Effects of mono and divalent cations on total and partial reactions catalysed by pig kidney Na,K-ATPase

Divalent cation interactions with Na,K-ATPase cytoplasmic cation sites: implications for the para-nitrophenyl phosphatase reaction mechanism

The interactions of divalent cations with the adenosine triphosphatase (ATPase) and para-nitrophenyl phosphatase (pNPPase) activity of the purified dog kidney Na pump and the fluorescence of fluorescein isothiocyanate (FITC)-labeled pump were determined. Sr(2+) and Ba(2+) did not compete with K(+) for ATPase (an extracellular K(+) effect). Sr(2+) and Ba(2+) did compete with Na(+) for ATPase (an intracellular Na(+) effect) and with K(+) for pNPPase (an intracellular K(+) effect). These results suggest that Ba(2+) or Sr(2+) can bind to the intracellular transport site, yet neither Ba(2+) nor Sr(2+) was able to activate pNPPase activity; we confirmed that Ca(2+) and Mn(2+) did activate. As another measure of cation binding, we observed that Ca(2+) and Mn(2+), but not Ba(2+), decreased the fluorescence of the FITC-labeled pump; we confirmed that K(+) substantially decreased the fluorescence. Interestingly, Ba(2+) did shift the K(+) dose-response curve. Ethane diamine inhibited Mn(2+) stimulation of pNPPase (as well as K(+) and Mg(2+) stimulation) but did not shift the 50% inhibitory concentration (IC(50)) for the Mn(2+)-induced fluorescence change of FITC, though it did shift the IC(50) for the K(+)-induced change. These results suggest that the Mn(2+)-induced fluorescence change is not due to Mn(2+) binding at the transport site. The drawbacks of models in which Mn(2+) stimulates pNPPase by binding solely to the catalytic site vs. those in which Mn(2+) stimulates by binding to both the catalytic and transport sites are presented. Our results provide new insights into the pNPPase kinetic mechanism as well as how divalent cations interact with the Na pump.

Magnesium deficiency in critical illness

Magnesium (Mg) deficiency commonly occurs in critical illness and correlates with a higher mortality and worse clinical outcome in the intensive care unit (ICU). Magnesium has been directly implicated in hypokalemia, hypocalcemia, tetany, and dysrhythmia. Moreover, Mg may play a role in acute coronary syndromes, acute cerebral ischemia, and asthma. Magnesium regulates hundreds of enzyme systems. By regulating enzymes controlling intracellular calcium, Mg affects smooth muscle vasoconstriction, important to the underlying pathophysiology of several critical illnesses. The principle causes of Mg deficiency are gastrointestinal and renal losses; however, the diagnosis is difficult to make because of the limitations of serum Mg levels, the most common assessment of Mg status. Magnesium tolerance testing and ionized Mg2+ are alternative laboratory assessments; however, each has its own difficulties in the ICU setting. The use of Mg therapy is supported by clinical trials in the treatment of symptomatic hypomagnesemia and preeclampsia and is recommended for torsade de pointes. Magnesium therapy is not supported in the treatment of acute myocardial infarction and is presently undergoing evaluation for the treatment of severe asthma exacerbation, for the prevention of post-coronary bypass grafting dysrhythmias, and as a neuroprotective agent in acute cerebral ischemia.

Breakdown of Na+/K+-exchanging ATPase phosphoenzymes formed from ATP and from inorganic phosphate during Na+-ATPase activity

The reactivity towards Na+ and K+ of Na+/K+-ATPase phosphoenzymes formed from ATP and Pi during Na+-ATPase turnover and that obtained from Pi in the absence of ATP, Na+ and K+ was studied. The phosphoenzyme formed from Pi in the absence of cycling and with no Na+ or K+ in the medium showed a biphasic time-dependent breakdown. The fast component, 96% of the total EP, had a decay rate of about 4 s(-1) in K+-free 130 mm Na+, and was 40% inhibited by 20 mm K+. The slow component, about 0.14 s(-1), was K+ insensitive. Values for the time-dependent breakdown of the phosphoenzymes obtained from ATP and from Pi during Na+-ATPase activity were indistinguishable from each other. In K+-free medium containing 130 mm Na+, the decays followed a single exponential with a rate constant of 0.45 s(-1). The addition of 20 mm K+ markedly increased the decays and made them biphasic. The fast components had a rate of approximately 220 s-1 and accounted for 92-93% of the total phosphoenzyme. The slow components decayed at a rate of about 47-53 s(-1). A second group of experiments examined the reactivity towards Na+ of the E2P forms obtained with ATP and Pi when the enzyme was cycling. In both cases, the rate of dephosphorylation was a biphasic function of [Na+]: inhibition at low [Na+], with a minimum at about 5 mm Na+, followed by recovery at higher [Na+]. Although qualitatively similar, the phosphoenzyme formed from Pi showed slightly less inhibition and more pronounced recovery. These results indicate that forward and backward phosphorylation during Na+-ATPase turnover share the same intermediates.

ATP-ADP exchange reaction catalyzed by Na+,K+-ATPase: dephosphorylation by ADP of the E1P enzyme form

We studied the effects of Mg2+ and of ADP and other nucleoside diphosphates on the dephosphorylation of the E1P form of the partially purified pig kidney Na+,K+-ATPase at 20-22 degrees C. We report for the first time the rate of the reversal of ATP phosphorylation. The experiments were done on enzyme subjected to controlled chymotrypsin digestion consisting of a homogenous population of a truncated catalytic subunit. Under this condition the whole cycle is E1 <-- (f1.ATP, b1) --> E1ATP <-- (f2, b2) --> E1P.ADP <-- (fd, bd.ADP) --> E1P-(f3) --> E1. The values of f1, b1, f2, and f3 were independently estimated in the absence of ADP; those of fd, bd, and b2 were obtained from the fit of ADP-dependent dephosphorylation data to the differential equation set. When f2 = 0 or b1 is very large, the model predicts that dephosphorylation by ADP gives a single exponential; in all other cases it predicts a biphasic dephosphorylation in a semilogarithmic plot. The fast phase is governed by b2.ADP and the slow one by b1. This was experimentally verified. Also, ADP stimulates E1P breakdown without release of Pi, thus leading to ATP synthesis. The data indicate that the true substrate for ATP synthesis is free ADP, while Mg2+ inhibits mainly by a reduction in the free [ADP]; in addition, E1P has a very low affinity for MgADP. The nucleotide structure is also very important; all ADP analogues tested were much less effective than ADP due to a reduced affinity for the E1P and a poor capacity to reverse phosphorylation.

Phosphoryl Group Exchange between ATP and ADP Catalyzed by H+-ATPase from Oat Roots

ATP-ADP exchange was estimated in the presence of plasma membrane H+-ATPase of oat (Avena sativa) roots partially purified with Triton X-100 by measuring [14C]ATP formation from [14C]ADP. Most studies were done at 0[deg]C. At pH 6.0 the exchange showed: (a) Mg2+ requirement with a biphasic response giving maximal activity at 152 [mu]M and (b) insensitivity to ionic strength, [Na+], and [K+]. ATP and ADP dependence were analyzed with a model in which nucleotide-enzyme interactions are at rapid-random equilibrium, whereas E1ATP [left right arrow] E1P-ADP transitions occur in steady state. The results indicated competition between ADP and ATP for the catalytic site, whereas ATP interaction with the ADP site was extremely weak. At 0[deg]C the exchange showed a 3-fold pH increase, from pH 5.5 to 9.0. At an alkaline pH the reaction was not affected by sodium azide and carbonyl cyanide p-trifluometoxyphenyl-hydrazone, had a biphasic response to Mg2+ (maximal at 513 [mu]m), and was insensitive to ionic strength. At 20[deg]C ATP-ADP exchange was pH insensitive. At both temperatures ATP hydrolysis displayed a bell-shaped response, with a maximum around pH 6.0 to 6.5. Because no adenylate kinase activity was detected under any condition, these results demonstrate the existence of an ATP-ADP exchange reaction catalyzed by the plant H+-ATPase.

Effects of magnesium and ATP on pre-steady-state phosphorylation kinetics of the Na+,K(+)-ATPase

The aim of the present work was to elucidate the role played by ATP and Mg2+ ions in the early steps of the Na+,K(+)-ATPase cycle. The approach was to follow pre-steady-state phosphorylation kinetics in Na(+)-containing K(+)-free solutions under variable ATP and MgCl2 concentrations. The experiments were performed with a rapid mixing apparatus at 20 +/- 2 degrees C. The concentrations of free and complexes species of Mg2+ and ATP were calculated on the basis of a dissociation constant of 0.091 +/- 0.004 mM, estimated with Arsenazo III under identical conditions. A simplified scheme were ATP binds to the ENa enzyme, which is phosphorylated to MgEPNa and consequently dephosphorylated returning to the ENa form, was used. In the absence of ADP and phosphate four rate constants are relevant: k1 and k-1, the on and off rate constants for ATP binding; k2, the transphosphorylation rate constant and k3, the constant that governs the dephosphorylation rate. The values obtained were: k1 = 0.025 +/- 0.003 microM-1 ms-1 for both free ATP and ATPMg; k-1 = 0.038 +/- 0.004 ms-1 for free ATP and 0.009 +/- 0.002 ms-1 for ATPMg; k2 = 0.199 +/- 0.005 ms-1; k3 = 0.0019 +/- 0.0002 ms-1. The model that seems best to explain the data is one where (i) the role of true substrate can be played equally well by free ATP or ATPMg, and (ii) free Mg2+, an essential activator, acts by binding to a specific Mg2+ site on the enzyme molecule.

Role of protein conformation changes and transphosphorylations in the function of Na+/K(+)-transporting adenosine triphosphatase: an attempt at an integration into the Na+/K+ pump mechanism

The particular aim of the review on some basic facets of the mechanism of Na+/K(+)-transporting ATPase (Na/K-ATPase) has been to integrate the experimental findings concerning the Na(+)- and K(+)-elicited protein conformation changes and transphosphorylations into the perspective of an allosterically regulated, phosphoryl energy transferring enzyme. This has led the authors to the following summarizing evaluations. 1. The currently dominating hypothesis on a link between protein conformation changes ('E1 in equilibrium with E2') and Na+/K+ transport (the 'Albers-Post scheme') has been constructed from a variety of partial reactions and elementary steps, which, however, do not all unequivocally support the hypothesis. 2. The Na(+)- and K(+)-elicited protein conformation changes are inducible by a variety of other ligands and modulatory factors and therefore cannot be accepted as evidence for their direct participation in effecting cation translocation. 3. There is no evidence that the 'E1 in equilibrium with E2' protein conformation changes are moving Na+ and K+ across the plasma membrane. 4. The allosterically caused ER in equilibrium with ET ('E1 in equilibrium with E2') conformer transitions and the associated cation 'occlusion' in equilibrium with 'de-occlusion' processes regulate the actual catalytic power of an enzyme ensemble. 5. A host of experimental variables determines the proportion of functionally competent ER enzyme conformers and incompetent ET conformers so that any enzyme population, even at the start of a reaction, consists of an unknown mixture of these conformers. These circumstances account for the occurrence of contradictory observations and apparent failures in their comparability. 6. The modelling of the mechanism of the Na/K-ATPase and Na+/K+ pump from the results of reductionistically designed experiments requires the careful consideration of the physiological boundary conditions. 7. Na+ and K+ ligandation of Na/K-ATPase controls the geometry and chemical reactivity of the catalytic centre in the cycle of E1 in equilibrium with E2 state conversions. This is possibly effected by hinge-bending, concerted motions of three adjacent, intracellularly exposed peptide sequences, which shape open and closed forms of the catalytic centre in lock-and-key responses. 8. The Na(+)-dependent enzyme phosphorylation with ATP and the K(+)-dependent hydrolysis of the phosphoenzyme formed are integral steps in the transport mechanism of Na/K-ATPase, but the translocations of Na+ and K+ do not occur via a phosphate-cation symport mechanism.(ABSTRACT TRUNCATED AT 400 WORDS)

Physiology of magnesium metabolism and the important role of magnesium in potassium deficiency

Magnesium, the second most abundant intracellular cation, has several critically important roles in the body. In addition to energy production and maintaining electrolyte balance, magnesium is essential for normal neuromuscular function as well as calcium and potassium transport. Evidence suggests that a deficit of magnesium is closely interrelated to potassium deficiency and refractory potassium repletion. Although the consequences of hypokalemia are widely documented and recognized, it is only recently that the importance of magnesium deficiency as a cause of potassium depletion has gained clinical attention. Because of the association between hypokalemia and ventricular ectopy/sudden death, familiarity with the causes of magnesium loss, as well as enhanced identification and treatment, appear to be important.

The sided action of Na+ on reconstituted shark Na+/K+-ATPase engaged in Na+-Na+ exchange accompanied by ATP hydrolysis. II. Transmembrane allosteric effects on Na+ affinity

The objective of the present investigation was to characterize the ATP-dependent Na+-Na+ exchange, with respect to cation sensitivity on the two aspects of the Na+/K+-pump protein. In order to accomplish this, we used Na+/K+-ATPase reconstituted with known orientation in the proteoliposomes. Activation by cytoplasmic Na+ shows cooperative interaction between three sites. The apparent intrinsic site constants displayed transmembrane dependence on the extracellular Na+ concentration. However, the apparent K0.5 for cytoplasmic Na+ is independent of the extracellular Na+ concentration. The activation by extracellular Na+ at a fixed cytoplasmic Na+ concentration is biphasic with a component which saturates at a concentration of about 1-2 mM extracellular Na+, a plateau phase up to 20 mM, and another component which tends to saturate at about 80 mM followed by a slight deactivation at higher concentrations of Na+. The apparent K0.5 value for extracellular Na+ is also found to be independent of the Na+ concentration on the opposite side of the membrane. The activation by extracellular Na+ can be explained by the negative cooperativity in the binding of extracellular Na+, but positive cooperativity in the rate of dephosphorylation of enzyme species with one and three sodium ions bound extracellularly. Na+ bound to E2-PNa has a transmembrane effect on the cooperativity between binding of cytoplasmic Na+, and E2-PNa2 does not dephosphorylate. K0.5/Vm for cytoplasmic as well as for extracellular Na+ decreases with an increase in the trans Na+ concentration in the non-saturating concentration range. The experiments indicate that at a step in the reaction simultaneous binding of extracellular and cytoplasmic Na+ occurs.

Binding of manganese ions to the Na+/K+-ATPase during phosphorylation by ATP

The aim of the present work was to study the Mg2+-Na+/K+-ATPase interaction that was proposed to lead to the formation of a stable Mg-enzyme complex during phosphorylation from ATP. Instead of Mg we used Mn, which can replace Mg as essential activator of Na+/K+-ATPase activity. The amounts of steady-state Mn bound to the enzyme were estimated at 0 degree C on the basis of the 54Mn remaining in the effluent after passing the reaction mixture through a cation exchange resin column. As a function of the MnCl2 concentration, the amount of Mn retained by the enzyme in the absence and presence of ATP showed a saturable and a linear component; the slope of the linear component was the same in both instances (0.016 nmol/mg per microM). The ATP-dependent Mn binding could be adjusted to a hyperbolic function with a Km of 0.76 microM. The ratio [ATP-dependent E-Mn]/[E-P] measured at 5 microM MnCl2 and 5 microM ATP was not different from 1.0, both in native (Mn-E2-P) as well as in a chymotrypsin treated enzyme (Mn-E1-P). When the Mn.E-P complex was allowed to react with KCl (E2-P form) or ADP (E1-P form), the enzyme was dephosphorylated and simultaneously lost the strongly bound Mn in such a way that the ratio [ATP-dependent E-Mn]/[E-P] remained 1:1. These results show the existence of strongly bound Mn ions to Na+/K+-ATPase during phosphorylation by ATP. That binding is (i) of high affinity for Mn, (ii) probably on a single site, and (iii) with a stoichiometry Mn-Pi of 1:1.

Some total and partial reactions of Na+/K+-ATPase using ATP and acetyl phosphate as a substrate

Acetyl phosphate, as a substrate of (Na+ + K+)-ATPase, was further characterized by comparing its effects with those of ATP on some total and partial reactions carried out by the enzyme. In the absence of Mg2+ acetyl phosphate could not induce disocclusion (release) of Rb+ from E2(Rb); nor did it affect the acceleration of Rb+ release by non-limiting concentrations of ADP. In K+-free solutions and at pH 7.4 sodium ions were essential for ATP hydrolysis by (Na+ + K+)-ATPase; when acetyl phosphate was the substrate a hydrolysis (inhibited by ouabain) was observed in the presence and absence of Na+. In liposomes with (Na+ + K+)-ATPase incorporated and exposed to extravesicular (intracellular) Na+, acetyl phosphate could sustain a ouabain-sensitive Rb+ efflux; the levels of that flux were similar to those obtained with micromolar concentrations of ATP. When the liposomes were incubated in the absence of extravesicular Na+ a ouabain-sensitive Rb+ efflux could not be detected with either substrate. Native (Na+ + K+)-ATPase was phosphorylated at 0 degrees C in the presence of NaCl (50 mM for ATP and 10 mM for acetyl phosphate); after phosphorylation had been stopped by simultaneous addition of excess trans-1,2-diaminocyclohexane-N,N,N',N' tetraacetic acid and 1 M NaCl net synthesis of ATP by addition of ADP was obtained with both phosphoenzymes. The present results show that acetyl phosphate can fuel the overall cycle of cation translocation by (Na+ + K+)-ATPase acting only at the catalytic substrate site; this takes place via the formation of phosphorylated intermediates which can lead to ATP synthesis in a way which is indistinguishable from that obtained with ATP.

Distinction between the intermediates in Na+-ATPase and Na+,K+-ATPase reactions. II. Exchange and hydrolysis kinetics at micromolar nucleotide concentrations

The ATP hydrolysis rate and the ADP-ATP exchange rate of (Na+ + K+)-ATPase from ox brain were measured at 10 microM Mg2+free and at micromolar concentrations of free ATP and ADP. (1) In the absence of K+, substrate inhibition of the hydrolysis rate was observed. It disappeared at low Na+ and diminished at increasing concentrations of ADP. This was interpreted in terms of free ATP binding to E1P. In support of this interpretation, free ATP was found to competitively inhibit ADP-ATP exchange. (2) In the presence of K+, substrate activation of the hydrolysis rate was observed. Increasing (microM) concentrations of ADP did not give rise to competitive inhibition in contrast to the situation in the absence of K+ (cf. 1, above). This was interpreted to show that at micromolar substrate, some low-affinity, high-turnover Na+ + K+ activity is possible, provided the Mg2+ concentration is low. (3) While small concentrations of K+ increased the hydrolysis rate (cf. 2) they decreased the rate of ADP-ATP exchange. To elucidate this phenomenon, parallel measurements of exchange and hydrolysis rates were performed over a wide range of ATP and ADP concentrations, with and without K+. If, in the presence and absence of K+, ADP (and ATP competing) are binding to the same phosphorylated intermediate for the backward reaction, it places quantitative restrictions on the ratio of rate constants with and without K+. The results did not conform to these restrictions, and the discrepancy is taken as evidence for the necessity for a bicyclic scheme for the action of the (Na+ + K+)-ATPase. (4) An earlier statement concerning the nature of the phosphoenzyme obtained in the presence of Na+ and K+ is amended.

Distinction between the intermediates in Na+-ATPase and Na+,K+-ATPase reactions. I. Exchange and hydrolysis kinetics at millimolar nucleotide concentrations

Parallel measurements in steady-state of ATP hydrolysis rate (vhydr) and the simultaneous reverse reaction, i.e., the ADP-ATP exchange rate (vexch), allowed the determination of a kinetic parameter, KE, containing only the four rate constants needed to characterize the enzyme intermediates involved in the sequence (Formula: see text). In order to compare the properties of these enzyme intermediates under different sets of conditions, KE was measured at varying K+ and Na+ concentrations in the presence of millimolar concentrations of ATP, ADP and MgATP, using an enzyme preparation that was partially purified from bovine brain. (1) In the presence of Na+ (150 mM), K+ (20-150 mM) was found to increase the exchange rate and decrease the ATP hydrolysis rate at steady-state. As a result, KE increased at increasing K+. However, the value of KE found by extrapolation to K+ = 0 was 7-times lower than the value actually measured in the absence of K+. This finding indicates that one of the intermediates, EATP or EP, or both, when formed in the presence of Na+ alone, are different from the corresponding intermediate(s) formed in the presence of Na+ + K+ (at millimolar substrate concentration). (2) In the presence of 150 mM K+, Na+ (5-30 mM) was found to increase the ADP/ATP exchange as well as the ATP hydrolysis rate at steady-state. The ratio of the two rates was constant. This finding, when interpreted in terms of KE, indicates that Na+ does not have to leave the enzyme for ATP release to be accelerated by K+ in the backward reaction. This also is in opposition to the usual versions of the Albers-Post model, which does not have simultaneous presence of Na+ and K+.