, Ca2+ was competitive with Mg2+. With increasing Ca2+ concentration, negative cooperativity was observed, suggesting the existence of multiple divalent cation sites with equivalent affinities for Mg2+, but varying affinities for Ca2+. 4. In the presence of ouabain, the maximum inhibition of Mg2+-dependent phosphorylation by Ca2+ was 50%. With saturating Pi, Mg2+, and ouabain, the number of sites binding ouabain was equal to the number of sites phosphorylated. Although Ca2+ halved phosphorylation and reduced the affinity for ouabain about 100-fold, it did not affect the number of ouabain sites. 5. We suggest that the enzyme is an alpha-oligomer and that the half-of-the-sites reactivity for phosphorylation in the presence of Pi, Mg2+, ouabain, and optimal Ca2+ is caused by (a) ouabain-induced increase in the affinities of both protomers for Mg2+ and (b) the inability of Ca2+ to replace Mg2+ on one of the protomers.

Origins of the different sensitivities of (Na+ + K+)-dependent adenosinetriphosphatase preparations to ouabain

Properties of the Na+, K+-ATPase preparations from rat and dog kidney medullae were compared. The two enzymes, with a 1000-fold difference in ouabain sensitivities, had similar subunit compositions and similar K0.5 values and Hill coefficients for substrates and activators of several catalytic activities; suggesting that the structural differences of the two are limited to the ouabain binding domains. Experiments on the interactions of ouabain, Pi, and Mg2+ with the enzymes showed that the two enzymes differed (a) in their inherent affinities for ouabain; and (b) in that Mg2+ binding increased affinity for ouabain to a greater extent in the dog enzyme than in the rat enzyme.

(Na+ + K+)-ATPase : phosphorylation-dependent cross-linking of the alpha-subunits in the presence of Ca2+ and o-phenanthroline

In previous studies we had demonstrated that in the presence of 0.25 mM Cu2+ and 1.25 mM o-phenanthroline, cross-linking of the alpha-subunits of Na+ + K+)-dependent adenosine triphosphatase was induced by the addition of Na+ + ATP, and that the formation of the alpha,alpha-dimer was preceded by that of phosphoenzyme. The purpose of the present studies was the further evaluation of the role of phosphoenzyme in the process of cross-linking. Na+ + UTP did not induce cross-linking unless Mg2+ was also added. In contrast, Na+ + ATP-induced cross-linking did not require the addition of Mg2+. The different effects of ATP and UTP in the absence of added Mg2+ could be accounted for by the presence in the enzyme preparation of bound Mg2+ which supported enzyme phosphorylation by ATP but not by UTP. When the enzyme was phosphorylated by Pi, in the presence of Mg2 and ouabain, and the exposed to Cu2+ and o-phenanthroline, the alpha,alpha-dimer was obtained. Under these conditions, Na+ blocked both phosphorylation and cross-linking. These results indicate that it is the formation of phosphoenzyme per se that leads to conformational transitions favorable to cross-linking. They also suggest that Cu2+ and o-phenanthroline participate in the cross-linking reaction, but not in the phosphorylation reactions. In the digitonin-treated enzyme, Na+ and ATP induced the formation of phosphoenzyme, but not that of alpha,alpha-dimer. These findings indicate that in addition to phosphorylation, a proper orientation o alpha-subunits in an oligomer is also necessary for cross-linking.

Relationship between the positive inotropic effect of ouabain and its inhibitory effects on Na+, K+ -ATPase and active transport of Rb+ in the dog heart

Two previous reports have indicated that in the dog if sustained positive inotropy without arrhythmias is induced by ouabain infusion, inhibition of myocardial Na+, K+-activated adenosine triphosphatase is not observed. Another previous study has shown that in similar experiments induction of sustained inotropy is accompanied by inhibition of the active uptake of Rb+. To resolve the apparent inconsistencies between these findings, we repeated the experiments and measured the inhibition of the enzyme and the inhibition of Rb+ uptake after the induction of sustained inotropy with ouabain. Utilizing three different ouabain dosage regimens in open-chested dogs, we obtained three different quasi-steady state plasma levels of ouabain. At the highest level, ouabain toxicity accompanied by enzyme inhibition and inhibition to RB+ uptake was observed. With the intermediate levels, positive inotropy without significant toxicity was also accompanied by enzyme inhibition and the inhibition of Rb+ uptake. At the lowest ouabain plasma levels, sustained positive inotropy without significant inhibition of either enzyme activity or +b+ uptake was obtained. The data indicate that in this preparation either a small degree of enzyme inhibition, which is difficult to detect is sufficient to produce pronounced positive inotropy, or that the inhibition of the enzyme is not required for the induction of positive inotropy.

(Na+ + K+)-ATPase: effects of detergents on the cross-linking of subunits in the presence of Cu2+ and o-phenanthroline

When (Na+ + K+)-ATPase is reacted with Cu2+ or Cu2+-phenanthroline, cross-linking of the two subunits (alpha and beta) occurs. The major products are alpha,beta- and alpha,alpha-dimers. The alpha,beta-dimer is unstable in the presence of EDTA, but becomes stable when it is first exposed to digitonin or Triton X-100. Conversion of alpha-CU2+-beta to alpha-S-S-beta is suggested. If the enzyme that is pretreated with these detergents is used, only the stable alpha,beta-dimer is obtained, and the formation of alpha,alpha-dimer is inhibited. The data are consistent with alpha 2 beta 2 quaternary structure of the enzyme.

Fossil Alpha-Particle Recoil Tracks: A New Method of Age Determination

The discovery of a new type of fossil nuclear track in mica is reported. This track is produced by the recoil nucleus accompanying the alpha-particle decay of uranium and thorium impurities. The tracks are very short and can be seen with phase contrast microscopy. Measurement of fossil alpha-recoil track densities, coupled with determinations of the thorium and uranium contents, provides a new dating technique analogous to the previously discovered "fission-track method." The primary advantage of the alpha-recoil method is a several-thousand-fold increase in sensitivity over the fission-track technique. The alpha-recoil method should also prove useful in studying the problem of extinct isotopes in meteorites.