The reaction mechanism of the sodium pump

ATP-dependent Ca uptake into plant membrane vesicles

Membrane vesicles were extracted from etiolated and light-grown plants, a plant cell suspension culture, and an alga. Upon addition of ATP and Mg(2+), active Ca(2+) uptake into the vesicles against a concentration gradient was shown. The dependence of this uptake on ATP and Mg(2+) concentrations, pH, and temperature is described. In the absence of oxalate, equilibrium between Ca(2+) uptake and efflux was reached after about 30 min. In the presence of oxalate, Ca(2+) accumulation continued for at least 120 min. Ca(2+) efflux from preloaded vesicles did not depend on ATP. Addition of the ionophores A23187 and Ro 20-0006/006 caused an immediate release of accumulated free Ca(2+). ATP-dependent Ca(2+) uptake was not inhibited by 10 muM oligomycin.

The ATPase activity of saponin-treated rat erythrocytes: regulation by monovalent cations, calcium, ouabain, and furosemide

The ATPase activities were studied in rat erythrocytes permeabilized with saponin. The concentrations of calcium and magnesium ions were varied within the range of 0.1-60 microM and 50-370 microM, respectively, by using EGTA-citrate buffer. The maximal activity of Ca2(+)-ATPase of permeabilized erythrocytes was by one order of magnitude higher, whereas the Ca2(+)-binding affinity was 1.5-2 times higher than that in erythrocyte ghosts washed an isotonic solution containing EGTA. Addition of the hemolysate restored the kinetic parameters of ghost Ca2(+)-ATPase practically completely, whereas in the presence of exogenous calmodulin only part of Ca2(+)-ATPase activity was recovered. Neither calmodulin nor R24571, a highly potent specific inhibitor of calmodulin-dependent reactions, influenced the Ca2(+)-ATPase activity of permeabilized erythrocytes. At Ca2+ concentrations below 0.7 microM, ouabain (0.5-1 mM) activated whereas at higher Ca2+ concentrations it inhibited the Ca2(+)-ATPase activity. Taking this observation into account the Na+/K(+)-ATPase was determined as the difference of between the ATPase activities in the presence of Na+ and K+ and in the presence of K+ alone. At physiological concentration of Mg2+ (370 microM), the addition of 0.3-1 microM Ca2+ increased Na+/K(+)-ATPase activity by 1.5-3-fold. Higher concentrations of this cation inhibited the enzyme. At low Mg2+ concentration (e.g., 50 microM) only Na+/K(+)-ATPase inhibition by Ca2+ was seen. It was found that at [NaCl] less than 20 mM furosemide was increased ouabain-inhibited component of ATPase in Ca2(+)-free media. This activating effect of furosemide was enhanced with a diminution of [Na+] upto 2 mM and did not reach the saturation level unless the 2 mM of drug was used. The activating effect of furosemide on Na+/K(+)-ATPase activity confirmed by experiments in which the ouabain-inhibited component was measured by the 86Rb+ influx into intact erythrocytes.

Chemical modification as an approach to elucidation of sodium pump structure-function relations

Chemical modification of specific residues in enzymes, with the characterization of the type of inhibition and properties of the modified activity, is an established approach in structure-function studies of proteins. This strategy has become more productive in recent years with the advances made in obtaining primary sequence information from gene-cloning technologies. This article discusses the application of chemical modification procedures to the study of the Na(+)-K(+)-ATPase protein. A wide array of information has become available about the kinetics, enzyme structure, and various conformational states as a result of the combined use of inhibitors, ligands, modifiers, and proteolytic enzymes. We will review a variety of reagents and approaches that have been employed to arrive at structure-function correlates and discuss critically the limits and ambiguities in the type of information obtained from these methodologies. Chemical modification of the Na(+)-pump protein has already provided a body of data and will, we anticipate, guide the efforts of mutagenesis studies in the future when suitable expression systems become available.

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+.

A fluorometric assay for the potassium-dependent phosphatase activity of the (Na+ + K+)-adenosine triphosphatase

A fluorometric assay for the K+-dependent phosphatase activity of the (Na+ + K+)-ATPase in both purified and membrane-bound forms is described. The assay utilizes 3-O-methylfluorescein phosphate as substrate and measures the fluorescence of the 3-O-methylfluorescein produced by hydrolysis of the substrate. The assay described is an order of magnitude more sensitive than the assay employing p-nitrophenylphosphate, the substrate most commonly used to measure this activity. The assay is also suitable for the specific measurement of (Na+ + K+)-ATPase activities in membranes which contain high levels of other ATPase activities.

(Na+ + K+)-ATPase: confirmation of the three-pool model for the phosphointermediates of Na+-ATPase activity. Estimation of the enzyme-ATP dissociation rate constant

The dephosphorylation kinetics of acid-stable phosphointermediates of (Na+ + K+)-ATPase from ox brain, ox kidney and pig kidney was studied at 0 degree C. Experiments performed on brain enzyme phosphorylated at 0 degree C in the presence of 20-600 mM Na+, 1 mM Mg2+ and 25 microM [gamma-32P]ATP show that irrespectively of the EP-pool composition, which is determined by Na+ concentration, all phosphoenzyme is either ADP- or K+-sensitive. After phosphorylation of kidney enzymes at 0 degree C with 1 mM Mg2+, 25 microM [gamma-32P]ATP and 150-1000 mM Na+ the amounts of ADP- and K+-sensitive phosphoenzymes were determined by addition of 1 mM ATP + 2.5 mM ADP or 1 mM ATP + 20 mM K+. Similarly to the previously reported results on brain enzyme, both types of dephosphorylation curves have a fast and a slow phase, so that also for kidney enzymes a slow decay of a part of the phosphoenzyme, up to 80% at 1000 mM Na+, after addition of 1 mM ATP + 20 mM K+ is observed. The results obtained with the kidney enzymes seem therefore to reinforce previous doubts about the role played by E1 approximately P(Na3) as intermediate of (Na+ + K+)-ATPase activity. Furthermore, for both kidney enzymes the sum of ADP- and K+-sensitive phosphoenzymes is greater than E tot. In experiments on brain enzyme an estimate of dissociation rate constant for the enzyme-ATP complex, k-1, is obtained. k-1 varies between 1 and 4 s-1 and seems to depend on the ligands present during formation of the complex. The highest values are found for enzyme-ATP complex formed in the presence of Na+ or Tris+. The results confirm the validity of the three-pool model in describing dephosphorylation kinetics of phosphointermediates of Na+-ATPase activity.

Studies on the bivalent-cation-activated ATPase activities of highly purified human platelet surface and intracellular membranes

Membrane-bound Ca2+-ATPases are responsible for the energy-dependent transport of Ca2+ across membrane barriers against concentration gradients. Such enzymes have been identified in sarcoplasmic reticulum of muscle tissues and in non-muscle cells in both surface membranes and endoplasmic-reticulum-like intracellular membrane complexes. In a previous study using membrane fractionation by density-gradient and free-flow electrophoresis, we reported that the intracellular membranes of human blood platelets were a major storage site for Ca2+ and involved in maintaining low cytosol [Ca2+] in the unactivated cell. In the present report we demonstrated that the intracellular membranes also exhibit a high-affinity Ca2+-ATPase which appears to be kinetically associated with the Ca2+-sequestering process. We found that both the surface membrane and the intracellular membrane exhibited a basal Mg2+-ATPase activity, but Ca2+ activation of this enzyme was confined only to the intracellular membrane. Use of Ca2+-EGTA buffers to control the extravesicle [Ca2+] allowed a direct comparison of the Ca2+-ATPase and the Ca2+-uptake process over a Ca2+ range of 0.01 microM to 1.0 mM, and it was found that both properties were maximally expressed in the range of external [Ca2+] 1-50 microM, with concentrations greater than 100 microM showing substantial inhibition. Double-reciprocal plots for the Ca2+-ATPase activity and Ca2+ uptake gave apparent Km values for Ca2+ of 0.15 and 0.13 microM respectively. However, similar plots for ATP with the enzyme revealed a discontinuity (two affinity sites, with Km 20 and 145 microM), whereas plots for the Ca2+ uptake gave a single Km value for Ca2+, 1.1 microM. Phosphorylation studies during Ca2+ uptake using [gamma-32P]ATP revealed two components of 90 and 95 kDa phosphorylated at extravesicle [Ca2+] of 3 microM. The Ca2+-ATPase activity, Ca2+ uptake and phosphorylation were all almost completely inhibited in the presence of 500 microM-Ca2+. Similar studies using mixed membranes revealed four other phosphoproteins (50, 40, 20 and 18 kDa) formed in addition to the 90 and 95 kDa components. The findings are discussed in the context of platelet Ca2+ mobilization for function and the mechanisms whereby Ca2+ homoeostasis is controlled in the unactivated cell.

The activation of the sodium pump in pig red blood cells by internal and external cations

A study has been made with pig red blood cells of the activation of the sodium pump by internal and external cations. Cell Na and K concentrations were altered using a PCMBS cation loading procedure. The procedure was characterised for resultant ionic conditions, maintenance of ATP levels and fragility. The activation of the sodium pump by external K was measured in cells suspended in choline (Na-free) solutions. External Cs was used as a substitute for K and elicited lower rates of pump activity. Both the Vmax and apparent Km for 42K influx and 134Cs influx increased as internal Na concentration was raised (within the non-saturating range). Vmax/apparent Km ratios for cation influx were constant. Raising external Cs concentration exerted a similar influence on pump activation by internal Na: both the maximum pump velocity and the apparent Na-site dissociation constant (K'Na) increased. The results provide evidence for a transmembrane connection between cation binding sites on opposite faces of the membrane and are consistent with a consecutive model for the sodium pump in pig red blood cells.

Erythrocyte heat production in human obesity: microcalorimetric investigation of sodium-potassium pump and cell metabolism

Changes in overall cellular metabolism, induced by specific inhibition of the Na-K-pump, were determined in erythrocytes from 33 normal and 25 obese subjects. Cellular metabolism was determined by measurement of heat production rates in erythrocytes suspended in plasma with and without the cardioactive glycoside, ouabain. Specific inhibition with ouabain induced the same decrease of the heat production rate in the two groups (14 +/- 5 mW/L for normal subjects and 13 +/- 5 mW/L for obese subjects). In neither group was there a correlation between the ouabain-inhibitable rate of metabolism and body weight. The present study results do not give support to the suggestion that a defect in the Na-K-pump activity would exist in the erythrocyte of human obese subjects and could not therefore be of importance in the pathogenesis of the disease.

Kinetic parameters and mechanism of active cation transport in HeLa cells as studied by Rb+ influx

On incubation of HeLa cells in chilled isotonic medium, intracellular Na+ (Nac+) increased and K+ (Kc+) decreased with time, reaching steady levels after 3 h. The steady levels varied in parallel with the extracellular cation concentrations ([Na+]e, [K+]e). The cell volumes and the protein and water contents, respectively, of cells kept for 3 h in chilled media of various [Na+]e and [K+]e were not significantly different. Ouabain-sensitive Rb+ influx took place at the initial rate for a certain period which depended on [Na+]c at the beginning of the assays. The existence of two external K+ loading sites per Na+/K+-pump was demonstrated. The affinities of the sites for Rb+ as a congener of K+ were almost the same. Na+e inhibited ouabain-sensitive Rb+ influx competitively, whereas K+ was not inhibitory. Kinetic parameters were determined: the K 1/2 for Rbe+ in the absence of Na+e was 0.16 mM and th Ki for Na+e was 36.8 mM; the K 1/2 for Na+c was 19.5 mM and the Ki for K+c seemed to be extremely large. The rate equation of the ouabain-sensitive Rb+ influx suggests that Na+ and K+ are exchanged alternately through the pump by a binary mechanism.

Differences in phosphorylation of the two large subunits of brine shrimp Na,K-ATPase

Analysis of purified Na,K-ATPase from brine shrimp nauplii revealed two molecular forms of the alpha subunit separable by sodium dodecyl sulfate-polyacrylamide gel electrophoresis [G.L. Peterson, R.D. Ewing, S.R. Hootman, and F.P. Conte (1978) J. Biol. Chem. 253:4762]. The molecular form with lower mobility is designated alpha 1 and the one with higher mobility, alpha 2, in a neutral or alkaline gel system. Differences in Na+-dependent, K+-sensitive phosphorylation of these two molecular forms have been investigated by directly measuring the radioactivity present in each phosphoprotein after separation of the two forms by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. In the presence of Na+,Mg2+, and ATP, when the ATP concentration is above 1 microM, both alpha subunits are phosphorylated, although the phosphoprotein content of alpha 1 is considerably greater than that of alpha 2. Below 1 microM ATP, the phosphoprotein content of alpha 2 is even further reduced. These striking differences in phosphorylation at low ATP concentrations are not due to a greater instability of the alpha 2 phosphoprotein during the long electrophoresis times or during fixation, staining, and destaining. The proportion of total phosphoprotein content in alpha 2, as well as the relationship between phosphoprotein content and ATP concentration, is unchanged when the radioactive analysis is performed on frozen gels that have been electrophoresed for shorter times, even though the actual amount of phosphorylation is 15 times greater than with fixed gels. Since the concentration of alpha 1 and alpha 2 vary during development [G.L. Peterson, L. Churchill, J.A. Fisher, and L.E. Hokin (1982) J. Exp. Zool. 221:295], the differences in phosphorylation may be relevant to differences in Na,K-ATPase activity during different development stages.

Photoaffinity labeling of the ouabain binding site in Na, K-ATPase in developing brine shrimp

Analysis of purified Na,K-ATPase from brine shrimp nauplii by sodium dodecyl sulfate-polyacrylamide gel electrophoresis reveals two large (alpha) subunits [G.L. Peterson, R.D. Ewing, S.R. Hootman, and F.P. Conte (1978) J. Biol. Chem. 253:4762]. The band with lower mobility in a neutral or alkaline gel is designated alpha 1 and the band with higher mobility alpha 2. Ouabain prevents dephosphorylation of both alpha 1 and alpha 2 as documented by gel analysis, but a higher concentration of ouabain is required to prevent dephosphorylation of alpha 2. The photoaffinity label, [3H]4'(2-ethyldiazomalonyl) digitoxigenin monodigitoxiside, specifically labels alpha in a ouabain-protectable manner without labeling other contaminating proteins in the preparation. Greater than 93% of the total ouabain-protectable labeling of the alpha subunits is associated with alpha 1. The photoaffinity label, [3H]4"' (2-ethyldiazomalonyl) digitoxin, specifically labels alpha 1 and beta in a ouabain-protectable manner without labeling other contaminating proteins. These data show that in the brine shrimp the third digitoxose residue of digitoxin binds in a region in which the alpha 1 and beta chains are in close proximity. Less than 5% of the specific ouabain-protectable labeling of total alpha is associated with alpha 2. These studies indicate that cardioactive steroids have higher affinity for the alpha 1 subunit.

[The problem of the cellular receptor for cardiac glycosides (author's transl)]

This review concerns the Na+, K+ -ATPase as well as the Na+, K+ -pump in the intact membrane and the highly specific inhibition of this transport system by cardiac glycosides. The interaction between glycoside and enzyme and the regulation of the kinetics of glycoside binding by ATP, K+, Na+, Mg2+ and Ca2+ are described. Emphasis is placed on the significance of the Na+, K+ -pump as the pharmacological receptor for cardiac glycosides. The problem encountered and progress made in attempting to correlate the inotropic action of cardiac glycosides with the binding of these drugs to the heart muscle and with the inhibition of the Na+, K+ -pump are reported. Recent results concerning increases of the intracellular Na+ concentration which are obtained by a partial inhibition of the Na+, K+ -pump and which are followed by an elevation of the intracellular Ca2+ -activity are reviewed. The discovery of a digitalis-like endogenous activity corresponds to the high specificity of the receptor for cardiac glycosides.

The current concept for the cardiac glycoside receptor

This brief review emphasizes the significance of the Na+,K+-ATPase or the Na+,K+ pump of the intact membrane as the pharmacological receptor for cardiac glycosides. The properties of transport enzyme and the regulation of glycoside binding are described. An outline is given of the problems encountered and of the progress made in attempting to correlate the inotropic action of cardiac glycosides with the binding of these drugs to the heart muscle and with the inhibition of the Na+,K+ pump. Furthermore, the correlation of intracellular Ca2+ activity an Na+ concentration with the inhibition of the Na+,K+ pump is discussed. The existence of a digitalis-like endogenous activity may also indicate an important role of the Na+,K+ pump as a receptor for a physiological regulatory control of cardiac contractility.

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

(1) The kinetics of the phosphorylated enzymic intermediates of (Na+ + K+)-ATPase from ox brain, which are formed by incubation of the enzyme with 25 microM AT32P, 150 mM Na+ and 1 mM Mg2+, have been studied in dephosphorylation experiments at 1 degree C. The dephosphorylation of the 32P-labelled enzyme was initiated by addition of either 1 mM unlabelled ATP, 2.5 mM ADP or 1 mM unlabelled ATP + ADP in concentrations from 25 to 1000 microM. (2) In the absence of ADP the dephosphorylation curve was linear in a semilogarithmic plot almost from t = 0, whereas by addition of ADP a biphasic behaviour was obtained. The slope of the slow phase of dephosphorylation was virtually independent of the ADP concentration. (3) The results were analysed by the mathematical equation corresponding to the simplest possible model for the interconversion and breakdown of the phosphointermediates: (formula: see text) where alpha, beta, H and G are functions of all the rate constants and H and G furthermore are functions of the initial values for [E1P] and [E2P]. (4) The analysis confirmed the model and enabled the determination of all the rate constants. (5) k-1 was found to be equal to k'-1 + k"-1 . [ADP] indicating an ADP-independent 'spontaneous' dephosphorylation of E1P. The rate constant for this process was close to that for dephosphorylation of E2P, i.e., k'-1 congruent to k3. Also the value of k"-1 was determined. (6) k3 was found to be at least 10 . k-2. The implication of this for the role of the E1P to E2P transition in the Na+ + K+)-stimulated ATP hydrolysis will be discussed in detail in the following paper (Plesner, I.W., Plesner, L., Nørby, J.G. and Klodos, I. (1981) Biochim. Biophys. Acta 643, 483--494). (7) A refinement of the model, accounting for the effect of Na+ on the steady-state ratio between [E1P] and [E2P] is proposed: (formula: see text). At [Na+] = 150 mM as used here, E1P(Na) and E'1P are assumed to be in rapid equilibrium. (8) Comparison of our results with those of others underlines the general validity of the conclusions of the present paper.

Electrostatic control by lipids upon the membrane-bound (Na+ + K+)-ATPase

In this paper, the membrane-bound (Na+ + K+)-ATPase from bovine brain is shown to be controlled by electrostatic alterations of the charged lipids surrounding the enzyme. The properties under investigation are the enzymatic activity, activation energy and the response of the enzymatic system to temperature. Arrhenius plots of the ATPase activity are biphasic with a break at temperature Ti. The temperature Ti, the activation energies at temperatures above and below Ti, and the enzymatic activity at any constant temperature have been shown to depend upon the concentrations of alkali and alkaline-earth metal ions in the solution. These electrolyte dependencies are ascribed to changes of electrostatic conditions at the lipids surrounding the ATPase. If the higher electrostatic screening ability of divalent ions is taken into account, the results in the presence of mono- and divalent ions become virtually the same. As a result of this work, it is concluded that electrostatic alterations are transmitted to the ATPase from the lipids of the membrane in which the enzyme is embedded. Inhibition and activation of the enzyme by mono-and divalent metal ions may thus be explained without any auxiliary hypothesis, particularly without postulating specific binding sites for the different ionic species at the protein. In addition, the specific lipid requirement of the ATPase may be understood better in the light of this interpretation.

Comparison between ATP-supported and GTP-supported phosphate turnover of the calcium-transporting sarcoplasmic reticulum membranes

The study deals with the interrelationship of the phosphate-transferring activities of the calcium-transporting sarcoplasmic reticulum membrane vesicles: the phosphate exchange between nucleoside triphosphate (NTP) and nucleoside diphosphate (NDP) (NTP-NDP exchange), the calcium-dependent NTase, and the phosphorylation of NDP by inorganic phosphate in the presence of NTP (NTP-Pi exchange). Different nucleotides were used as phosphate donors and acceptors. It is demonstrated for the phosphate transfer from ITP to GDP that the NTP-NDP exchange exhibits ping-pong kinetics with Mg-ITP and unliganded GDP as substrates. The apparent affinities of the enzyme for the nucleoside diphosphate and triphosphate species are deduced according to this mechanism. The enzyme's affinity for the nucleoside triphosphates and diphosphates depends on its functional state being considerably lower under conditions of NTP-NDP exchange than during NTP splitting or NTP synthesis. ATP and GTP are split with the same low rates when calcium-activated NTPase is inhibited by high internal calcium concentrations after calcium transport has reached steady state. The rates of the NTP-NDP exchange reactions, however, differ by a factor of about 10 being approximately equal to 3 mumol . mg-1 . min-1 for ATP-ADP and only approximately equal to 0.3 mumol . mg-1 . min-1 (22 degrees C) for GTP-GDP. When the sarcoplasmic reticulum vesicles are made calcium-permeable, the calcium transport ATPase is turned on and the rates of GTP and ATP splitting increase about tenfold. Yet, while the rate of ATP-ADP exchange is little reduced, the rate of GTP-GDP exchange drops by approximately 50%. The persisting exchange activity of calcium-permeable vesicles demonstrates that high internal calcium concentrations are not required for the transfer of the protein-bound phosphoryl group to NDP during NTP-NDP exchange.

Modulation of ouabain binding and potassium pump fluxes by cellular sodium and potassium in human and sheep erythrocytes

1. Erythrocytes were treated with nystatin to alter internal Na (Nai) and K (Ki) composition. Although the rates of K pumping and [3H]ouabain binding were altered dramatically, the relationship between glycoside binding and K pump inhibition was unaffected. 2. Human cells with high Nai and low Ki exhibited an increased rate of ouabain binding as compared to high Ki, low Nai cells; this paralleled the stimulated K pump activity of high Nai cells. 3. At constant Ki, increasing internal Na stimulated K pump and ouabain binding rates concomitantly. 4. At low Nai, increasing Ki inhibited both K pumping and ouabain binding. However, at high Nai, increasing Ki from 4 to 44 mM stimulated the rate of glycoside binding, parallel to its effect of increasing the rate of active K influx. 5. Anti-L, an isoantibody to low K (LK) sheep red cells, increased the rate of ouabain binding via its stimulation of K pump turnover. Since the latter effect is the result of affinity changes at the internal cation activation site(s) of the pump (Lauf, Rasmusen, Hoffman, Dunham, Cook, Parmelee & Tosteson, 1970), the antibody's effect on ouabain binding reflected the positive correlation between the rates of K pump turnover and glycoside binding. 6. These data provide the first evidence in intact cells for the occurrence of a Nai-induced conformational change in the Na/K pump during its normal operational cycle.

Pyruvate kinase-catalyzed ATP-formation in human red blood cell membranes

Previous studies on the linkage between enzymatically catalyzed ATP-generating reactions in the red blood cell membrane and the sodium and potassium transport in the control of overall glycolysis of human erythrocytes were controversial. In this study a significant amount of pyruvate kinase activity is shown to be localized within the membrane. Membrane fragments produce 20.5 mumol of ATP per 10(10) membranes per hour from phosphoenolpyruvate and ADP. The kinetics of the membrane-localized pyruvate kinase do not differ from those of the enzyme from hemolysates. The results clearly document the presence of the second ATP-generating enzyme of glycolysis, pyruvate kinase, in human red blood cell membranes. The main fraction of the enzyme is deeply hidden in the lipid layers of the membrane. It can be demasked by mechanical desintegration of membranes at high levels of activity. It is suggested that the amount of the membrane-localized fraction of pyruvate kinase is related to the clinical severity of the hemolytic process in pyruvate kinase deficiency.

Elementary steps of the (Na+ + K+)-ATPase mechanism, studied with formycin nucleotides

1. Formycin triphosphate (FTP), a fluorescent analogue of ATP, is a substrate for (Na+ + K+)-ATPase (ATP phosphohydrolase, EC 3.6.1.3), with properties similar to those of ATP. 2. FTP and formycin diphosphate (FDP) bind to the enzyme with high affinity and, on binding, the nucleotide fluorescence is enhanced 3-4-fold. It is therefore possible, with a stopped-flow fluorimeter, to measure the rates of binding and release of FTP and FDP under conditions in which turnover does not occur. 3. When the enzyme-FTP complex is exposed to conditions permitting turnover (Mg2+, Na+ +/- K+), changes in fluorescence occur which can be explained by supposing that they reflect the interconversion of states with or without bound nucleotides. A rapid fall in fluorescence, that we attribute to the rapid release of FDP from newly phosphorylated enzyme, is followed by a steady state in which low fluorescence suggests that little nucleotide is bound. Eventually, exhaustion of FTP allows rebinding of FDP to the enzyme, which is signalled by a rise in fluorescence. 4. The estimated rate of FDP release from newly formed phosphoenzyme is unaffected by the presence of K+ (0-2 mM) or the concentration of FTP (1-20 micron). 5. Experiments with [gamma-32P]FTP show that about 1 mol of 32P is incorporated per mol of enzyme. The rate of phosphorylation of the enzyme by [gamma-32P]FTP has been measured with a rapid-mixing-and-quenching apparatus. 6. Kinetic data from the fluorescence and phosphorylation experiments show that the behaviour of the enzyme, at least at the low nucleotide concentrations employed, is consistent with the Albers-Post model, and is difficult to reconcile with models in which K+ acts at or before the step in which FDP is released during turnover.

Interaction between cell sodium and the amiloride-sensitive sodium entry step in rabbit colon

Ouabain abolishes the short-circuit current (Isc) and decreases the transepithelial conductance (Gt) of rabbit colon. In contrast, amphotericin B elicits a maximum Isc and markedly increases Gt. However, in both instances the amiloride-sensitive Na entry step is completely blocked, presumably due to an increase in cell Na. Conversely, when Na-depleted tissues are suddenly exposed to 140 mM Na, the amiloride-sensitive Isc and the amiloride-sensitive component of Gt (alphaGNa) increase abruptly to their maximum values and then decline to steady-state plateaus with a half time of approximately 6 min; throughout the decline (Isc/alphaGNa) = ENa is constant at a value of 95 mV. In the presence of amphotericin B, the Isc abruptly rises to the same maximum but does not decline. These findings indicate that in the presence of 140 mM Na the conductance of the amiloride-sensitive Na entry step can vary from a maximum value of approximately 1.6 mmhos/cm2 when cell Na is depleted, to zero when cell Na is abnormally elevated (e.g., in the presence of ouabain or amphotericin B). Our findings are consistent with a system in which the pathway responsible for transcellular Na transport parallels another cellular compartment with which it communicates. The Na capacity of the active transport pathway appears to be very small so that this compartment fills rapidly after exposure of Na-depleted cells to 140 mM Na, and active transepithelial Na transport is initiated and reaches steady-state levels quickly. The Na capacity of the second compartment is much larger; the Na content of this compartment appears to be responsible for the negative feedback effect on the permeability of the amiloride-sensitive entry step.

Effect of intravesicular monovalent cations on the steady state of the phosphoenzyme of adenosine triphosphatase dependent on sodium and potassium ions in inside-out plasma-membrane vesicles

Phosphorylation of the ATPase dependent on Na+ and K+ is promoted through the synergistic action of cations on both sides of the membrane. This phenomenon has been observed in plasma membrane vesicles isolated from sheep-kidney outer medulla which accept ATP from the outside surface (inside-out) and which are tight for sodium ions. In these inside-out vesicles phosphorylating capacity is low even in the presence of 100 mM extravesicular sodium chloride as is turnover of the enzyme. The level of the phosphoenzyme and the transient release of inorganic phosphate from the phosphoenzyme increases several-fold when sodium chloride is allowed to equilibrate over the membrane, 25 mM intravesicular NaCl is necessary to obtain the half-maximum level of the phosphoenzyme. This result shows that intravesicular (= extracellular) low affinity sites are involved in the phosphorylation. Intravesicular potassium ions modify the activating action of Na+ on the phosphorylation by increasing the steady state of the phosphoenzyme at low intravesicular sodium ion concentrations. This suggests that Na+ and K+ compete with each other for the intravesicular cation-binding site.

Substrate inhibition of the (Na+, K+)-ATPase in the presence of excess Mg2+

1. High concentrations of ATP inhibit completely the activity of (Na+, K+)-ATPase (ATP phosphohydrolase, EC 3.6.1.3) prepared from sheep brain. 2. The inhibition depends on the concentration of total ATP, i.e. complexed ATP+ free ATP. 3. The inhibition by high ATP concentrations persists in the absence of K+, and is then independent of the Na+ concentration between 2 and 140 mM Na+. 4. Raising the K+ concentration at 20 mM Na+ increases the ATP concentration required for the maximal hydrolysis rate. 5. The Hill number for the inhibition process is about three. 6. The inhibition by ATP is temperature-dependent, in that as the temperature is increased, higher ATP concentrations are required for inhibition.

Kinetic evaluation of the Na-K pump reaction mechanism

1. The ouabain-sensitive K influx was measured at varying external K concentrations ([K]o) and at several fixed internal Na concentrations ([Na]c). The cells were nominally K-free and the solutions Na-free. Both the apparent maximal velocity (VM) and the apparent Michaelis constant for K (KK) increased as Nac increased. The ratio app. VM/app. KK increased with increasing Nac. 2. The ouabain-sensitive Cs influx was measured at varying external Cs concentrations and at several fixed Nac in K-free cells and Na-free solutions. Both app. VM and app. Kcs increased as Nac increased and the ratio app. VM/app. Kcs increased with increasing Nac. 3. The data were evaluated in terms of ping-pong model and a simultaneous model for the pump reaction mechanism. The simultaneous model described the data adequately and the ping-pong models did not. 4. The K influx was measured at varying external K concentrations in solutions containing Na and at a low and high Nac; the cells contained K. The relation between the pump rate and the external K concentration was sigmoid. A Hill equation was fitted to the data. KK was higher in the high Nac cells, but the Hill coefficient (n) was not altered as Nac increased. 5. The K influx was measured at varying internal Na concentrations and two fixed external K concentrations; the cells contained K. The relation between the pump rate and Nac was sigmoid. When a Hill equation was fitted to the data, it was found that KNac was higher at the high external K concentration, but n was the same at both K concentrations.

Accelerating effect of 2,4,6-trinitrophenol on the glycolytic rate of human red cells

A number of instantaneous changes occurred when picrate was added to a suspension of human red cells in steady state with respect to glycolysis and ion distribution across the membrane at pH 7.40. The rate of glycolysis increased, without change in glycolytic quotient, to a new steady-state value, the effect reaching a maximum of 1.75 times the rate of the control at 0.5 mM picrate. Inorganic phosphate (P(i)) was released at a relatively constant rate, increasing with picrate concentration to 1.0 mmol P(i)/liter cells x h at 5-6 mM picrate. The steady- state concentrations of ATP and 1,3-diphosphoglycerate (1,3-DPG) decreased to new stable values within 15-45 min after the addition of picrate. The ATP level was affected only at picrate concentrations of 1 mM or more, and the level of ATP stabilized at 75 percent of the control values at 4 mM of picrate. In contrast, 1,3-DPG concentrations decreased to 40 percent of the control value of 0.5 mM picrate. Higher concentrations of picrate resulted in only a small additional decrease in the stationary concentration of 1,3-DGP. A net efflux of cellular potassium at constant rate took place. This net efflux was an almost linear function of picrate concentration in the range of 0.1-3 mM. At the latter concentration the net efflux amounted to about 2.7 meq/liter cells x h and a further increase in picrate concentration caused only a minor increase in the potassium efflux. Possible mechanisms for the effects of picrate on human red cell glycolysis are discussed.

The sarcoplasmic calcium pump - a most efficient ion translocating system

In contrast to the sodium-potassium transporting plasma membranes, the sarcoplasmic membranes (SR) are highly specialized structures into which only two major intrinsic proteins, a calcium transporting protein and a calcium binding protein are embedded. The calcium transporting protein is a highly asymmetric molecule. It binds two calcium ions with a very high affinity at its external, and two calcium ions with low affinity at the internal section of the molecule. ATP is bound with high afffinity to an external binding site, inducing a conformational change. When the vesicular membranes are exposed to solutions containing Ca++, Mg++ and ATP, ATP is hydrolyzed and simultaneously calcium ions are translocated from the external medium into the vesicular space. When calcium ions are translocated in the opposite direction, ATP is synthesized. The calcium-ATP ratio for ATP cleavage as well as for ATP synthesis is 2. Thus, the SR membranes can transform reversibly chemical into osmotical energy. Inward and outward movements of calcium ions are relatively slow processes connected with the appearance and disappearance of different phosphorylated intermediates. One phosphorylated intermediate is formed by phosphoryltransfer from ATP when calcium ions are present in the medium. In contrast, when calcium ions are absent from the external medium, two different intermediates can be formed by the incorporation of inorganic phosphate. Only when calcium ions present in the internal space of the vesicles are released, the incorporation of inorganic phosphate gives rise to an intermediate who phosphoryl group can be transferred to ADP.

Models for the active transport of cations...the steady-state analysis

We summarise the progress that has been made in the analysis of active transport models, at the steady-state level. The two general classes of such model, counter-and co-transport, can be treated by a kinetic analysis which makes no assumptions as to the symmetry or asymmetry of the systems nor as to the presence of any particular rate-limiting steps. Precisely the same formalism is obeyed for primary active transport as for secondary active transport. Both are merely a generalisation of facilitated diffusion, in that they follow directly from accepted properties of carrier models. How affinities of such carriers for their substrates affect the efficiency of active transport is discussed and it is shown that in a number of cases, the affinity changes that the carrier demonstrates arise from inherent properties of the free carrier and not from any "high energy" properties of the chemical reactants. Methods of obtaining the kinetic parameters of the system from experimental data are reviewed, together with methods for testing and characterising the different transport models.

Differential lipid control of (Na+ + K+)-ATPase in homeotherms and poikilotherms

1. (Na+ + K+)-ATPase from homeotherms and poikilotherms demonstrate non-linear thermal dependence for ATP hydrolysis. Apparent energies of activation from crab nerve preparations are less than those of brain or kidney preparations from beef, rabbit, sheep or ground squirrel. 2. Crab nerve (Na+ + K+)-ATPase is less sensitive to inhibition by ouabain than that from beef or ground squirrel; lower rates of [3H]-ouabain binding and reduced amount of drug bound at equilibrium are found. 3. K+-activated acyl-phosphatase is similar in all preparations. 4. Fluorescence polarization of 12-AS labelled membranes demonstrate greater mobility of crab nerve lipids compared to beef brain which has a thermal transition at 20-25 degrees C. Crab nerve is linear in this range.