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

The effect of K+ on the equilibrium between the E2 and the K+-occluded E2 conformation of the (Na+ + K+)-ATPase

The rate of the transition from the E2 form to the E1 form of (Na+ + K+)-ATPase (ATP phosphohydrolase, EC 3.6.1.3) has been monitored by the fluorescence changes of eosin. The equilibrium between E1 and E2 is poised towards E2 in the absence of added cations. A stopped-flow tracing of the transition from E2 in the presence of 2 microM K+ (contamination) to E1 (in 150 mM Na+) is multiexponential with a large, rapidly decaying component (t 1/2 about 50 ms) and a smaller component which has a t 1/2 of about 2 s. KCl in microM concentrations decreases the amplitude of the rapidly decaying component and increases the amplitude of the slow component. The stopped-flow tracings can be satisfactorily fitted by a sum of three exponentials. An apparent Kd for K+ of about 5 microM is obtained for the conversion of the rapidly decaying component to the slowly decaying component. The experiments show that the E2 form is a mixture of at least two enzyme conformations. One E2 conformation - without K+ bound, (E2) - is transferred rapidly to the E1 conformation when Na+ is added, whereas the other E2-conformation--with K+ bound with an apparent high affinity, Kocc E2--is transferred slowly to the E1 conformation.

The effect of intracellular calcium on the sodium pump of human red cells

The inhibitory effect of cytoplasmic Ca on Na-pump-mediated Na-K exchange was investigated in intact red cells under conditions of constant cell volume, membrane potential and inorganic ion composition. The ionized cytoplasmic Ca concentration ( [Ca2+]i) was controlled using the ionophore A23187. In normal cells, ouabain-sensitive 24Na efflux was inhibited with an apparent affinity for [Ca2+]i which depended on the concentration of A23187; 50% inhibition required 20-40 microM and 160-300 microM-cytoplasmic Ca2+ with 10 microM and 0.63 microM-A23187 respectively. Cytoplasmic Ca also affected cell ATP content which fell rapidly on addition of A23187 and subsequently increased, steadied or continued to fall more slowly depending on the Ca and A23187 concentrations. Half-maximal fall required 5-15 microM and 110-170 microM-cytoplasmic Ca2+ at 10 microM and 0.63 microM-A23187 respectively. Removal of Ca from the cells failed to reverse either the Na pump inhibition or the fall in cell ATP. In ATP-enriched cells cytoplasmic Ca caused inhibition of ouabain-sensitive 24Na efflux in an A23187-dependent manner with apparent affinities for [Ca2+]i similar to those observed in the normal cells. Inhibition was complete at high [Ca2+]i. As in the normal cells, the ATP content of the cells fell in the presence of cytoplasmic Ca, but always remained above 1.2 m-mole/l. cells. This was higher than the ATP content of Ca-free normal intact cells. A23187 had no effect on the inhibition by Ca of ouabain-sensitive ATPase activity in isolated red cell membrane preparations. Both under conditions near optimal for Na-K-ATPase activity and under conditions resembling those in the cytoplasm, inhibition was half-maximal at about 25 microM-Ca2+ and in the latter case complete at below 400 microM-Ca2+. The apparent ATP-dependence of ouabain-sensitive Na efflux in the presence of cytoplasmic Ca was distinctly different in the normal and ATP-enriched cells but in both groups of cells it was similar for data obtained with high and low concentrations of A23187. The data for Na pump inhibition by cytoplasmic Ca in the intact cells were well fitted by several kinetic models involving either [Ca2+]i or CaATP as the inhibitory species and a low affinity dependence of pump activity on MgATP or total ATP. However, for any model, the apparent affinities for CaATP or for Ca2+ required to fit the ATPase data were 2.5-10 times higher than those required to fit the data for Na efflux.(ABSTRACT TRUNCATED AT 400 WORDS)

Modification of the conformational equilibria in the sodium and potassium dependent adenosinetriphosphatase with glutaraldehyde

Glutaraldehyde treatment of electroplax membrane preparations of Na,K-ATPase leads to irreversible changes in the enzymic behavior of the protein, which are not due to modification of the active site. When the glutaraldehyde treatment is carried out in a medium containing K+ and without Na+, the "K+-modified enzyme" so produced shows the following changes in enzymic properties: The steady-state phosphorylation by ATP and the rate of ATP-ADP exchange are decreased to approximately 40% of control, while Na,K-ATPase activity decreases to approximately 15% of control. Phosphatase activity is decreased very little, but the potassium activation parameters of the reaction are changed, from K0.5 approximately equal to 5 mM and nH = 1.9 in control to K0.5 approximately equal to 0.5 mM and nH = 1 in K+-modified enzyme. KI(app) for nucleotide inhibition of phosphatase activity is increased significantly. Changes in the cation dependence of the ATPase reaction are also observed. All of these effects can be explained by assuming that the cross-linking of surface groups in protein subunits when they are in conformation E2 shifts the intrinsic conformational equilibrium of the enzyme toward E2. We considered the simplest mathematical model for the coupling between K+ binding and the conformational equilibrium, with equivalent potassium sites that must be simultaneously in the same state. If one assumes that the potassium activation of phosphatase activity in the K+-modified enzyme reflects the affinity for K+ of E2, the behavior of the phosphatase activity in the native enzyme can be fit if there are only two potassium sites, whose affinity is 80-fold higher in E2 than in E1, and the equilibrium constant for E2 in equilibrium E1 is about 250. The same sites can explain the activation of dephosphorylation during ATP hydrolysis. Independent of the model chosen, potassium ions must be required for the catalytic action of form E2 and cannot be merely "allosteric activators". The enzyme modified with glutaraldehyde in a medium containing Na+ also has interesting properties, but their rationalization is less straightforward. The Na,K-ATPase activity is inhibited more than the "partial reactions", as in the K+-modified enzyme. We suggest that this is a generally expected result of modifications of the enzyme.

The effects of Na+ and K+ on the conformational transitions of (Na+ + K+)-ATPase

The equilibrium between the E1 form and the E2 form of the (Na+ + K+)-ATPase is poised towards E2 in the absence of cations when monitored by the fluorescence of eosin. Na+ converts the enzyme to the E1 form with a K0.5 of 1.9 mM (pH 7.4 and 2 microM eosin). The titration curves indicate that more than one Na+ is necessary. K0.5 for K+ for reversal of the Na+ effect is 0.8 mM with 10 mM Na+, 5.8 mM K+ with 30 mM Na+ and 37.7 mM with 100 mM Na+. In the absence of K+ the rate of transition from E2 to E1 is rapid when Na+ is added, t1/2 is about 50 ms at 6 degrees C (pH 7.4). K+ in low concentrations decreases the rate with a K0.5 for K+ of about 10 microM, and t1/2 is about 2 s at 1 mM K+. K+ in concentrations from 1 to 50 mM (the highest tested) increases the rate of transition from E2 to E1. E2 thus consists of a non-K+ form (E2), a K+ both with K+ bound with apparent high affinity, the 'occluded' form (KoccE2), and a form with K+ bound both at high and low affinity sites (KE2). K+ bound to the low-affinity sites facilitates the transition from KoocE2 to E1. So does an increase in the Na+ concentration, but Na+ is less effective than K+. The rate of transition from E1 to E2 is rapid when K+ is added to enzyme in 20 mM Na+. The rate increases with the K+ concentration, and the effect of K+ does not saturate at the concentrations of K+ sufficient to displace the equilibrium fully to E2. Na+ decreases the rate of transition from E1 to E2. A model is suggested for the effect of the cations on the transition between the different forms of the enzyme. The physiological implications of the effect of K+ at a low-affinity site are discussed.

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.

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.

Inhibition of the sodium pump in guinea-pig ventricular muscle by dihydro-ouabain: effects of external potassium and sodium

The inhibition of the electrogenic pump current in quiescent guinea-pig ventricular muscle by dihydro-ouabain (DHO) was studied with the three-micro-electrode voltage-clamp technique described previously (Daut, 1982c). From dose-response curves of the drug-induced current change (ID) the equilibrium dissociation constant of the binding of DHO to the Na-K pump (KD) and the electrogenic pump current flowing in the steady state (Ip) were inferred (Daut & Rüdel, 1982b). The external K concentration ([K]o) was varied between 2 and 4.5 mM (substituted by Na). KD was found to increase with increasing [K]o. A plot of log KD versus log [K]o gave a straight line with a slope of about 1.5. The time constants of the onset (tau on) and decay (tau off) of ID are supposed to represent the chemical kinetics of binding and unbinding of the drug (Daut & Rüdel, 1981, 1982b). Tau on was found to be inversely related to [K]o whereas tau off was found to be independent of [K]o. Ip was found to be independent of [K]o. This was interpreted to indicate that in the steady state Ip is mainly determined by the passive influx of Na into the cell, which may be relatively insensitive to small changes in [K]o. The effects of [K]o on the drug-induced current change are consistent with competitive inhibition of the binding of DHO to the Na-K pump. It is suggested that K ions and cardiac glycosides compete for extracellular binding sites on the same conformation of the Na-K pump. The external Na concentration ([Na]o) was varied between 147 and 49 mM (substituted by choline or Tris). Reduction of [Na]o produced a proportional decrease of Ip. This may be a consequence of the accompanying reduction of passive Na influx and the resulting decrease in intracellular Na activity (alpha iNa). Reduction of [Na]o markedly increased KD. This effect may be mediated by competition between Na and K at the K-loading sites of the pump and/or by separate modulatory Na-binding sites. It is concluded that the well known effects of external Na and K on the positive inotropic action of cardiac glycosides can be fully accounted for by the marked changes in the apparent binding affinity of the drug reported here.

Interaction of fluorescein isothiocyanate with the (H+ + K+)-ATPase

Fluorescein isothiocyanate was used to covalently label the gastric (H+ + K+)-ATPase. FITC treatment of the enzyme inhibited the ATPase activity while largely sparing partial reactions such as the associated p-nitrophenylphosphatase activity. ATP protected against inhibition suggesting the ligand binds at or near an ATP binding site. At 100% inhibition the stoichiometry of binding was 1.5 nmol FITC per mg Lowry protein a value corresponding to maximal phosphoenzyme formation. Binding occurred largely to a peptide of 6.2 isoelectric point, although minor labelling of a peptide of pI 5.6 was also noted. Fluorescence was quenched by K+, Rb+ and Tl+ in a dose-dependent manner, and the K0.5 values of 0.28, 0.83 and 0.025 mM correspond rather well to the values required for dephosphorylation at a luminal site. Vanadate, a known inhibitor of the gastric ATPase produced a slow Mg2+-dependent fluorescent quench. Ca2+ reversed the K+-dependent loss of fluorescence and inhibited it when added prior to K+. This may relate to the slow phosphorylation in the presence of ATP found when Ca2+ was substituted for Mg2+ and the absence of K+-dependent dephosphorylation. The results with FITC-modified gastric ATPase provide evidence for a conformational change with K+ binding to the enzyme.

(Na+,K+)-ATPase of mammalian brain: effects of temperatures on cation and ATP interactions regulating phosphatase activity

The effects of temperature on interactions between univalent cations or ATP and the p-nitrophenylphosphatase activity associated with brain (Na+,K+)-ATPase were examined. The apparent affinity for K+ activation under conditions favoring the moderate affinity site was temperature dependent, increasing with decreasing temperature. A comparison of univalent cations showed that the negative apparent delta H and delta S for cation binding increased with increasing apparent cation affinity. In contrast to the case with the moderate affinity sites, apparent affinity for the high affinity K+ site was independent of temperature. As temperature decreased, properties of moderate affinity site binding approached those of the high affinity site. The temperature dependence of ATP inhibition was opposite to that for K+ activation, with positive apparent delta H and delta S. The apparent delta H and delta S for cation binding approached those for the overall conformational change to K+-sensitive enzyme as cation affinity increased. These data suggest that E2, the K+-sensitive form of (Na+,K+)-ATPase, is stabilized by forces that require a decrease in entropy, explaining the predominant existence of E1 at physiologic temperatures. A conformational change leading to stabilization of E2 at higher temperatures can be produced by binding of univalent cations to a moderate affinity, presumably intracellular, site. This effect is counteracted by ATP. ATP also appears to alter the selectivity of this site to favor Na+ over K+ binding.

Stimulation and inhibition by ATP and orthophosphate of the potassium-potassium exchange in resealed red cell ghosts

The potassium:potassium (K-K) exchange through the sodium pump has been measured as the ouabain-sensitive 86Rb uptake by Na-free ghosts resealed to contain various concentrations of ATP, orthophosphate and K. The exchange is activated by increasing either internal or external K+ (Rb+) ion concentration. The activation curves can be described by simple Michaelis kinetics as: exchange = Vmax [K]/(Kapp + [K]). Increasing ATP concentration increases the apparent affinity for external K ions but decreases the apparent affinity for internal K (Ki+). Increasing [ATP] from 1 microM to 1 mM typically increases the Kapp for Ki+ from less than 1 mM to about 30 mM. Increasing ATP first activates the exchange but, after an optimal concentration is reached, further increase of ATP inhibits. The value of ATP concentration which gives the maximum flux depends on the internal and external K+ concentrations. The higher [Ki], the greater the optimal ATP concentration. Increasing external K (Rb) decreases the optimal ATP concentration. Increasing the concentration of orthophosphate (Pi) activates the exchange at high ATP but inhibits at low ATP concentration. A concentration of Pi which stimulates the exchange at high external K (Rb) can inhibit at low external K (Rb). These findings are in agreement with a consecutive or ping-pong model of the K-K exchange. We suggest that previous experiments have not shown the inhibitory effects of ATP and Pi because of the particular range of concentrations investigated.

Kinetic studies on the (Na+ + K+)-dependent ATPase evidence for coexisting sites for Na+, K+ and Mg2+

Na+-ATPase activity of a dog kidney (Na+ + K+)-ATPase enzyme preparation was inhibited by a high concentration of NaCl (100 mM) in the presence of 30 microM ATP and 50 microM MgCl2, but stimulated by 100 mM NaCl in the presence of 30 microM ATP and 3 mM MgCl2. The K0.5 for the effect of MgCl2 was near 0.5 mM. Treatment of the enzyme with the organic mercurial thimerosal had little effect on Na+ -ATPase activity with 10 mM NaCl but lessened inhibition by 100 mM NaCl in the presence of 50 microM MgCl2. Similar thimerosal treatment reduced (Na+ + K+)-ATPase activity by half but did not appreciably affect the K0.5 for activation by either Na+ or K+, although it reduced inhibition by high Na+ concentrations. These data are interpreted in terms of two classes of extracellularly-available low-affinity sites for Na+: Na+-discharge sites at which Na+-binding can drive E2-P back to E1-P, thereby inhibiting Na+-ATPase activity, and sites activating E2-P hydrolysis and thereby stimulating Na+-ATPase activity, corresponding to the K+-acceptance sites. Since these two classes of sites cannot be identical, the data favor co-existing Na+-discharge and K+-acceptance sites. Mg2+ may stimulate Na+-ATPase activity by favoring E2-P over E1-P, through occupying intracellular sites distinct from the phosphorylation site or Na+-acceptance sites, perhaps at a coexisting low-affinity substrate site. Among other effects, thimerosal treatment appears to stimulate the Na+-ATPase reaction and lessen Na+-inhibition of the (Na+ + K+)-ATPase reaction by increasing the efficacy of Na+ in activating E2-P hydrolysis.

Effect of magnesium ions on the high-affinity binding of eosin to the (Na+ + K+)-ATPase

(1) The fluorescence of eosin Y in the presence of (Na+ + K+)-ATPase is enhanced by Mg2+. The enhancement by Mg2+ is larger than that obtained with Na+ (Skou, J.C. and Esmann, M. (1981) Biochim. Biophys. Acta 647, 232-240). Mg2+ shifts the excitation maximum from 518 to 524 nm, the emission maximum from 538 to 542 nm. Also a shoulder appears at about 490 nm on the excitation curve, as was also observed with Na+. (2) The Mg2+-dependent enhancement of fluorescence can be reversed by K+ as well as by ATP. In the presence of Mg2+ + Pi (i.e. under conditions of phosphorylation), the fluorescence enhancement can be reversed by ouabain. With Mg2+ and a low concentration of K+ (i.e. conditions for vanadate binding), the enhancement of fluorescence can be reversed by vanadate. (3) There is a low-affinity binding of eosin which increases with the Mg2+ concentration. This is observed as a slight increase in the fluorescence when the excitation wavelength is above 520 nm. The low-affinity binding is K+-, ATP-, ouabain- and vanadate-insensitive. (4) Scatchard analysis of the binding experiments suggests that there are two high-affinity eosin-binding sites per 32P-labelling site in the presence of 5 mM Mg2+ both of which are ouabain-, vanadate- and ATP-sensitive. With 5 mM Mg2+ + 0.25 Pi, the Kd values are 0.14 microM and 1.3 microM, respectively. With 5 mM Mg2+, 150 mM Na+, the Kd values are 0.45 microM and 3.2 microM, respectively. With 5 mM Mg2+, the addition of K+ gives a pronounced decrease in affinity but does not decrease the number of binding sites (which remains at two per 32P-labelling site). With 5 mM Mg2+ + 150 mM K+, the affinities of the two binding sites become identical, at a Kd of 17 microM. (5) The rate of conformational transitions was measured using the stopped-flow method. The rate of the transition from the Mg2+-form to the K+-form is high. Oligomycin has only a small (if any) effect on the rate. Addition of Na+ in the presence of Mg2+ does not appreciably change the rate of conversion to the K+-form, giving a rate constant of about 110 s-1. However, the addition of oligomycin in the presence of Mg2+ + Na+ had a profound effect: the rate of conversion to the K+-form was decreased by a factor of 2000 to about 0.063 s-1. This suggests that the conformation with Mg2+ alone is different from the conformation with Na+ alone. (6) The effects of K+, ouabain, vanadate and ATP on the high-affinity binding of eosin suggest that the two eosin molecules bound per 32P-labelling site are bound to ATP sites.

Fluorescent labeling of (Na+ + K+)-ATPase by 5-iodoacetamidofluorescein

5-Iodoacetamidofluorescein (5-IAF) covalently labels dog kidney (Na+ + K+)-ATPase with approximately 2 moles incorporated per mole of enzyme. ATPase and K+-phosphatase activities are fully retained after reaction, and the kinetic parameters for Na+, K+, Mg2+, ATP and p-nitrophenyl phosphate are likewise not significantly affected. The fluorescence of the bound 5-IAF is increased by ATP, Na+, and Mg2+, and decreased by K+. These fluorescence changes likely reflect ligand-induced stabilization of the E1 or E2 states of the enzyme.

Occlusion of rubidium ions by the sodium-potassium pump: its implications for the mechanism of potassium transport

1. The occlusion of rubidium ions by Na, K-ATPase has been investigated by suspending enzyme prepared from pig kidney outer medulla in media containing low concentrations of (86)Rb, forcing the suspensions rapidly through small columns of cation-exchange resin, and measuring the amounts of radioactivity emerging from the columns.2. When the suspension media contained 2 mM-ATP or ADP, or 15 mM-NaCl, the amounts of radioactivity emerging from the columns were greatly (and similarly) reduced, presumably because both nucleotides and sodium ions stabilized the enzyme in the E(1) form. (See p. 19 for definition of E(1) and E(2)). The extra radioactivity carried through the columns when nucleotides and sodium were absent was taken as a measure of the amount of rubidium occluded within the enzyme (in the E(2) form) when it emerged from the resin.3. By varying the flow rate, and therefore the time spent by the enzyme on the resin, and relating this to the amount of radioactivity emerging from the columns, we have been able to estimate the rate constant for the conformational change (E(2) --> E(1)) that allows the occluded rubidium ions to escape. At 20 degrees C, and in the absence of nucleotides, it is about 0.1 S(-1).4. The rate constant for rubidium release was the same in a sodium-containing as in a potassium-containing medium. The opposite effects of sodium and potassium ions on the poise of the equilibrium between the E(1) and the E(2) forms of the enzyme must, therefore, be due solely to opposite effects of these ions on the rate of conversion of E(1) to E(2).5. The rate constant for rubidium release was greatly increased by ATP and by ADP. Both nucleotides appeared to act at low-affinity sites and without phosphorylating the enzyme.6. Orthovanadate, in the presence of magnesium ions, stabilized the enzyme in the occluded-rubidium (E(2)Rb) form.7. Ouabain, in the presence of magnesium ions, prevented the occlusion of rubidium ions.8. We have measured the amount of rubidium occluded by the enzyme as a function of rubidium concentration, and estimate that at saturating rubidium concentrations about three rubidium ions can be occluded per phosphorylation site (or per ouabain-binding site).9. We have found that the occluded-rubidium form of the enzyme can also be formed by allowing rubidium ions to catalyse the hydrolysis of phosphoenzyme generated by the addition of ATP to enzyme suspended in a high-sodium medium.10. The properties of the occluded-rubidium form of the enzyme, and of the two routes that can lead to its formation, suggest that an analagous occluded-potassium form plays a central role in the transport of potassium ions through the sodium-potassium pump. This hypothesis is supported by a detailed consideration of the probable magnitudes of the rate constants of the individual reactions making up the two routes.

The effect of pH, of ATP and of modification with pyridoxal 5-phosphate on the conformational transition between the Na+-form and the K+-form of the (Na+ +K+)-ATPase

An increase in pH decreases the Na+ concentration (Na+ +K+ = 150 mM) necessary for half-maximum activation of the (Na+ +K+)-ATPase at non-saturating concentrations of ATP just as an increase in the concentration of ATP at a given pH. It also decreases the concentration of Na+ necessary for transformation from the K+-form to the Na+-form at equilibrium conditions (Na+ +K+ = 150 mM). An increase in pH increases the rate of the transformation from the K+-form to the Na+-form of the system and decreases the rate of the reverse reaction. The pH effect on the conformation suggests that the K+-form is a protonated form and the Na+-form a deprotonated one. The similarity between the effect of an increase in pH with non-saturating concentrations of ATP and that of an increase in ATP at a given pH suggests that ATP exerts its effect on the transformation from the K+ - to the Na+-form by a decrease in pK values of the system, i.e., by releasing protons, a Bohr effect. Enzyme modified by reaction with pyridoxal 5-phosphate terminated by NaBH4 behaves at a given pH as if it were non-modified enzyme but at a higher pH. The 'pH effect' is seen after modification by pyridoxal 5-phosphate in the presence of ATP, of Na+ without and with ATP, of K+ with ATP but not in the presence of K+ alone. The modification has also a 'pH effect' on the rate of the transformation from the K+ -form to the Na+ -form and on the reverse reaction. There are at least two different pyridoxal 5-phosphate-reactive groups (amino groups), one which can be protected by ATP and which is of importance for activity and another which is not protected by ATP and which is of importance for the pH effect on the conformation. The effect of a protonation-deprotonation of amino groups on the conformation is explained by an involvement of the amino groups in salt bridge formation in between and inside the polypeptide chains, a hemoglobin-like situation. The protonated K+ -form is then a tense T-structure with a high K+, low Na+ affinity and the deprotonated Na+ -form a relaxed, R-structure with high Na+, low K+ affinity. ATP facilitates deprotonation by decreasing pK values. Oligomycin has 'pH effect' on the K0.5 for Na+ under equilibrium and steady-state conditions, but oligomycin has no effect on the rate of the transformation from the K+ -form to the Na+ -form, but gives a pronounced decrease of the rate of the reverse reaction, indicating that oligomycin does not react with the K+ -form but with the Na+ -form of the system and prevents the protonation, the E1 to E2 transformation.

ATPases: common and unique features within a group of enzymes

An attempt is made to survey ATPases with respect to features common to all or some of them and features peculiar to each individual enzyme of the group. Clues are presented for a tentative classification of ATPases and a simple system is suggested for the designation of interaction of ATPases with ions which is often used as the main feature of identification of individual ATPases.

Inhibition of the sodium pump by inorganic phosphate in resealed red cell ghosts

1. Ouabain-sensitive Rb influx was measured into K-free resealed red cell ghosts. The effects of inorganic phosphate (Pi) were examined. 2. Phosphate decreased the magnitude of the influx. Increasing Pi lowered the apparent affinity for both ATP and external rubidium ions. The effects of Pi on the affinity for external Rb were greatest at low ATP concentrations. 3. In the nominal absence of phosphate, increasing ATP from 1 to 100 microM had little effect on the Rb influx from solutions of low (1 microM) rubidium concentrations. In the presence of Pi, ATP increased Rb uptake markedly, even from solutions of low Rb concentration. 4. The above interactions between Pi, ATP and external Rb are consistent with a consecutive scheme for the Na pump in which phosphate is released after potassium binds at the external surface and before ATP binds to release potassium ions to the internal solution. 5. Previous failures to find an effect of phosphate on either the affinity for ATP or that for external potassium (rubidium) ions are shown to be equally consistent with the model. The lack of change of apparent affinity is shown to result from the restricted range of concentrations used in these previous experiments.

Effects of atp or phosphate on passive rubidium fluxes mediated by Na-K-ATPase reconstituted into phospholipid vesicles

1. The passive Rb fluxes mediated by the Na-K pump in reconstituted vesicles, described by Karlish & Stein (1982), are affected by ATP or by phosphate acting separately.2. Rb-Rb exchange through inside-out pumps is stimulated by ATP at low concentrations and is inhibited at high concentrations. There are mutual effects of Rb at cytoplasmic sites and ATP. The higher is the Rb concentration, the greater is the degree of stimulation and the less is the inhibition of exchange by ATP, and the higher are the concentrations of ATP required to produce effects. ATP stimulates Rb-Rb exchange maximally by about 5-fold.3. There are similar effects of ATP on zero-trans net Rb uptake through inside-out pumps. However, much lower degrees of stimulation and greater inhibition of the net flux by ATP are observed, and much lower concentrations of ATP are required for these effects, by comparison with those on Rb-Rb exchange.4. Rb uptake on inside-out pumps in Na-loaded vesicles shows only inhibition by ATP.5. Phosphate effects require the presence of Mg(0) ions. At low Mg(0) concentrations (up to 100 muM) phosphate moderately stimulates Rb uptake into Rb-free or Rb-loaded vesicles (about 50%), but has no effect on Rb uptake into Na-loaded vesicles. At millimolar concentrations of Mg(0) ions, phosphate strongly inhibits the Rb uptake into Rb-free or Na-loaded vesicles but has no effect on Rb uptake into Rb-loaded vesicles.6. The separate effects of ATP and of phosphate are explained in terms of the model proposed by Karlish & Stein (1982), modified to take into account stimulation of the conformational transition E(2)(Rb)(occ) --> E(1) Rb by ATP, and stimulation of the conformational transition E(2)(Rb)(occ) --> E(2) Rb by phosphate due to phosphorylation of the protein.

Combined effects of ATP and phosphate on rubidium exchange mediated by Na-K-ATPase reconstituted into phospholipid vesicles

1. Phospholipid vesicles reconstituted with Na-K-ATPase show an (ATP+phosphate)-stimulated Rb-Rb exchange, with properties similar to the K-K exchange of human red cells. This includes a rate 15-20% of the rate of active ATP-dependent Na-K exchange.2. We have studied activation of this Rb-Rb exchange by ATP at fixed phosphate concentrations and by phosphate at fixed ATP concentrations. It is found for both ATP and phosphate that with low concentrations of the fixed ligand an increase in concentration of the complementary ligand produces first stimulation and then inhibition of Rb-Rb exchange. At high concentrations of the fixed ligand the complementary ligand shows only saturation behaviour.3. The pattern of activation and of inhibition by ATP and by phosphate is affected by the Rb(0) concentration in the exterior medium, in that higher concentrations of Rb(0) counteract inhibitory effects of high concentrations of ATP and phosphate.4. (ATP+phosphate)-stimulated Rb-Rb exchange is activated by Rb(0) in the exterior medium along a sigmoid curve. An increase of Rb(i) within the vesicles, which raises the maximal velocity of Rb-Rb exchange, is accompanied by a smaller increase in the Rb(0) concentration required for half-maximal stimulation of the Rb-Rb exchange.5. The data are interpreted in terms of a model similar to those proposed by Karlish & Stein (1982a,b), but extended to include simultaneous effects of ATP and phosphate. Inhibitions by high concentration of ATP or phosphate arise as a result of stabilization of E(1) ATP or E(2)-P forms respectively, in the presence of low concentrations of the complementary ligand. With high concentrations of the fixed ligand, saturation behaviour of the varying ligand is observed because the occluded Rb forms become the dominant transport intermediates. The occluded Rb forms bind both ATP and phosphate weakly and independently. The effects of ATP together with phosphate are accounted for by a simple combination of their separate effects on the Rb-Rb exchange.6. We suggest that the functional role of the occluded Rb form E(2) (Rb)(occ) in active transport is to minimize passive cation leaks through the system and allow control of the direction of cation movements by binding of physiological ligands such as ATP or phosphate.

Passive rubidium fluxes mediated by Na-K-ATPase reconstituted into phospholipid vesicles when ATP- and phosphate-free

1. Phospholipid vesicles reconstituted with Na-K-ATPase from pig kidney, show slow passive pump-mediated (86)Rb fluxes in the complete absence of ATP and phosphate.2. The Rb fluxes are inhibited in vesicles prepared from enzyme pre-treated with either ouabain or vanadate ions. Rb fluxes through Na-K pumps oriented inside-out or right-side out by comparison with the normal cellular orientation can be distinguished by effects of vanadate on one or both sides of the vesicle.3. (86)Rb uptake into Rb-loaded vesicles represents a (86)Rb-Rb exchange. The maximal rate of exchange through inside-out and right-side out oriented pumps is equal, suggesting a random arrangement of the pumps across the vesicle membrane. This Rb-Rb exchange is half-saturated on inside-out and right-side out pumps at about 0.6 and 0.2 mM-external Rb respectively.4. (86)Rb uptake into Rb-free vesicles represents a net Rb flux. The Rb uptake through inside-out pumps has a maximal rate about equal to the Rb-Rb exchange, half-saturates at an external Rb concentration of roughly 0.5 mM, and shows evidence for co-operativity. Net Rb uptake through right-side out pumps is very slow, and half-saturates at roughly 0.1 mM external Rb.5. K ions at low concentrations in the exterior medium stimulate (86)Rb uptake, but at high concentrations, inhibit. Na ions in the exterior medium always inhibit (86)Rb uptake. The result suggests that K ions are transported in co-operative fashion together with Rb ions, while Na ions block the Rb fluxes.6. The presence of Rb congeners at the vesicle interior raises the (86)Rb uptake through inside-out pumps with the decreasing order of effectiveness: Li > Na > Cs > K > Rb. Stimulation by Na ions involves a Rb-Na exchange.7. Turnover numbers were estimated from parallel measurement of Na/K pump mediated fluxes and amount of covalent phosphoenzyme. In units of moles of ion per mole of phosphoenzyme per second at 20 degrees C the following values were obtained: ATP-dependent Na-Rb exchange, 43; (ATP+phosphate)-stimulated Rb-Rb exchange, 7. For (ATP+phosphate)-independent fluxes: Rb-Rb exchange 0.25; net Rb uptake 0.15 and Rb-Na exchange 0.65.8. Mg ions in the exterior medium inhibited both net and exchange Rb fluxes through inside-out pumps in a manner antagonistic with respect to Rb. Mg and vanadate ions inhibit the Rb fluxes in a synergistic fashion.9. The results are interpreted in terms of a model in which net and exchange (86)Rb fluxes occur via conformational transitions between form E(1) which binds Rb at the cytoplasmic face of the protein, the form E(2) (Rb)(occ) containing occluded Rb ions and a form E(2) which binds Rb at the extracellular face of the protein. A kinetic analysis allows us to identify rate-limiting steps of the transport cycle by making use of our transport data in combination with values of rate-constants for conformational transitions observed directly in isolated Na-K-ATPase.

Vanadate binding to the (Na + K)-ATPase

A particulate (Na + K)-ATPase preparation from dog kidney bound [48V]-ortho-vanadate rapidly at 37 degrees C through a divalent cation-dependent process. In the presence of 3 mM MgCl2 the Kd was 96 nM; substituting MnCl2 decreased the Kd to 12 nM but the maximal binding remained the same, 2.8 nmol per mg protein, consistent with 1 mol vanadate per functional enzyme complex. Adding KCl in the presence of MgCl2 increased binding, with a K0.5 for KCl near 0.5 mM; the increased binding was associated with a drop in Kd for vanadate to 11 nM but with no change in maximal binding. Adding NaCl in the presence of MgCl2 decreased binding markedly, with an I50 for NaCl of 7 mM. However, in the presence of MnCl2 neither KCl nor NaCl affected vanadate binding appreciably. Both the nonhydrolyzable, beta, gamma-imido analog of ATP and nitrophenyl phosphate, a substrate for the K-phosphatase reaction that this enzyme also catalyzes, decreased vanadate binding at concentrations consistent with their acting at the low-affinity substrate site of the enzyme, the presence of KCl increased the concentration of each required to decrease vanadate binding. Oligomycin decreased vanadate binding in the presence of MgCl2, whereas dimethyl sulfoxide and ouabain increased it. With inside-out membrane vesicles from red blood cells vanadate inhibited both the K-phosphatase and (Na + K)-ATPase reactions; however, with the K-phosphatase reaction extravesicular K+ (corresponding to intracellular K+) both stimulated catalysis and augmented vanadate inhibition, whereas with the (Na + K)-ATPase reaction intravesicular K+ (corresponding to extracellular K+) both stimulated catalysis and augmented vanadate binding.

Radiation inactivation of (Na+ + K+)-ATPase. A small target size for the K+-occluding mechanism

Radiation inactivation of partially purified (Na+ + K+)-ATPase (ATP phosphohydrolase, EC 3.6.1.3) from pig kidney outer medulla shows that the target size for Rb+ occlusion by the enzyme (in the absence of phosphorylation) is much smaller than the target size for p-nitrophenyl phosphatase activity, which is itself smaller than the reported target size for (Na+ + K+)-ATPase activity.

The steady-state kinetic mechanism of ATP hydrolysis catalyzed by membrane-bound (Na+ + K+)-ATPase from ox brain

The expressions for the kinetic constants corresponding to the steady state model for hydrolysis of ATP catalyzed by (Na+ + K+)-ATPase proposed recently are analyzed with the object of determining the rate constants. The theoretical background for the necessary procedures is described. The results of this analysis are: (1) A small class (four) of rate constants are determined directly by the previously published values of the kinetic constants. (2) For a somewhat larger class of rate constants upper and lower bounds may be established. For several rate constants the upper and lower bounds differ by less than a factor 1.6 (for the "(Na+ + K+)-enzyme", i.e. the enzyme activity with K+ and millimolar substrate concentration) and 1.2 (for the "Na+-enzyme",i.e. the activity at micromolar substrate concentrations). (3) Experiments on inhibition by K+ of the Na+-enzyme at various Mg2+ concentrations are reported and analyzed. With the additional assumption that the rate constants governing the addition to ATP of Mg2+ is independent of whether or not ATP is bound to an enzyme molecule, a set of consistent values for all the 23 rate constants in the mechanism may be obtained. (4) The values of some rate constants lend further support to the contention discussed in a previous paper that the enzyme hydrolyzes ATP along two kinetically distinct pathways, depending on the presence of K+ and on the concentration of substrate, without the necessity of having more than one active substrate site per enzyme unit at any time. (5) The results show that while the two enzyme forms, the "Na+-enzyme" E1 and the "K+-enzyme" E2K, add substrate with (second order) rate constants of the same order of magnitude (differing only by a factor of four in favor of the former), the rate constants for the reverse processes differ by a factor of 100, being largest for the K+-enzyme. This is the main reason for the large difference in the Michaelis constants for the two forms reported previously. (6) Compatibility of the model with the well-known rapid dephosphorylation of the phosphorylated enzyme in the presence of K+ requires the presence, at non-zero steady state concentration, of an enzyme-potassium-phosphate intermediate, which is acid labile and is therefore not detected as a phosphorylated enzyme using conventional methods.

Interpretation of current-voltage relationships for "active" ion transport systems: I. Steady-state reaction-kinetic analysis of class-I mechanisms

This paper develops a simple reaction-kinetic model to describe electrogenic pumping and co- (or counter-) transport of ions. It uses the standard steady-state approach for cyclic enzyme- or carrier-mediated transport, but does not assume rate-limitation by any particular reaction step. Voltage-dependence is introduced, after the suggestion of Läuger and Stark (Biochim. Biophys. Acta 211:458-466, 1970), via a symmetric Eyring barrier, in which the charge-transit reaction constants are written as k12 = ko12 exp(zF delta psi/2RT) and k21 = ko21 exp(-zF delta psi/2RT). For interpretation of current-voltage relationships, all voltage-independent reaction steps are lumped together, so the model in its simplest form can be described as a pseudo-2-state model. It is characterized by the two voltage-dependent reaction constants, two lumped voltage-independent reaction constants (k12, k21), and two reserve factors (ri, ro) which formally take account of carrier states that are indistinguishable in the current-voltage (I-V) analysis. The model generates a wide range of I-V relationships, depending on the relative magnitudes of the four reaction constants, sufficient to describe essentially all I-V datas now available on "active" ion-transport systems. Algebraic and numerical analysis of the reserve factors, by means of expanded pseudo-3-, 4-, and 5-state models, shows them to be bounded and not large for most combinations of reaction constants in the lumped pathway. The most important exception to this rule occurs when carrier decharging immediately follows charge transit of the membrane and is very fast relative to other constituent voltage-independent reactions. Such a circumstance generates kinetic equivalence of chemical and electrical gradients, thus providing a consistent definition of ion-motive forces (e.g., proton-motive force, PMF). With appropriate restrictions, it also yields both linear and log-linear relationships between net transport velocity and either membrane potential or PMF. The model thus accommodates many known properties of proton-transport systems, particularly as observed in "chemiosmotic" or energy-coupling membranes.

Role of the plasma membrane proton pump in pH regulation in non-animal cells

Possible methods by which eukaryotic cells can regulate intracellular pH (pHi) in response to experimental acid loading were investigated by using as a model cell the fungus Neurospora. Attention was focused on the role of membrane transport in such regulation, starting from the fact that this organism possesses a powerful electrogenic proton extrusion pump. Intracellular acidification was forced by introducing butyric acid into the recording medium, and subsequent changes in pHi and membrane potential were determined with intracellular microelectrodes. In separate experiments, membrane current-voltage curves were obtained and resolved--by an explicit kinetic model--into distinct pump and leak components. Decreased pHi causes increased outward pumping of H+ ions, in a manner quantitatively consistent with their role as a substrate for the proton pump. This increased pumping is often manifest as a transient hyperpolarization at the onset of cytoplasmic acidification. With a considerably slower time course, decreased pHi also produces a large increase in membrane leak conductance, which brings about net membrane depolarization and further stimulates the pump (by virtue of the reduced back electromotive force). Although the identity of the ion responsible for increased leak conductance is not yet known, the evident modulation of conductance seemingly plays an important role in stabilizing the intracellular pH: Stimulation of the pump alone would have little net effect on pHi because it would result simply in enhanced backflux of H+ (to which the membrane is most permeable in normal circumstances). An increased leak to nonprotons, however, would allow the pump to accomplish net H+ ejection.

Molecular considerations relevant to the mechanism of active transport

A small group of closely related proteins is responsible for all active transport in animal cells, and inorganic cations are the only substances transported by these enzymes. They share a common kinetic mechanism in which two fundamental conformations participate, each receiving and dispatching substrates from its unique side of the membrane. During transport, the cations must pass through their enzyme to cross the membrane and intense interest is currently focused on the possibility that the path which they follow lies within the interface between two discrete subunits in a dimeric structure. Although 'half-of-sites' behaviour, consistent with this hypothesis, has been reported, it is now known that systematic errors were responsible for this mistaken conclusion. The number of protomers which comprise a functional unit of active transport has not been determined.

Mechanistic implications of the potassium-potassium exchange carried out by the sodium-potassium pump

1. The magnitude of the K-K exchange carried out by the Na pump in human red cells was measured as a function of the external K concentration in cells with high and low intracellular K. The apparent K1/2 for external K and the Vm increased by the same proportion when intracellular K was raised. The result suggests that the K-K exchange is part of a ping-pong reaction mechanism. 2. The velocity of the exchange increases monotonically with intracellular phosphate and ATP concentration; neither substrate inhibits at very high concentration. 3. These results seem to require that the enzyme species which carries out the exchange is both phosphorylated and combined with ATP.

Ligand-dependent reactivity of (Na+ + K+)-ATPase with showdomycin

Showdomycin inhibited pig brain (Na+ + K+)-ATPase with pseudo first-order kinetics. The rate of inhibition by showdomycin was examined in the presence of 16 combinations of four ligands, i.e., Na+, K+, Mg2+ and ATP, and was found to depend on the ligands added. Combinations of ligands were divided into five groups in terms of the magnitude of the rate constant; in the order of decreasing rate constants these were: (1) Na+ + Mg2+ + ATP, (2) Mg2+, Mg2+ + K+, K+ and none, (3) Na+ + Mg2+, Na+, K+ + Na+ and Na+ + K+ + Mg2+, (4) Mg2+ + K+ + ATP, K+ + ATP and Mg2+ + ATP, (5) K+ + Na + + ATP, Na+ + ATP, Na+ + K+ + Mg2+ + ATP and ATP. The highest rate was obtained in the presence of Na+, Mg2+ and ATP. The apparent concentrations of Na+, Mg2+ and ATP for half-maximum stimulation of inhibition (KS0.5) were 3 mM, 0.13 mM and 4 MicroM, respectively. The rate was unchanged upon further increase in Na+ concentration from 140 to 1000 mM. The rates of inhibition could be explained on the basis of the enzyme forms present, including E1, E2, ES, E1-P and E2-P, i. e., E2 has higher reactivity with showdomycin than E1, while E2-P has almost the same reactivity as E1-P. We conclude that the reaction of (Na+ + K+)- ATPase proceeds via at least four kinds of enzyme form (E1, E2, E1 . nucleotide and EP), which all have different conformations.

The magnesium dependence of sodium-pump-mediated sodium-potassium and sodium-sodium exchange in intact human red cells

1. The magnesium content of human red blood cells was controlled by varying the magnesium concentration in the medium in the presence of the ionophore A23187. The new magnesium levels attained were very stable, which allowed the magnesium dependence of the sodium pump to be investigated.2. The effects of magnesium were shown to occur at the inner surface of the red cell membrane for the range of magnesium concentrations tested (10(-7) to 6 x 10(-3)m).3. At intracellular ionized magnesium concentrations below 0.8 mm the activation of ouabain-sensitive sodium-potassium exchange by internal ionized magnesium could be resolved into two or three components: (a) a small component, about 5% of the maximum flux, which is apparently independent of the ionized magnesium concentration below 2 mum, (b) a saturating component with a K((1/2)) of between 30 and 45 mum, and possibly (c) a component which increases linearly with ionized magnesium concentration and which only becomes apparent at concentrations above 0.1 mm.4. At intracellular ionized magnesium concentrations below 0.8 mm, activation of ouabain-sensitive sodium-sodium exchange by internal ionized magnesium could be resolved into two components: (a) a small component, about 6% of the maximal flux, which is apparently independent of the ionized magnesium concentration below 2 mum, and (b) a saturating component with a K((1/2)) of about 9 mum. At ionized magnesium concentrations between about 0.2 and 0.8 mm the rate of sodium-sodium exchange remained constant at the maximal level.5. The intracellular concentration of ATP decreased and the ADP concentration increased as the magnesium content of the cells was reduced from the normal level. A small increase in ATP and a small decrease in ADP was seen when the magnesium content was increased above the normal level. The variation in the ATP: ADP ratio from 2.5 at very low magnesium levels to about 6 at normal magnesium levels can account, at least in part, for the different K((1/2)) values of sodium-potassium and sodium-sodium exchange.6. When the concentration of ionized magnesium was increased above about 0.8 mm both sodium-potassium and sodium-sodium exchange were inhibited. Sodium-sodium exchange was more strongly inhibited than sodium-potassium exchange.7. The possible sites of action of magnesium in the sodium pump cycle are discussed.