The connexion between active cation transport and metabolism in erythrocytes

Reconstitution of (Na+ + K+)-ATPase proteoliposomes having the same turnover rate as the membranous enzyme

Membranous (Na+ + K+)-ATPase from the electric eel was solubilized with 3-[3-cholamidopropyl)-dimethylammonio)-1-propanesulfonate (Chaps). 50 to 70% of the solubilized enzyme was reconstituted in egg phospholipid liposomes containing cholesterol by using Chaps. The obtained proteoliposomes consisted of large vesicles with a diameter of 134 +/- 24 nm as the major component, and their protein/lipid ratio was 1.25 +/- 0.07 g protein/mol phospholipid. The intravesicular volume of these proteoliposomes is too small to consistently sustain the intravesicular concentrations of ligands, especially K+, during the assay. The decrease in K+ concentration was cancelled by the addition of 20 microM valinomycin in the assay medium. The low value of the protein/lipid ratio suggests that these proteoliposomes contain one Na+/K+-pump particle with a molecular mass of 280 kDa per one vesicle as the major component. In these proteoliposomes, the specific activity of the (Na+ + K+)-ATPase reaction was 10 mumol Pi/mg protein per min, and the turnover rate of the ATP-hydrolysis was 3500 min-1, the same as the original enzyme under the same assay condition. The ratio of transported Na+ to hydrolyzed ATP was 3, the same as that in the red cell. The proteoliposomes could be disintegrated by 40-50 mM Chaps without any significant inactivation. This disintegration of proteoliposomes nearly tripled the ATPase activity compared to the original ones and doubled the specific ATPase activity compared to the membranous enzyme, but the turnover rate was the same as the original proteoliposomes and the membranous enzyme. This disintegration of proteoliposomes by Chaps suggests the selective incorporation of the (Na+ + K+)-ATPase particle into the liposomes and the asymmetric orientation of the (Na+ + K+)-ATPase particle in the vesicle.

Glucose uptake in porcine carotid artery: relation to alterations in active Na+-K+ transport

In vascular smooth muscle (VSM), aerobic lactate production can account for as much as 30% of the basal rate of ATP production. Generally, glucose transport is thought to be the rate-limiting step for glycolysis in unstimulated VSM. In this work we provide evidence that the intracellular concentration of glucose is negligible in porcine carotid artery, indicating that glucose transport is rate limiting for its utilization. Since aerobic glycolysis appears to be coupled to active Na+-K+ transport in this tissue, we examined the effects of altering ion transport on glucose transport. Glucose uptake and 3-O-methyl-D-glucose transport were accelerated, though intracellular glucose remained negligible in artery rings that were incubated with 80 mM KCl, which is known to stimulate active Na+-K+ transport, as well as aerobic glycolysis and mechanical activity. On the other hand, inhibitors of active Na+-K+ transport (ouabain, Na+-free media), which also elicit mechanical activity, had little effect on sugar transport but significantly inhibited aerobic glycolysis and caused an intracellular accumulation of glucose. Our results indicate the following: 1) that glucose transport is regulated in VSM; 2) that the intracellular concentration of Ca2+ does not appear to regulate sugar transport, since changes in glucose and 3-O-methyl-D-glucose transport are not always seen in association with increased mechanical activity, and 3) that the decrease in aerobic glycolysis associated with the inhibition of active Na+-K+ transport is not due to a decrease in glucose transport but rather to an inhibition of glucose utilization.

Rb+ influx in response to changes in energy generation: effect of the regulation of the ATP content of HeLa cells

The effect of changes in energy metabolism on Rb+ influx was studied in HeLa cells. Irrespective of whether ATP production was controlled by varying the activity of glycolysis or of oxidative metabolism on addition of certain combinations of glucose, carbonylcyanide m-chlorophenylhydrazone, monoiodoacetic acid, and quercetin, Rb+ influx changed as a linear function of the ATP content, which varied in a wide range up to the normal level (15-20 nmol/mg protein or 3-4 mM). The difference between results obtained by these procedures was not significant. As the intracellular Na+ content varied at different ATP contents, the Na+ content was adjusted to similar levels by chilling the cells with varying ATP contents. However, a linear relation was still observed. A similar dependence was also obtained for cytoplasmic ATP, which would be more closely connected with the Na,K-pump than total ATP. The ratio of ouabain-sensitive Rb+ influx to the corresponding part of lactate production was nearly 2 in the presence of 2 mM glucose. From these results it is concluded that (1) active Rb+ influx, which is chiefly maintained by energy generated through glycolysis, can also be supported by oxidative metabolism; (2) Rb+ influx is regulated linearly as a function of the cellular ATP content up to the control level; but does not increase if ATP is raised still further; and (3) 2 Rb+ ions move concomitantly at the expense of one ATP molecule.

Partial purification and properties of the inhibitors of Na, K-ATPase and ouabain-binding in bovine central nervous system

Endogenous inhibitors of Na,K-ATPase and ouabain-binding were partially purified from bovine central nervous system, and some of their properties were studied. They were eluted as low-molecular-weight fractions by gel filtration. They could be adsorbed by both Amberlite IR 120 and Amberlite IRA 400 at acidic and basic pH, respectively, indicating that they could act as both anions and cations at different pH. These inhibitors of ouabain-binding appeared to affect specific binding of ouabin, and Scatchard plot analysis showed that the inhibition was competitive, suggesting that they could bind to the same site as ouabain, presumably to Na,K-ATPase itself. The inhibitory activities were heat stable, but charring inactivated them completely.

Binding of sodium and potassium to the sodium pump of pig kidney evaluated from nucleotide-binding behaviour

Using a rate-dialysis technique at 0-2 degrees C, the affinities of Na+ and K+ for the sodium pump of pig kidney outer medulla were determined from their effects on the binding of ADP to the enzyme. Since all experiments were carried out in the presence of Tris, the enzyme in absence of its specific ligands was assumed to be in a 'sodium-like' conformation. The model used in the analysis of the results assumed the enzyme to be a dimeric structure with two identical high-affinity nucleotide-binding sites. It is concluded from the data that the effects of Na+ and K+ on the binding of nucleotide to either subunit of a nucleotide-free enzyme are identical. The two subunits, taken together, have five identical and non-interacting K+-binding sites (Kdiss = 0.5 mM) whose occupation antagonizes nucleotide binding. The binding of a nucleotide molecule to a nucleotide-free enzyme results in the abolition of K+ binding to two of the five K+-binding sites. The binding of the second molecule of nucleotide prevents the binding of three more K+ ions to the enzyme. These results can explain the K+-induced curvature observed in nucleotide-binding isotherms in Scatchard plots. The two subunits, taken together, have five identical and non-interacting Na+-binding sites (Kdiss = 0.5 mM) whose occupation antagonizes the effects of K+ on nucleotide binding, but does not affect nucleotide binding directly. A few experiments carried out at 18 degrees C indicate that the model applies also at this temperature. It is likely that the cation sites investigated are intracellular ones and it is concluded that the binding of each cation to its site induces a specific conformational change in the neighbourhood of the site itself without affecting the regions around the remaining cation binding sites.

A metabolic osmotic model of human erythrocytes

A metabolic osmotic model of red blood cells is presented which takes into account the main reaction steps of glycolysis and the passive and active fluxes of ions across the cell membrane. Cellular energy metabolism and osmotic behaviour are linked by the ATP consumption for the active transport of cations as well as by the osmotic action of the glycolytic intermediate 2,3-diphosphoglycerate (2,3-DPG). The model is based on a system of differential equations describing the metabolic reactions and transport processes. Further, two algebraic conditions for the osmotic equilibrium and the electroneutrality of the cell are considered. Using realistic system parameters the model allows the calculation of a great number of dependent variables, among them the cell volume, the concentrations of metabolites and ions and the transmembrane potential. Only stationary states are considered. The parameter dependence of important model variables is characterized by control coefficients. The main results are: (a) The volume of erythrocytes is mainly determined by the permeabilities of the leak fluxes of cations, the content of hemoglobin and the activity of the hexokinase-phosphofructokinase system of glycolysis; (b) Changes of volume affect the glycolytic rate mainly by changing the concentration of ATP which is a regulator of glycolysis; (c) A change in the membrane area may affect the other cell properties only if it is connected with variations of the number of active and leak sites of the membrane.

Functional compartmentalization of oxidative and glycolytic metabolism in vascular smooth muscle

The metabolism of vascular smooth muscle is characterized by an unusual component of aerobic glycolysis. Lactate production, even under fully oxygenated conditions, is of similar magnitude to the rate of oxygen consumption when compared on a molar basis. Although the underlying mechanisms are unknown, the ratio of glycolytic to oxidative metabolism has been suggested to be an index of vascular myopathy. Measurements of the rate of O2 consumption (JO2), lactate production (Jlac), and isometric force in porcine coronary arteries were made under conditions known to alter both active force (delta Po) and Na+-K+ transport. As previously reported, JO2 was strongly correlated with delta Po; Jlac, however, was correlated with conditions that alter Na+-K+ transport. Under conditions known to inhibit Na+-K+ transport (10(-5) M ouabain, absence of extracellular K+ or Na+), Jlac was inhibited even though delta Po and JO2 were increased. The coupling of Na+-K+ transport with aerobic glycolysis was not dependent on tonicity or the major anion species, nor was it an effect simply on tissue lactate permeability as indicated by studies of tissue lactate content. Metabolic measurements made at similar levels of delta Po indicate that Jlac is markedly inhibited by ouabain, whereas JO2 shows little effect. Thus the unusual aerobic glycolysis observed in vascular smooth muscle appears to be linked to Na+-K+ transport processes and not to some nonspecific metabolic deficiency. Experiments on both systemic and pulmonary arteries from rat, rabbit, dog, and pig indicate that these results are not limited solely to porcine coronary vessels.

The concentration dependence of active K+ transport in the turkey erythrocyte. Hill analysis and evidence for positive cooperativity between ion binding sites

A mathematical model is presented which describes the theoretical relationship between ligand concentration and physiological response for systems in which the response is dependent upon simultaneous occupancy of two receptor ligand-binding sites. The treatment considers both the possibility of intrinsic differences between the binding sites with regard to ligand affinity, as well as the possibility of mutually induced changes in affinity resulting from allosteric interactions. Unlike the Monod-Wyman-Changeux formulation for allosteric enzymes, the general model put forward here makes double occupancy an absolute requirement for enzymatic function. It is shown that such a model leads to the prediction of a curvilinear Hill plot from which one can obtain an explicit estimate of the degree of allosteric interaction between the two ligand binding sites as well as the Gibbs standard free energy change for the overall binding reaction. It is then shown that, in the specific instance of Na, K-ATPase-mediated K+ transport by the turkey erythrocyte, the configuration of the Hill curve describing the rate of ouabain-sensitive K+ transport as a function of external K+ concentration conforms closely to that predicted by the model described above. The results are of particular interest because they indicate a strongly cooperative interaction between the two K+ binding sites on the transport protein such that occupancy of one site results in an enhancement of the affinity of the other site for K+ by a minimum of 15- to 20-fold. Finally, we consider in detail a model of the Monod-Wyman-Changeux type in which, by contrast, both singly and doubly occupied forms of the enzyme are assumed to be catalytically active, and which we analogously extend to allow for the possibility of asymmetry between the two ligand binding sites. Although it is shown that the two models can not be differentiated from each other in the present experimental system, they yield virtually identical estimates for the degree of positive cooperativity between the two K+ binding sites.

The effect of digoxin on 86rubidium uptake by erythrocytes from mothers and babies

1 The uptake of 86rubidium by erythrocytes from mothers and babies has been used as a model system to investigate possible differences in sensitivity to digoxin in the very young. 2 While total 86rubidium uptake was not significantly different between mothers and babies, the digoxin-sensitive proportion was higher in neonatal erythrocytes. Neonatal cells were less sensitive to digoxin, demonstrated by the requirement for a larger amount of digoxin to inhibit 86rubidium uptake and this was accompanied by an increase in numbers of specific erythrocyte binding sites for digoxin. 3 These results provide further evidence in support of the hypothesis of decreased sensitivity to digoxin in the very young.

The link between metabolism and active transport of sodium in human red cell ghosts

Reconstituted human red blood cell ghosts have been used to assay various metabolic intermediates for their role in driving the Na:K pump. ATP was identified as the primary substrate of the pump. The main evidence was based on 1) the finding that the only requirement for activation of the pump was the presence of ATP, whether incorporated directly or generated by an ATP-yielding reaction; 2) the discriminating effects of various metabolic inhibitors; 3) the competition between the pump and hexokinase reaction for ATP; and 4) the difference in effects of adenosine and inosine in activating the pump in energy-depleted ghosts. ADP was found to affect the Na:K pump due to the presence of an adenylate kinase and perhaps because of an effect on the phosphoryl potential. The sidedness of action of the cardiotonic steroid, strophanthidin, was evaluated and found to inhibit the Na:K pump only from the outside of the membrane. Inhibition of the pump by strophanthidin was also found to spare ATP in reconstituted ghosts provided the nonspecific phosphatase activity was suppressed.

Effects of anesthetic alcohols on membrane transport processes in human erythrocytes

1. Anesthetic alcohols (pentanol, hexanol and heptanol) were found to increase the fluidity of red cell membrane lipids as monitored by the fluorescence depolarization of diphenylhexatriene. The relative potency of the alcohols was found to be parallel to their relative membrane/water partition coefficients. 2. Hexanol had biphasic effect on erythritol uptake by simple diffusion by red cells. At concentrations less than 9 mM, there was an approximately linear increase in erythritol permeability with increasing alcohol concentration. 3. The facilitated transport of uridine was markedly inhibited by hexanol. Hexanol at 6 mM produced a 65% inhibition of uridine (4 mM) uptake. Hexanol decreased both the apparent Km and V values for the equilibrium exchange of uridine. 4. The facilitated transport of galactose was only slightly inhibited by hexanol. 5. Hexanol was without effect on the passive and active fluxes of Na+ and K+ in red cells with altered cation contents. Cells that were slightly depleted of K+ and cells that were highly K+ -depleted were both insensitive to hexanol.

Observations on the mechanism for the active extrusion of lithium in mammalian non-myelinated nerve fibres

1. A study has been made of the O2 consumption, and the corresponding efflux of labelled phosphate, from the non-myelinated fibres of the desheathed rabbit vagus nerve at 37 degrees C in Locke solutions in which various ions were substituted for Na, and also in the presence of ouabain. 2. Switching from Na-Locke solution to Li-Locke solution produced a small transient decrease in the resting O2 consumption (of 14%), which rapidly recovered to its original value. This was accompanied by an initial brief rise followed by a maintained fall in the resting phosphate efflux. 3. In Li-Locke solution, ouabain (100 microM) produced a fall in the resting O2 consumption of 40%, i.e. similar to that produced in Na-Locke solution. Any depression of the resting phosphate efflux was absent or small. 4. In choline-Locke solution, in Tris-Locke solution, in K-Locke solution or in sucrose-Locke solution the resting O2 consumption, which fell by 30-40%, was insensitive to the addition of ouabain (100 microM). 5. Addition of either Na ions or of Li ions partially restored respiration in choline-Locke solution, Li being an order of magnitude less effective than Na. 6. In choline-Locke solution the internal K content was not affected by ouabain. However, if Li (77 mM) was present in the bathing solution ouabain (100 microM) produced a 30% fall in the internal K content. 7. It is concluded that these effects of Li, and their alteration by ouabain, reflect the activity of a mechanism for the active extrusion of Li ions. It is suggested that the mechanism for the active extrusion of Li is the same as that for Na. 8. There also seems to be a site for Li that controls the phosphate efflux and which is half-maximally activated with external Li concentrations of about 2-4 mM.

Activity of erythrocyte Na,K-ATPase in manic patients

Erythrocyte membrane Na,K-ATPase activity of manic patients was studied. The activity of patients in acute manic phase was higher than that of normal controls, while that of patients in normal phase was not. Even in the same patient, the activity in acute phase was higher than that in normal phase. Mg-ATPase activities did not differ between controls and patients. These findings clearly indicate a possible correlation between change in clinical phase in manic patients and change of erythrocyte membrane Na,K-ATPase activity.

Oxygen consumption and cellular ion transport: evidence for adenosine triphosphate to O2 ratio near 6 in intact cell

Oxygen (O2) consumption and net K+ uptake were measured simultaneously upon reintroduction of K+ into a K+-depleted suspension of renal tubules. The K+/O2 stoichiometries of 11.8 +/- 0.2 and 8.4 +/- 0.6 were obtained for reduced nicotinamide adenine dinucleotide- and flavoprotein-linked substrates, respectively. These values complement classical K+ to adenosine triphosphate (ATP) and ATP/O2 stoichiometries, thereby demonstrating a remarkably efficient coupling between the processes of Na+- and K+-dependent adenosinetriphosphatase-mediated ion transport and oxidative phosphorylation within the intact cell.

Inherited defect in a Na+, K-co-transport system in erythrocytes from essential hypertensive patients

The Na+ and K+ electrochemical gradients across cell membranes are believed to be maintained by the action of a Na+, K+-pump. In human erythrocytes this pump exchanges internal Na+ for external K+ in approximately a 1.5 ratio. Thus, when Na+-loaded/K+-depleted erythrocytes are incubated in physiological conditions they tend to recover their original low Na+/high K+ content. Surprisingly, in erythrocytes from healthy donors the net Na+ extrusion/K+ influx ratio exceeds the 1.5 ratio predicted for Na+, K+-pump-mediated fluxes whereas it is similar to this value in erythrocytes from essential hypertensive patients and some of their descendants. We now report that this difference is due to the presence of a Na+, K+-co-transport system in normal erythrocytes which extrudes both internal Na+ and K+ and is functionally deficient in erythrocytes of essential hypertensive patients and some of their descendants. No difference in passive Na+ permeability could be detected between normotoensive and hypertensive subjects.

86Rb+ fluxes in Chinese hamster ovary cells as a function of membrane cholesterol content

Steady-state fluxes of 86Rb+ (as a tracer for K+) were measured in Chinese hamster ovary cells (CHO-K1) and a mutant (CR1) defective in the regulation of cholesterol biosynthesis; the membrane cholesterol content of this mutant was varied by growing it on a range of cholesterol supplements to lipid-free medium (Sinensky, M. (1978) Proc. Natl. Acad. Sci. U.S. 75, 1247--1249). Analogous to previous findings in ascites tumor cells, 86Rb+ influx in the parent strain was differentiated into a ouabain-inhibitable 'pump' flux, furosemide-sensitive, chloride-dependent exchange diffusion, and a residual 'leak' flux. On the basis of this flux characterization, 86Rb+ pump and leak fluxes were measured in the mutant as a function of membrane cholesterol content. Pump and leak fluxes, when expressed per ml cell water, were independent of the cholesterol content of the mutant. Moreover, 86Rb+ fluxes in the mutant were equal to those in the parent strain. Our data imply that the flux behavior of K+ in the steady state is independent of the ordering of membrane lipid acyl chains.

The relationship of K+ efflux at the inner surface of the isolated frog skin epithelium to the short circuit current

Isolated frog skins (without chorion) were incubated with 42K+ Ringers' solution, bathing the internal surface for 2 h. All the K+ contained in the frog skin was equilibrated in specific activity with external 42K+. The kinetics of the washout of 42K+ from the internal surface of the skin exhibits one fast and one slow exponential component. Amiloride reduces the release of 42K+ corresponding to both components without affecting the K+ content of the skin. Ouabain increases the loss of 42K+ of the slow component by 200%. Since the total K+ in the skin decreases to 25% of its original value both compartments are affected. The results suggest that two distinct functional compartments exist defined by two 42K+ release ratios and that because of the large K+ contents of these compartments both are intracellular. The relation with the transepithelial Na+ transport and the morphological identification of these compartments is discussed.

Properties of Na-K pump in primary cultures of kidney cells

Activities related to Na-K transport were measured in cell cultures of ground squirrel kidney cortex in order to compare these cells with those of intact kidney and of continuous cell lines. A microsomal preparation containing plasma membrane Na,K-ATPase from fresh kidney showed twice the activity of a similar preparation from 72-hour cultured cells. Na,K-ATPase of homogenates of 72-hour cells showed one-third to one-fourth the specific activity of that from 6-hour cultured cells. The associated K-dependent phosphatase activity also declined as a function of time in culture. The ouabain-sensitive influx of K into 6-hour cultured cells was twice as great as the K influx into 72-hour cells. The number of sites binding 3H-ouabain in intact cultured cells declined 81% on a cell protein basis between 6 and 72 hours in culture. This decline in ouabain binding sites was relatively greater than that of K influx, so that the K turnover number increased over this same time period. The decline in ouabain-sensitive K influx during culture was complementary to an increase in furosemide-sensitive K influx. Measurements of unidirectional and net K fluxes showed that there were three components of K influx into 3-day cultured cells: ouabain-sensitive Na:K exchange, furosemide-sensitive K:K exchange, and K diffusion. In the 6-hour cultures, however, there was no furosemide-sensitive K:K exchange. Thus, after three days in culture ground squirrel kidney cells lose a feature characteristic of the original parent cells (high Na,K-ATPase activity), and gain a feature common to many undifferentiated cultured cells (furosemide-sensitive K:K exchange).

Monovalent cation transport in myotonic dystrophy. Na-K pump ratio in erythrocytes

Myotonic dystrophy is a dominantly-inherited disorder which affects skeletal muscle in combination with several other systems. Because of abnormalities in red blood cells, a universal membrane defect has been proposed as the primary disturbance. Erythrocyte cation pump ratios have also been reported to be abnormal. Hyperinsulinemia and glucose intolerance are present in a large number of patients. Since dramatic effects of insulin on membrane cation transport have been shown in several tissues, notably skeletal muscle, we wished first to confirm reports of altered pump ratio in these patients and then to evaluate the effects of insulin on cation fluxes. However, in our experiments myotonic dystrophy patients had normal pump ratios when compared with disease controls.

Metabolic signals produced by purine ribonucleosides stimulate proinsulin biosynthesis and insulin secretion

Inosine, guanosine and adenosine strongly stimulated proinsulin biosynthesis and insulin secretion in isolated mouse pancreatic islets. None of the purine ribonucleosides stimulated insulin secretion in rat islets, although as reported [jain & Logothetopoulos (1977) Endocrinilogy 100, 923-927] inosine and guanosine, but no adenosine, were potent stimulants of proinsulin biosynthesis in this species. The purine bases had no effect in either species. D-Ribose, which enhanced proinsulin biosynthesis at 0.3 and 0.6 mM but not at 5mM in rat pancreatic islets [jain & Logothetopoulos (1977) Endocrinology 100, 923-927], produced no secretory signals in rat islets and was without any effect on proinsulin biosynthesis and insulin secretion in mouse islets. The rates of oxidation of 14C-labelled purine ribonucleosides and D-ribose in islets of the two species correlated well with their effectiveness as inducers of insulin secretion and proinsulin biosynthesis. Specific inhibitors of purine ribonucleoside phosphorylase, adenosine deaminiase and of purine ribonucleoside transport suppressed the stimulatory effects of nucleosides in pancreatic islets without altering the effect of D-glucose. The same inhibitors also markedly diminished the oxidation rats of the labelled purine ribonucleosides. The experiments clearly indicate that porinsulin biosynthesis and insulin secretion are modulated through metabolic signals and not through interactions of intact substrate molecules with cell receptors.

Gliding edge dislocations in proteins as a mechanism for active ion transport

A mechanism for the translocation of ions across biological membranes is proposed in which transconformational changes occur in the membrane-bound ion pump. In particular, it is suggested that such conformational changes are effected by means of the propagation of crystal defects in the form of edge-dislocation glide.

The interaction of monovalent cations with the sodium pump of low-potassium goat erythrocytes

1. The activation by Na ions and the effect of the anti-L antibody on the sodium pump of low-potassium type (LK) erythrocytes, have been studied by measuring ouabain-sensitive ATPase activity of red cell membranes of LK goats. The experimental data were first corrected for incomplete occupation of the external K sites of the pump, using a saturation function obtained from influx experiments.2. Double-reciprocal plots of the corrected rates against Na concentration at various fixed K concentrations, yield a pattern of competitive K inhibition when it is assumed that three equivalent sodium sites take part in the internal activation of LK-(Na+K)-ATPase. The dissociation constant of Na at each site (K(m)) lies between 10 and 20 mM and that of K as competitive inhibitor (K(i)), between 1.5 and 4.5 mM.3. The maximal rate of hydrolysis of LK goat (Na + K)-ATPase is not different from those usually obtained with the high-potassium type (HK) red cell enzyme. Then, the low pumping rate of LK erythrocytes in physiological conditions is only reflecting the poor Na affinity, both absolute and relative, at the internal Na sites of their sodium pumps.4. The stimulation of the ouabain-sensitive ATPase activity by sensitization of the membranes with anti-L serum, is mediated by a threefold reduction of the K(m)/K(i) ratio at each site. K(m) decreases by a factor of 10, but there is also a smaller diminution of K(i). The maximal rate of hydrolysis, however, is unchanged by the anti-L treatment. The least-squares fitting of the pooled data by the rate equation, converges better with less than three and more than two equivalent sodium sites.5. The affinity sequence at two external K sites of the LK goat erythrocyte sodium pump, determined in the presence of 100 mM external Na, is Rb > K > Cs. It is obtained from the concentration dependence in influx experiments, and is the same as reported for human red cells.6. Cubic-root Dixon plots of the corrected ouabain-sensitive ATPase activity against the concentration of K and its congeners, show the sequence Tl > K > Rb > Na > Cs for the affinities at the internal cation sites of the LK sodium pump. Anti-L treatment decreases the relative magnitude of Na and Cs selectivities, it being not certain whether a Rb-Na transition then occurs.7. The results are discussed in terms of possible mechanisms whereby the sodium pump of LK and HK red cells may adjust the properties of their cation sites upon translocation of monovalent cations.

Membrane compartmentalized ATP and its preferential use by the Na,K-ATPase of human red cell ghosts

This paper describes work which begins to define the molecular organization in the region of the membrane that comprises the functional domain of the Na:K pump. The membrane-bound phosphoglycerate kinase (PGK) and Na, K-ATPase appear to be directly linked via a compartmentalized form of ATP. Evidence for the membrane pool of ATP is based on the labeling characteristics of the phosphoproteins by [gamma-(32)P]ATP of ghosts incubated under various conditions. Preincubation of ghosts in the presence of ATP at 37 degrees C, but not at 0 degrees C, completely obscures the formation of the Na-phosphoprotein in ghosts washed and subsequently incubated in the presence of [gamma-(32)P]ATP. In contrast to the Na component, the Mg component of phosphorylation is only slightly altered by preincubation with ATP. ATPase activity measured as (32)P(i) liberated during the subsequent incubation at 0 degrees C, reflects completely the differential effects of preincubation with ATP on (32)P incorporation into phosphoprotein. ATP placed within the pool by preincubation can be removed by operating the Na, K-ATPase or the PGK reaction in the reverse direction by use of exogenous substrates. Alternatively, the membrane pool of ATP can be formed also from exogenous substrates by running the PGK reaction in the forward direction. These results, while providing direct support for a membrane compartment of ATP, also indicate the location of this compartment in relation to the PGK and the Na, K-ATPase. In addition, these results also imply that the Mg and Na components are different enzymatic entities since substrate ATP can be derived from separate sources.

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.