Cation interactions with different functional states of the Na+, K+-ATPase

Steady-state analysis of enzymes with non-Michaelis-Menten kinetics: The transport mechanism of Na+/K+-ATPase

Procedures to define kinetic mechanisms from catalytic activity measurements that obey the Michaelis-Menten equation are well established. In contrast, analytical tools for enzymes displaying non-Michaelis-Menten kinetics are underdeveloped, and transient-state measurements, when feasible, are therefore preferred in kinetic studies. Of note, transient-state determinations evaluate only partial reactions, and these might not participate in the reaction cycle. Here, we provide a general procedure to characterize kinetic mechanisms from steady-state determinations. We described non-Michaelis-Menten kinetics with equations containing parameters equivalent to k_cat and K_m and modeled the underlying mechanism by an approach similar to that used under Michaelis-Menten kinetics. The procedure enabled us to evaluate whether Na⁺/K⁺-ATPase uses the same sites to alternatively transport Na⁺ and K⁺ This ping-pong mechanism is supported by transient-state studies but contradicted to date by steady-state analyses claiming that the release of one cationic species as product requires the binding of the other (ternary-complex mechanism). To derive robust conclusions about the Na⁺/K⁺-ATPase transport mechanism, we did not rely on ATPase activity measurements alone. During the catalytic cycle, the transported cations become transitorily occluded (i.e. trapped within the enzyme). We employed radioactive isotopes to quantify occluded cations under steady-state conditions. We replaced K⁺ with Rb⁺ because ⁴²K⁺ has a short half-life, and previous studies showed that K⁺- and Rb⁺-occluded reaction intermediates are similar. We derived conclusions regarding the rate of Rb⁺ deocclusion that were verified by direct measurements. Our results validated the ping-pong mechanism and proved that Rb⁺ deocclusion is accelerated when Na⁺ binds to an allosteric, nonspecific site, leading to a 2-fold increase in ATPase activity.

Different approaches to the mechanism of the sodium pump

The way in which the sodium pump uses energy from the hydrolysis of ATP to perform osmotic and electrical work is not yet understood. We attempt to bring together the results of a number of different approaches to this problem. One approach has been to correlate biochemical changes and ionic fluxes, both when the pump operates normally and when it operates in various abnormal 'modes' in particular unphysiological conditions. A second approach has been to expose fragments of cell membrane to (gamma-32P)ATP and to study the properties of components of the membrane that become labelled. It is now clear that 32P can be transferred to the beta-carboxy group of an aspartyl residue in a pump polypeptide, but there is controversy about the interrelations of different forms of this polypeptide and its role, if any, in the normal functioning of the pump. A third approach has been to attempt to purify the pump and to determine the properties of the pure enzyme. It seems that the pump contains a polypeptide (molecular weight about 100,000), which bears the phosphorylation site, and a smaller glycopeptide, but there is disagreement about the molecular ratios. The results of these and other approaches cannot yet be fitted into a satisfactory model for the sodium pump, but we shall consider some of the problems involved in this task.

Modification of the E1ATP binding site of Na+/K(+)-ATPase by the chromium complex of adenosine 5'-[beta,gamma-methylene]triphosphate blocks the overall reaction but not the partial activities of the E2 conformation

The chromium complex of adenosine 5'-[beta,gamma-methylene]triphosphate, Cr(H2O)4AdoPP[CH2]P, inactivates Na+/K(+)-ATPase from pig kidney at 37 degrees C with an inactivation velocity constant of 7.1 x 10(-3) min-1 by binding to the high-affinity ATP site (E1ATP site). The dissociation constant (Kd) of the analogue at this site is 26 microM, and of ATP 0.8 microM. Inactivation of the overall reaction of Na+/K(+)-ATPase by Cr(H2O)4AdoPP[CH2]P did not alter the activities of the E2 conformational state such as K(+)-activated p-nitrophenylphosphatase, 86Rb+ occlusion and [3H]ouabain binding by the 'backdoor' phosphorylation. However, [3H]ouabain binding via the forwards reaction from E1ATP in the presence of Na+ + Mg2+ is inhibited. K(+)-activated p-nitrophenylphosphatase activity of the Cr(H2O)4AdoPP[CH2]P-inactivated enzyme decreases when an MgATP analogue, the tetraammine cobalt complex of ATP, Co(NH3)4ATP, is used additionally to inactivate the E2ATP site. The enzyme activity of K(+)-activated phosphatase is also lost if the beta,gamma-bidentate chromium(III) complex of ATP, Cr(H2O)4ATP, which may form a stable E1-chromo-phosphointermediate, is used for the inactivation of Na+/K(+)-ATPase. We conclude that the phenomenon of a blockade of the overall reaction of Na+/K(+)-ATPase by the formation of a stable E1.CrAdoPP[CH2]P complex, leading thereby to a loss of the partial activities of the E1 conformation, but not of the E2 conformation, is consistent with the postulate of an (alpha beta)2 diprotomeric nature of the sodium pump. The observation, moreover, that treatment of the sodium pump with Cr(H2O)4ATP but not with Cr(H2O)4AdoPP[CH2]P leads to an inactivation of K(+)-activated phosphatase seems to indicate that the formation of a E1-phosphointermediate affects the E2ATP site.

Dependence of the electrogenic pump current of Xenopus oocytes on external potassium

The membrane potential of Xenopus oocytes showed a variable response to an increase of the K+ concentration in the bathing solution, [K+]e, from 2.5 mM to 20 mM. In 54% of the cases (n = 52) the cells hyperpolarized (by max. 70 mV). In the presence of 10(-5) M ouabain, all cells depolarized suggesting that the hyperpolarization was caused by an electrogenic Na+/K+ pump. In cells stored overnight in a Na+-free solution the transition from 2.5 to 20 mM [K+]e always caused depolarization indicating that the stimulation of the pump requires high internal sodium, [Na+]i. Cells stored overnight in a Na+-rich solution had a [Na+]i of 30.7 +/- 7 mM, i.e. the Na+/K+ pump was saturated with sodium (Lafaire and Schwarz 1986). With 9 such cells we determined the K+ activation of the Na+/K+ pump. The activation follows Hill kinetics with Imax = 90.5 nA, Ks = 2.3 mM, and n = 1.68.

Potassium-p-nitrophenyl phosphate interactions with (Na+ + K+)-ATPase. Their relevance to phosphatase activity

The K+-dependent p-nitrophenylphosphatase activity catalyzed by purified (Na+ + K+)-ATPase from pig kidney shows substrate inhibition (Ki about 9.5 mM at 2.1 mM Mg2+). Potassium antagonizes and sodium favours this inhibition. In addition , K+ reduces the apparent affinity for substrate activation, whereas p-nitrophenyl phosphate reduces the apparent affinity for K+ activation. In the absence of Mg2+, p-nitrophenyl phosphate, as well as ATP, accelerates the release of Rb+ from the Rb+ occluded unphosphorylated enzyme. With no Mg2+ and with 0.5 mM KCl, trypsin inactivation of (Na+ + K+)-ATPase as a function of time follows a single exponential but is transformed into a double exponential when 1 mM ATP or 5 mM p-nitrophenyl phosphate are also present. In the presence of 3 mM MgCl2, 5 mM p-nitrophenyl phosphate and without KCl the trypsin inactivation pattern is that described for the E1 enzyme form; the addition of 10 mM KCl changes the pattern which, after about 6 min delay, follows a single exponential. These results suggest that (i) the shifting of the enzyme toward the E1 state is the basis for substrate inhibition of the p-nitrophenylphosphatase activity of(Na+ + K+)-ATPase, and (ii) the substrate site during phosphatase activity is distinct from the low-affinity ATP site.

The effect of Na+ and K+ on the thermal denaturation of Na+ and + K+-dependent ATPase

To increase our understanding of the physical nature of the Na+ and K+ forms of the Na+ + K+-dependent ATPase, thermal-denaturation studies were conducted in different types of ionic media. Thermal-denaturation measurements were performed by measuring the regeneration of ATPase activity after slow pulse exposure to elevated temperatures. Two types of experiments were performed. First, the dependence of the thermal-denaturation rate on Na+ and K+ concentrations was examined. It was found that both cations stabilized the pump protein. Also, K+ was a more effective stabilizer of the native state than was Na+. Secondly, a set of thermodynamic parameters was obtained by measuring the temperature-dependence of the thermal-denaturation rate under three ionic conditions: 60 mM-K+, 150 mM-Na+ and no Na+ or K+. It was found that ion-mediated stabilization of the pump protein was accompanied by substantial increases in activation enthalpy and entropy, the net effect being a less-pronounced increase in activation free energy.

Kinetic characteristics of hamster (Mesocricetus auratus) brown fat (Na+/K+)-ATPase: effects of catecholamines

1. The (Na+/K+)-ATPase activity of brown fat membranes is increased by norepinephrine, the physiological mediator of thermogenesis in this tissue. 2. This increased ATPase activity was inhibited approximately 50% by either propranolol (a beta-adrenergic blocker) or phentolamine (an alpha-blocker). 3. The alpha-agonist, phenylephrine and the beta-agonist, isoproterenol, also stimulated the ATPase activity. 4. That these latter effects were receptor-specific is supported by the finding that: (a) l(-)isoproterenol stimulation was inhibited by propranolol but not by phentolamine; (b) d(+)isoproterenol had no stimulatory effect on the ATPase activity; and (c) the l(-)phenylephrine-induced increase was inhibited by phentolamine but not by propranolol. 5. (-)norepinephrine, l(-)isoproterenol and l(-)phenylephrine all decreased the apparent Km for K+ of the (Na+/K+)-ATPase but did not alter the apparent Km for ATP or the Vmax of the reaction.

Comparison of red cell and kidney (Na+ +K+)-ATPase at 0 degrees C

Human red cell and guinea pig kidney (Na+ +K+)-ATPase were phosphorylated at 0 degrees C. Using concentrations of ATP ranging from 10(-6) to 10(-8) M, ATP-dependent regulation of reactivity is observed with red cell but not kidney (Na+ +K+)-ATPase at 0 degrees C. In particular, with the red cell enzyme only, the following are observed: (i) the ratio of enzyme-bound ATP (E.ATP, measured by the pulse-chase method of Post, R.L., Kume, S., Tobin, T., Orcutt, B. and Sen, A.K. (1969) J. Gen. Physiol. 54, 306s-326s) to steady-state level of total phosphoenzyme (EP) decreases with decrease in ATP concentration and (ii) the apparent turnover of phosphoenzyme (ratio of Na+-stimulated ATP hydrolysis to level of total EP at steady state) also varies as a function of ATP concentration. In addition, when EP is formed at very low ATP (0.02 microM), and then EDTA is added, rapid disappearance of a fraction of EP occurs, presumably due to ATP resynthesis, only with the red cell enzyme. These differences in behaviour of the red cell and kidney enzymes are explained on the basis of the observed predominance of K+-insensitive EP in red cell, but K+-sensitive EP in kidney (Na+ +K+)-ATPase at 0 degrees C.

Effects of ATP and protons on the Na : K selectivity of the (Na+ + K+)-ATPase studied by ligand effects on intrinsic and extrinsic fluorescence

The effect of pH and of ATP on the Na : K selectivity of the (Na+ + K+)-ATPase has been tested under equilibrium conditions. The Na+ : K+-induced change in intrinsic tryptophan fluorescence and in fluorescence of eosin maleimide bound to the system has been used as a tool. 1 mol of eosin maleimide per mol of enzyme gives no loss in either ATPase or phosphatase activity and the fluorescence in the presence of Na+ is about 30% higher than in the presence of K+. Choline, protonated Tris, protonated histidine and Mg2+ have an 'Na+' effect on the extrinsic fluorescence, while Rb+, Cs+ and NH4+ have a 'K+' effect. Choline and protonated Tris have an Na+ effect on intrinsic fluorescence. A close correlation between the effect of Na+ compared to K+ on the fluorescence change and on Na+ activation of hydrolysis indicates that the observed changes in fluorescence are due to an effect of Na+ and of K+ on the internal sites of the system. The equilibrium between the two conformations, which are reflected by the difference in fluorescence with Na+ and K+, respectively, is highly influenced by the concentration of protons. At a given Na+ : K+ ratio, an increase in the proton concentration shifts the equilibrium towards the 'K+' fluorescence form while a decrease shifts the equilibrium towards the 'Na+' fluorescence form, i.e., protons increase the apparent affinity for K+ and vice versa, K+ increases pK values of importance for the Na+ : K+ selectivity. Conversely, a decrease in protons increases the apparent affinity for Na+ and vice versa, Na+ decreases the pK. ATP decreases the apparent pK for the protonation-deprotonation, i.e., ATP facilitates the deprotonation which accompanies Na+ binding. The results suggest two effects of ATP for the hydrolysis in the presence of Na+ and K+ : (i) at low ATP concentrations (K0.5 < 10 microM) on the K+-Na+ exchange on the internal sites and (ii) at higher, substrate, concentrations on the activation by K+ on the external sites.

Separate and combined effects of ouabain and extracellular potassium on renin secretion from rat renal cortical slices

1. Renin secretion of rat renal cortical slices was measured as a function of extracellular K and ouabain concentrations in the incubation medium.2. A sigmoid relationship was found between renin secretion and log K concentration over the range 1.0-4.0 mM. Secretion was maximal at about 2.25 mM-K and half-maximal at about 1.43 mM-K.3. In media containing 4.0 mM-K, ouabain at 10(-8), 10(-7), and 10(-6)M did not affect renin secretion. Higher concentrations of ouabain inhibited secretion. A sigmoid relationship was found between% inhibition of secretion and log ouabain concentration (10(-6)-10(-3)M). Inhibition was half-maximal at 2.3 x 10(-5)M and complete at 10(-3)M-ouabain.4. Lowering extracellular K concentration from 4.0 to 2.25 mM shifted the dose-effect curve of ouabain to the left. At 2.25 mM-K, inhibition of renin secretion was half-maximal at 10(-5)M-ouabain.5. The inhibitory effect of 2 x 10(-5)M-ouabain (twice the dose for 50% inhibition) in media containing 2.25 mM-K was nearly identical to the combined effect of lowering K to 1.43 mM (the concentration required for 50% inhibition) and adding 10(-5)M-ouabain. This observation suggests that ouabain and low extracellular K act at a common site, presumably on Na, K-ATPase activity, to inhibit renin secretion.6. Neither 10(-3)M-ouabain nor K-free medium inhibited renin secretion when the concentration of free Ca in the medium was lowered to < 10(-8)M. Therefore it is proposed that as a result of Na, K-ATPase inhibition, (a) intracellular Na increases, (b) intracellular Ca increases via Na-Ca exchange, provided that extracellular Ca exceeds 10(-8)M, and that (c) Ca accumulation, in some unknown manner, inhibits renin secretion from rat renal cortical slices.

Tryptophan fluorescence of (Na+ + K+)-ATPase as a tool for study of the enzyme mechanism

1. The protein fluorescence intensity of (Na+ + K+)-ATPase is enhanced following binding of K+ at low concentrations. The properties of the response suggest that one or a few tryptophan residues are affected by a conformational transition between the K-bound form E2 . (K) and a Na-bound form E1 . Na. 2. The rate of the conformational transition E2 . (K) leads to E . Na has been measured with a stopped-flow fluorimeter by exploiting the difference in fluorescence of the two states. In the absence of ATP the rate is very slow, but it is greatly accelerated by binding of ATP to a low affinity site. 3. Transient changes in tryptophan fluorescence accompany hydrolysis of ATP at low concentrations, in media containing Mg2+, Na+ and K+. The fluorescence response reflects interconversion between the initial enzyme conformation, E1 . Na and the steady-state turnover intermediate E2 . (K). 4. The phosphorylated intermediate, E2P can be detected by a fluorescence increase accompanying hydrolysis of ATP in media containing Mg2+ and Na+ but no K+. 5. The conformational states and reaction mechanism of the (Na+ + K+)-ATPase are discussed in the light of this work. The results permit a comparison of the behaviour of the enzyme at both low and high nucleotide concentrations.

Conformational transitions between Na+-bound and K+-bound forms of (Na+ + K+)-ATPase, studied with formycin nucleotides

1. Fluorescence measurements have shown that formycin triphosphate (FTP) or formycin diphosphate (FDP) bound to (Na+ + K+)-ATPase (ATP phosphohydrolase, EC 3.6.1.3) in Na+-containing media can be displaced by the following ions (listed in order of effectiveness): Tl+, K+, Rb+, NH4+, Cs+. 2. The differences between the nucleotide affinities displayed by the enzyme in predominantly Na+ and predominantly K+ media in the absence of phosphorylation, are thought to reflect changes in enzyme conformation. These changes can therefore be monitored by observing the changes in fluorescence that accompany net binding or net release of formycin nucleotides. 3. The transition from a K+-bound form (E2-(K)) to an Na+-bound form (E1-Na) is remarkably slow at low nucleotide concentrations, but is accelerated if the nucleotide concentration is increased. This suggests that the binding of nucleotide to a low-affinity site on E2-(K) accelerates its conversion to E1-Na; it supports the hypothesis that during the normal working of the pump, ATP, acting at a low affinity site, accelerates the conversion of dephosphoenzyme, newly formed by K+-catalysed hydrolysis of E2P, to a form in which it can be phosphorylated in the presence of Na+. 4. The rate of the reverse transformation, E1-Na to E2-(K), varies roughly linearly with the K+ concentration up to the highest concentration at which the rate can be measured (15 mM). Since much lower concentrations of K+ are sufficient to displace the equilibrium to the K-form, we suggest that the sequence of events is: (i) combination of K+ with low affinity (probably internal) binding sites, followed by (ii) spontaneous conversion of the enzyme to a form, E2-(K), containing occluded K+. 5. Mg2+ or oligomycin slows the rate of conversion of E1-Na to E2-(K) but does not significantly affect the rate of conversion of E2-(K) to E1-Na. 6. In the light of these and previous findings, we propose a model for the sodium pump in which conformational changes alternate with trans-phosphorylations, and the inward and outward fluxes of both Na+ and K+ each involve the transfer of a phosphoryl group as well as a change in conformation between E1 and E2 forms of the enzyme or phosphoenzyme.

ATPase and phosphatase activities from human red cell membranes. III. Stimulation of K+-activated phosphatase by phospholipase C

Treatment of red cell membranes with pure phospholipase C inactivates (Na+ + K+)-ATPase activity and Na+-dependent phosphorylation but increases K+-dependent phosphatase activity. When phospholipase A2 replaces phospholipase C, all activities are lost. Activation of K+-dependent phosphatase by treatment with phospholipase C is caused by an increase in the maximum rate of hydrolysis of p-nitrophenylphosphate and in the maximum activating effect of K+, the apparent affinities for substrate and cofactors being little affected. After phospholipase C treatment K+-dependent phosphatase is no longer sensitive to ouabain but becomes more sensitive to N-ethylmaleimide. In treated membranes Na+ partially replaces K+ as an activator of the phosphatase. Although ATP still inhibits phosphatase activity, neither ATP, nor ATP+Na+ are able to modify the apparent affinity for K+ of K+-dependent phosphatase in these membranes.

The pre-steady-state hydrolysis of ATP by porcine brain (Na+ + K+)-dependent ATPase

The hydrolysis of [gamma-32P]ATP by porcine brain (Na+ + K+)-stimulated ATP phosphohydrolase (EC 3.6.1.3) has been studied at 28 degree C in a rapid mixing quenched-flow apparatus. An "early burst" in the release of Pi from ATP has been observed when the enzyme is mixed with ATP, Na+ and a relatively high concentration of K+ (10 mM) but the burst is less pronounced with 0.5 mM K+. This "early burst" of Pi release is suppressed when the enzyme is pre-mixed with 10 mM K+ or 20% (v/v) dimethylsulphoxide before mixing with ATP and Na+, and premixing of enzyme with Na+ antagonizes this effect of dimethylsulphoxide. The results have been analysed by a non-linear least squares regression treatment and are consistent with a mechanism involving three steps, one of which may be a relatively slow change in enzyme conformation following release of Pi from its covalent linkage with the enzyme, in addition to formation of the enzyme-substrate complex. Rate constants (and S.E.) for these steps have been calculated and the roles of phospho-enzyme and other intermediates in the reaction mechanism of the transport ATPase are dicussed.

Substrate sites for the (Na+ + K+)-dependent ATPase

Kinetic studies on a rat brain (Na+ + K+)-dependent ATPase (EC 3.6.1.3) preparation demonstrated high-affinity sites for ATP, with a Km near 1 mum, and low affinity sites for ATP, with a Km near 0.5 mM. In addition, the dissociation constant for ATP at the low affinity sites was approached through the ability of ATP to modify the rate of photo-oxidation of the enzyme in the presence of methylene blue; a value of 0.4 mM was obtained. The temperature dependence of the Km values in these two concentration ranges also differed markedly, and the estimated entropy of binding was +27 cal/degree per mol at the high affinity sites, whereas it was -20 cal/degree per mol at the low affinity sites. Moreover, the relative affinities of various congeners of ATP as of the K+ -dependent phosphatase reaction of the enzyme indicated an interaction at the low-affinity sites for ATP: ATP, ADP, CTP, and the [beta-gamma] -imido analog of ATP all competed with Ki values near those for the ATPase reaction at the low affinity sites. Conversely, the Km for nitrophenyl phosphate as a substrate for the phosphatase reaction was near its Ki as a competitor at the low-affinity sites of the ATPase reaction. These observations are incorporated into a reaction scheme with two classes of substrate sites on a dimeric enzyme, manifesting idverse enzymatic and transport characteristics.

Mechanisms by which Li+ stimulates the (Na+ and K+)-dependent ATPase

The addition of LiCl stimulated the (Na+ + K+)-dependent ATPase activity of a rat brain enzyme preparation. Stimulation was greatest in high Na+/low K+ media and at low Mg-ATP concentrations. Apparent affinities for Li+ were estimated at the alpha-sites (moderate-affinity sites for K+ demonstrable in terms of activation of the associated K+-dependent phosphatase reaction), at the beta-sites (high-affinity sites for K+ demonstrable in terms of activation of the overall ATPase reaction), and at the Na+ sites for activation. The relative efficacy of Li+ was estimated in terms of the apparent maximal velocity of the phosphatase and ATPase reactions when Li+ was substituted for K+, and also in terms of the relative effect of Li+ on the apparent Km for Mg-ATP. With these data, and previously determined values for the apparent affinities of K+ and Na+ at these same sites, quantitative kinetic models for the stimulation were examined. A composite model is required in which Li+ stimulates by relieving inhibition due to K+ and Na+ (i) by competing with K+ for the alpha-sites on the enzyme through which K+ decreases the apparent affinity for Mg-ATP and (ii) by competing with Na+ at low-affinity inhibitory sites, which may represent the external sites at which Na+ is discharged by the membrane Na+/K pump that this enzyme represents. Both these sites of action for Li+ would thus lie, in vivo, on the cell exterior.

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+.

Interactions between K+ and ATP binding to the (Na+ + K+)-dependent ATPase

K+ appears to decrease the affinity of the (Na+ + K+)-dependent ATPase (ATP phosphohydrolase, EC 3.6.1.3) for its substrate, Mg2+ - ATP, and Mg2+ - ATP, in turn, appears to decrease the affinity of the enzyme for K+. These antagonisms have been investigated in terms of a quantitative model defining the magnitude of the effects as well as identifying the class of K+ sites on the enzyme involved. K+ increased the apparent Km for Mg2+ - ATP, an effect that was antagonized competitively by Na+. The data can be fitted to a model in which Mg2+ - ATP binding is prevented by occupancy of alpha-sites on the enzyme by K+ (i.e. sites of moderate affinity for K+ accessible on the "free" non-phosphorylated enzyme, in situ on the external membrane surface). By contrast, occupancy of these alpha-sites by Na+ has no effect on Mg2+ - ATP binding to the enzyme. On the other hand, Mg2+ - ATP decreased the apparent affinity of the enzyme for K+ at the alpha-sites, in terms of (i) the KD for K+ measured by K+-accelerated inactivation of the enzyme by F-, and (ii) the concentration of K+ for half-maximal activation of the K+-dependent phosphatase reaction (which reflects the terminal hydrolytic steps of the overall ATPase reaction). These data fit the same quantitative model. Although this formulation does not support schemes in which ATP binding effects the release of transported K+ from discharge sites, it is consistent with observations that K+ can inhibit the enzyme at low substrate concentrations, and that Li+, which has poor efficacy when occupying these alpha-sites, can stimulate enzymatic activity at high K+ concentrations by displacing the inhibitory K+.