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

Fluorescence measurements of nucleotide association with the Na(+)/K(+)-ATPase

The Na(+)/K(+)-ATPase, a membrane-associated ion pump, uses energy from the hydrolysis of ATP to pump 3 Na(+) ions out of and 2 K(+) into cells. The dependence of ATP hydrolysis on ATP concentration was measured using a fluorescence coupled-enzyme assay. The dependence on concentration of nucleotide association with the ATPase was examined using ADP and ATP-induced quenching of the fluorescence of ATPase labeled with Cy3-maleimide (Cy3-ATPase) or Alexa Fluor 546 carboxylic acid, succinimidyl ester (AF-ATPase). The kinetics of ATP hydrolysis in the presence of Na(+) and K(+) exhibited negative cooperativity with a Hill coefficient (n(H)) of 0.66 and a half-maximal concentration (K(0.5)) of 61 microM; in the absence of K(+), n(H) was 0.58 and K(0.5) was 13 microM. Nucleotide-induced fluorescence quenching exhibited negative cooperativity with an n(H) of 0.3-0.5. These results suggest that negative cooperativity observed in ATP hydrolysis is attributable to negative cooperativity in nucleotide association to the ATPase. Interaction between AF-ATPase and ATP labeled with Alexa Fluor 647 (AF-ATP) showed significant Förster resonance energy transfer (FRET). These results indicate that the ATPase exists as oligoprotomeric complexes in this preparation, and that this aggregation has significant effects on enzyme function.

The crustacean gill (Na+,K+)-ATPase: allosteric modulation of high- and low-affinity ATP-binding sites by sodium and potassium

The blue crab, Callinectes danae, tolerates exposure to a wide salinity range employing mechanisms of compensatory ion uptake when in dilute media. Although the gill (Na+,K+)-ATPase is vital to hyperosmoregulatory ability, the interactions occurring at the sites of ATP binding on the molecule itself are unknown. Here, we investigate the modulation by Na+ and K+ of homotropic interactions between the ATP-binding sites, and of phosphoenzyme formation of the (Na+,K+)-ATPase from the posterior gills of this euryhaline crab. The contribution of the high- and low-affinity ATP-binding sites to maximum velocity was similar for both Na+ and K+. However, in contrast to Na+, a threshold K+ concentration triggers the appearance of the high-affinity binding sites, displacing the saturation curve to lower ATP concentrations.Further, a low-affinity site for phosphorylation is present on the enzyme. These findings reveal notable differences in the catalytic mechanism of the crustacean (Na+,K+)-ATPase compared to the vertebrate enzyme.

The acid-activated signaling pathway: starting with Pyk2 and ending with increased NHE3 activity

On a typical Western diet, the body is faced with the generation of a metabolically derived acid load that must be excreted to maintain systemic acid-base balance. The kidney is responsible for this task and matches daily acid excretion with daily acid production. Multiple nephron segments are involved in the process, including the proximal tubule cell. This review discusses the acid-activated signaling pathway in the proximal tubule that senses a decrease in cell pH and then mediates stimulation of the apical membrane Na/H antiporter, isoform NHE3. NHE3 mediates secretion of the majority of protons involved in bicarbonate reclamation, is involved in ammonium secretion, and provides a source of luminal protons for titrating filtered titratable acids and secreted ammonia to ammonium.

Rapid eye movement sleep deprivation induces changes in the high-affinity binding of [3H]-ouabain to the rat cortical membranes

Rapid eye movement sleep (REMS) suppresses seizures. On the other hand, REMS deprivation (REMSD) increases brain susceptibility to seizures. Sodium-potassium/ATPase is involved in the control of brain excitability. Ouabain, a cardiotonic glycoside, binds to a regulatory extracellular allosteric site in the sodium-potassium/ATPase inhibiting/stimulating its activity depending on its concentration. Endogenous ouabain-like substances exist in the brain; therefore, changes in the ouabain binding site may be involved in the increased brain excitability induced by REMSD. Adult, Wistar male rats were deprived of REMS for 96 hours by the flower-pot method (REMSD). A stress control group was kept in the same environment on a larger platform (LP). A third group of rats was kept in the same room in their home-cages (CONTROL). After REMSD all rats were sacrificed by decapitation and their cerebral cortex dissected. High-affinity [3H]-ouabain binding was carried out in cortical crude membrane preparation using 8 concentrations of [3H]-ouabain (1-24 nM). The results show a statistically significant increase of KD in the REMSD rats compared to both CONTROL and LP groups. There were no statistically significant differences in the Bmax among the experimental groups. There was also no change either in cortical activity of K+ stimulated p-nitrophenylphosphatase, the dephosphorylation reaction of phosphorylated sodium-potassium/ATPase or in Mg2+-stimulated p-nitrophenylphosphatase. An increase in the KD of [3H]-ouabain binding to the sodium-potassium/ATPase in REMSD rats indicates a lower affinity to the endogenous inhibitors/stimulators of the enzyme. Therefore, this decreased affinity of the endogenous ouabain-like substances may be involved in the increased excitability induced by REMSD.

Kinetic studies on Na+/K+-ATPase and inhibition of Na+/K+-ATPase by ATP

Na+/K+-ATPase (EC 3.6.1.3) is an important membrane-bound enzyme. In this paper, kinetic studies on Na+/K+-ATPase were carried out under mimetic physiological conditions. By using microcalorimeter, a thermokinetic method was employed for the first time. Compared with other methods, it provided accurate measurements of not only thermodynamic data (deltarHm) but also the kinetic data (Km and Vmax). At 310.15K and pH 7.4, the molar reaction enthalpy (deltarHm) was measured as -40.514 +/- 0.9kJmol(-1). The Michaelis constant (Km) was determined to be 0.479 +/- 0.020 mM and consistent with literature data. The reliability of the thermokinetic method was further confirmed by colorimetric studies. Furthermore, a simple and reliable kinetic procedure was presented for ascertaining the true substrate for Na+/K+-ATPase and determining the effect of free ATP. Results showed that the MgATP complex was the real substrate with a Km value of about 0.5mM and free ATP was a competitive inhibitor with a Ki value of 0.253 mM.

The phosphatase activity of the isolated H4-H5 loop of Na+/K+ ATPase resides outside its ATP binding site

The structural stability of the large cytoplasmic domain (H(4)-H(5) loop) of mouse alpha(1) subunit of Na(+)/K(+) ATPase (L354-I777), the number and the location of its binding sites for 2'-3'-O-(trinitrophenyl) adenosine 5'-triphosphate (TNP-ATP) and p-nitrophenylphosphate (pNPP) were investigated. C- and N-terminal shortening revealed that neither part of the phosphorylation (P)-domain are necessary for TNP-ATP binding. There is no indication of a second ATP site on the P-domain of the isolated loop, even though others reported previously of its existence by TNP-N(3)ADP affinity labeling of the full enzyme. Fluorescein isothiocyanate (FITC)-anisotropy measurements reveal a considerable stability of the nucleotide (N)-domain suggesting that it may not undergo a substantial conformational change upon ATP binding. The FITC modified loop showed only slightly diminished phosphatase activity, most likely due to a pNPP site on the N-domain around N398 whose mutation to D reduced the phosphatase activity by 50%. The amino acids forming this pNPP site (M384, L414, W411, S400, S408) are conserved in the alpha(1-4) isoforms of Na(+)/K(+) ATPase, whereas N398 is only conserved in the vertebrates' alpha(1) subunit. The phosphatase activity of the isolated H(4)-H(5) loop was neither inhibited by ATP, nor affected by mutation of D369, which is phosphorylated in native Na(+)/K(+) ATPase.

Insights into the mechanism of erythrocyte Na+/K+-ATPase inhibition by nitric oxide and peroxynitrite anion

Evidence shows that Na(+)/K(+)-ATPase from kidney, brain and liver is inhibited by nitric oxide (NO) and peroxynitrite anion (ONO(2) (-)), but the mechanism is unknown. The aim of the present work was to study the inhibitory effect of NO and ONO(2) (-) on erythrocyte Na(+)/K(+)-ATPase. Erythrocyte membranes were isolated from male Wistar rats by hypotonic washing. The membranes (free from haemoglobin) were incubated for Na(+)/K(+)-ATPase activity measurement at various concentrations of ATP in the presence or absence of 400 microM SNAP (an NO donor) or 100 microM SIN-1 (an ONO(2)(-) donor). At these concentrations, SNAP and SIN-1 released about the same amount (100 microM) of NO or ONO(2)(-), respectively, as monitored by measuring NO(2)(-) + NO(3)(-). Both SNAP and SIN-1 decreased V(max) by ca. 75% but they did not decrease the apparent affinity of the Na(+)/K(+)-ATPase for the substrate (a decrease of K(m) was even observed after SNAP treatment). The pattern of this inhibition is compatible either with oxidation of SH groups directly involved in ATP binding but in a way that is not surmountable by increasing the substrate concentration ("non-competitive") or with oxidation of SH groups located outside the active site of the enzyme but important for the activity of the enzyme.

On the mechanism of activation of the plasma membrane Ca2+-ATPase by ATP and acidic phospholipids

The activation of purified and phospholipid-depleted plasma membrane Ca2+-ATPase by phospholipids and ATP was studied. Enzyme activity increased with [ATP] along biphasic curves representing the sum of two Michaelis-Menten equations. Acidic phospholipids (phosphatidylinositol (PI) and phosphatidylserine (PS)) increased Vmax without affecting apparent affinities of the ATP sites. In the presence of 20 microm ATP, phosphorylation of the enzyme preincubated with Ca2+ (CaE1) was very fast (kapp congruent with 400 s-1). vo of phosphorylation of CaE1 increased with [ATP] along a Michaelis-Menten curve (Km of 15 microm) and was phospholipid-independent. Without Ca2+ preincubation (E1 + E2), vo of phosphorylation was also phospholipid-independent, but was slower and increased with [ATP] along biphasic curves. The high affinity component reflected rapid phosphorylation of CaE1, the low affinity component the E2 --> E1 shift, which accelerated to a rate higher than that of the ATPase activity when ATP was bound to the regulatory site. Dephosphorylation of EP did not occur without ATP. Dephosphorylation increased along a biphasic curve with increasing [ATP], showing that ATP accelerated dephosphorylation independently of phospholipid. PI, but not phosphatidylethanolamine (PE), accelerated dephosphorylation even in the absence of ATP. kapp for dephosphorylation was 57 s-1 at 0 microM ATP; that rate was further increased by ATP. Steady-state [EP] x kapp for dephosphorylation varied with [ATP], and matched the Ca2+-ATPase activity measured under the same conditions. Apparently, the catalytic cycle is rate-limited by dephosphorylation. Acidic phospholipids stimulate Ca2+-ATPase activity by accelerating dephosphorylation, while ATP accelerates both dephosphorylation and the conformational change from E2 to E1, further stimulating the ATPase activity.

Photoinactivation of fluorescein isothiocyanate-modified Na,K-ATPase by 2'(3')-O-(2,4,6-trinitrophenyl)8-azidoadenosine 5'-diphosphate. Abolition of E1 and E2 partial reactions by sequential block of high and low affinity nucleotide sites

The Na,K-ATPase activity of the sodium pump exhibits apparent multisite kinetics toward ATP, a feature that is inherent to the minimal enzyme unit, the alpha beta protomer. We have argued that this should arise from separate catalytic and noncatalytic sites on the alpha beta protomer as fluorescein isothiocyanate (FITC) blocks a high affinity ATP site on all alpha subunits and yet the modified Na, K-ATPase retains a low affinity response to nucleotides (Ward, D. G., and Cavieres, J. D. (1996) J. Biol. Chem. 271, 12317-12321). We now find that 2'(3')-O-(2,4,6-trinitrophenyl)8-azido-adenosine 5'-diphosphate (TNP-8N3-ADP), a high affinity photoactivatable analogue of ATP, can inhibit the K+-phosphatase activity of the FITC-modified enzyme during assays in dimmed light. The inhibition occurs with a Ki of 140 microM at 20 mM K+; it requires the adenine ring as 2'(3')-O-(2,4 6-trinitrophenyl) (TNP)-UDP or TNP-uridine are less potent and 2,4,6-trinitrobenzene-sulfonate is ineffective. Under irradiation with UV light, TNP-8N3-ADP inactivates the K+-phosphatase activity of the fluorescein-enzyme and also its phosphorylation by [32P]Pi. The photoinactivation process is stimulated by Na+ or Mg2+, and is inhibited by K+ or excess TNP-ADP. In the presence of 50 mM Na+ and 1 mM Mg2+, TNP-8N3-ADP photoinactivates with a K0.5 of 15 microM. Furthermore, TNP-8N3-ADP photoinactivates the FITC-modified, solubilized alpha beta protomers, even more effectively than the membrane-bound fluorescein-enzyme. These results strongly suggest that catalytic and allosteric ATP sites coexist on the alpha beta protomer of Na,K-ATPase.

Glucose-induced intracellular ion changes in sugar-sensitive hypothalamic neurons

In the lateral hypothalamic area (LHA) of rat brain, approximately 30% of cells showed sensitivity to small changes in local concentrations of glucose. These "glucose-sensitive" neurons demonstrated four types of behavior, three of which probably represent segments of a continuous spectrum of recruitment in response to ever more severe changes in blood sugar. Type I cells showed maximum activity </=5.6 mM blood glucose but became completely silent at hyperglycemia of 10-12 mM (normoglycemia 7.6 +/- 0.3 mM; mean +/- SD). Type II and III neurons exhibited a wider range of response. Type IV cells (5-7% of glucose-sensitive neurons) paralleled the behavior of sugar-sensitive cells in ventromedial hypothalamic nucleus (VMH). In VMH, approximately 40% of cells responded to changes in blood glucose over a range of concentrations from 3.6 to 17 mM, by increasing their firing rate as sugar level rose and vice versa. Ionic shifts during increases in blood (brain) glucose levels were similar in LHA types I-III but fastest in I and slowest in III. [Na+]i fell by 5-9 mM, [K+]i rose by 6-8 mM, and plasma membrane hyperpolarized by 5 mV. [Ca2+]i declined by 15-20 nM in line with membrane hyperpolarization. In VMH and type IV LHA cells, [K+]i fell 3-8 mM and plasma membrane depolarized -3 to -5 mV as blood/brain glucose concentration increased from 7.6/2.4 to 17.6/4.2 mM, whereas [Ca2+]i increased from 125 to 180 nM as a consequence of falling membrane potential. During falls in blood/brain sugar concentration the effects in both VMH and LHA cells were reversed. The findings are consistent with the ionic shifts in types I-III LHA cells being dependent on alterations in Na/K-ATPase activity, whereas those in VMH and type IV LHA cells could be caused by modulation of ATP-dependent K+ channels. A possible mechanism for linking the effects of small changes in glucose to ATP generation, which could bring about the above phenomena, is the interposition of a "glucokinase-type" enzyme in a role similar to that which it has in glucose-sensing pancreatic beta-cells.

A 133Cs nuclear magnetic resonance study of endothelial Na(+)-K(+)-ATPase activity: can actin regulate its activity?

Using (133)Cs+ NMR, we developed a technique to repetitively measure, in vivo, Na(+)-K(+)-ATPase activity in endothelial cells. The measurements were made without the use of an exogenous shift reagent, because of the large chemical shift of 1.36 +/- 0.13 ppm between intra- and extracellular Cs+. Intracellularly we obtained a spin lattice relaxation time (T1) of 2.0 +/- 0.3 s, and extracellular T1 was 7.9 +/- 0.4 s. Na(+)-K+ pump activity in endothelial cells was determined at 12 +/- 3 nmol Cs+ x min(-1) x (mg Prot)[-1] under control conditions. When intracellular ATP was depleted by the addition of 5 mM 2-deoxy-D-glucose (DOG) and NaCN to about 5% of control, the pump rate decreased by 33%. After 80 min of perfusion with 5 mM DOG and NaCN, reperfusion with control medium rapidly reestablished the endothelial membrane Cs+ gradient. Using (133)Cs+ NMR as a convenient tool, we further addressed the proposed role of actin as a regulator of Na(+)-K+ pump activity in intact cells. Two models of actin rearrangement were tested. DOG caused a rearrangement of F-actin and an increase in G-actin, with a simultaneous decrease in ATP concentration. Cytochalasin D, however, caused an F-actin rearrangement different from that observed for DOG and an increase in G-actin, and cellular ATP levels remained unchanged. In both models, the Na(+)-K(+)-pump activity remained unchanged, as measured with (133)Cs NMR. Our results demonstrate that (133)Cs NMR can be used to repetitively measure Na(+)-K(+)-ATPase activity in endothelial cells. No evidence for a regulatory role of actin on Na(+)-K(+)-ATPase was found.

The role of (alpha beta) protomer interaction in determining functional characteristics of red cell Na,K-ATPase

We have examined the possibility that interaction of (alpha beta) protomers within a diprotomer is responsible for some anomalous characteristics of red cell Na,K-ATPase by examining their response to two inhibitors, FITC and H2DIDS, which bind covalently, and to ouabain, which debinds slowly from red cell pumps. The phenomena we examined were: (1) the biphasic curve relating Na,K-ATPase activity to ATP concentration, and (2) protection of Na pumps against vanadate inhibition by external Na. If interaction of (alpha beta) protomers within a diprotomer were responsible for these phenomena, random inactivation of (alpha beta) protomers should have resulted in a high proportion of (alpha beta) promtomers with an inhibited protomer as a partner, and therefore should have significantly altered the consequences of subunit interaction. With each inhibitor, 60-70% inhibition of ATPase activity did not alter the functional characteristics of the residual activity. We conclude that interaction of functional (alpha beta) protomers does not explain the phenomena which we investigated. This is consistent with our previous observation that Na,K pumps of red cell membranes exist as monomeric (alpha beta) protomers (Martin, D.W. and Sachs, V.R. (1992) J. Biol. Chem. 267, 23922-23929).

Characterization of K(+)-dependent and K(+)-independent p-nitrophenylphosphatase activity of synaptosomes

These experiments examined effects of several ligands on the K+ p-nitrophenylphosphatase activity of the (Na+,K+)-ATPase in membranes of a rat brain cortex synaptosomal preparation. K(+)-independent hydrolysis of this substrate by the synaptosomal preparation was studied in parallel; the rate of hydrolysis in the absence of K+ was approximately 75% less than that observed when K+ was included in the incubation medium. The response to the H+ concentrations was different: K(+)-independent activity showed a pH optimum around 6.5-7.0, while the K(+)-dependent activity was relatively low at this pH range. Ouabain (0.1 mM) inhibited K(+)-dependent activity 50%; a concentration 10 times higher did not produce any appreciable effect on the K(+)-independent activity. Na+ did not affect K(+)-independent activity at all, while the same ligand concentration inhibited sharply the K(+)-dependent activity; this inhibition was not competitive with the substrate, p-nitrophenyl phosphate. K(+)-dependent activity was stimulated by Mg2+ with low affinity (millimolar range), and 3 mM Mg2+ produced a slight stimulation of the activity in absence of K+, which could be interpreted as Mg2+ occupying the K+ sites. Ca2+ had no appreciable effect on the activity in the absence of K+. However, in the presence of K+ a sharp inhibition was found with all Ca2+ concentrations studied. ATP (0.5 mM) did not affect the K(+)-independent activity, but this nucleotide behaved as a competitive inhibitor to p-nitrophenylphosphate. Pi inhibited activity in the presence of K+, competitively to the substrate, so it could be considered as the second product of the reaction sequence.

Solubilized alpha beta Na,K-ATPase remains protomeric during turnover yet shows apparent negative cooperativity toward ATP

A prominent feature of the Na,K-ATPase reaction is an ATP dependence that suggests high- and low-affinity ATP requirements during the enzymic cycle. As only one ATP-binding domain has been identified in the alpha subunit and none has been identified in the beta subunit, it has seemed likely that the apparent negative cooperativity results from subunit interactions in an (alpha beta)2 diprotomer. To test this possibility, we have examined the behavior of solubilized alpha beta protomers of Na,K-ATPase down to 50 nM [gamma-32P]ATP. Active-enzyme analytical ultracentrifugation shows that the protomer is the active species and that no oligomerization occurs during turnover. However, we find that dual ATP effects can be clearly demonstrated and that nonhydrolyzable ATP analogs can stimulate the Na,K-ATPase activity of the soluble protomer. We conclude that the apparent negative cooperativity is inherent to the alpha beta protomer and that this should explain some of the complexities found with membrane-bound Na,K-ATPase and, perhaps, other P-type cation pumps.

ATP dependence of Na(+)-K+ pump of cold-sensitive and cold-tolerant mammalian red blood cells

1. The ATP concentration of intact, cold-tolerant (ground squirrel) red cells and cold-sensitive (guinea-pig and human) red cells was monitored by use of the firefly tail, luciferin-luciferase assay. ATP kinetics of the pump in intact red blood cells was investigated by altering cell [ATP] by progressive depletion of ATP in the presence of 2-deoxy-D-glucose and then by measurement of ouabain-sensitive K+ influx at each level of [ATP] at various temperatures between 37 and 5 degrees C. Na(+)-K(+)-ATPase activity of broken membranes was also determined in parallel experiments using ouabain-sensitive release of 32P from [gamma-32P]ATP as a measure of activity. 2. Without depletion, there is no immediate decrease in [ATP] of intact cold-sensitive cells at low temperature (5 degrees C) at times when there are marked differences in the activities of the Na(+)-K+ pump of cold-tolerant and cold-sensitive cells. 3. At 37 degrees C Na(+)-K(+)-ATPase of all three species exhibited two components of ATP dependence at 37 degrees C, one with high velocity, low affinity, the other with low velocity, high affinity. Affinities of both components rose with cooling. 4. A similar, two component pattern was observed in intact guinea-pig and human red cells at 37 degrees C, except that the segment corresponding to the high affinity component had an apparent Km (Michaelis-Menten constant) 3- to 4-fold higher than that of the broken membrane preparation. 5. Cooling intact guinea-pig and human red cells decreased the apparent affinity of the high velocity, low affinity component for ATP, so that at 20 degrees C the value of Km approached or exceeded the levels of physiological ATP concentration. Below 20 degrees C only one component with values corresponding to that of the low velocity, high affinity component could be observed. 6. In intact ground squirrel cells only the low affinity, high velocity component was apparent between 37 and 5 degrees C. Its affinity for ATP rose with cooling between 37 and 5 degrees C.

Investigation of subunit interactions by radiation inactivation: the case of Na+/K+-ATPase

The target size of Na+/K+-ATPase has been determined by radiation inactivation. To interpret the results, we have performed Monte Carlo simulations of the inactivation process. This seems to be a general method for the interpretation of such studies. The simulation revealed that radiation inactivation can distinguish between monoprotomeric and multiprotomeric models of enzyme action only if the measured reaction requires the actual co-operation of, rather than the mere co-existence of, different protomeres.

An Activation of Synaptosomal Na+, K+-ATPase by a Novel Dibenzoxazepine Derivative (BY-1949) in the Rat Brain: Its Functional Role in the Neurotransmitter Uptake Systems

In search of factors mitigating the final outcome of ischemic and epileptic brain damage, we tested a novel dibenzoxazepine derivative (BY-1949), as the compound has been shown to be effective under these two conditions. First, using rat brain, we assessed whether or not BY-1949 affects the Na+,K(+)-ATPase activity. Although in vitro applications of either BY-1949 or its three major metabolites did not cause any apparent effects, both acute and chronic oral administrations of the compound (10 mg/kg) invariably increased the Na+,K(+)-ATPase activity in the synaptosomal plasma membranes by increasing Vmax values. Second, it was shown by this study that the drug treatment caused marked increases in the uptake of both glutamic acid and gamma-aminobutyric acid into the synaptosomes. These results suggest that the activity against ischemic/epileptic brain damage by BY-1949 is explicable, at least partly, in terms of improvement of ionic derangements across the neural membranes via Na+,K(+)-ATPase activation.

Depolarization-induced changes in cellular energy production

Addition of high concentrations of KC1 to preparations of rat brain synaptosomes incubated with either glucose or pyruvate caused a transient stimulation of oxygen uptake. This increased respiration was insensitive to 1 mM ouabain and 10 microM ruthenium red but was dependent upon the presence of calcium. With 40 mM KCl in the incubation medium, the levels of high-energy phosphate compounds in the synaptosomes were unaltered, whereas pyridine nucleotides underwent a rapid, albeit small and temporary, oxidation. It is postulated that there is a calcium-dependent mechanism in synaptosomes through which the function of the mitochondrial respiratory chain or of oxidative phosphorylation is stimulated directly without the involvement of either adenine nucleotides or mitochondrial dehydrogenases.

Similarities between the effects of dimethyl sulfoxide and calmodulin on the red blood cell Ca2(+)-ATPase

The Ca2(+)-ATPase of the erythrocyte plasma membrane can be activated by calmodulin, acidic phospholipids, limited proteolysis and self-association. Recently, it has been shown that different organic solvents increase both the Vmax and the Ca2+ affinity of the enzyme (Benaim, G. and De Meis, L. (1989) FEBS Lett. 244, 484-486). In this report the effects of calmodulin and dimethyl sulfoxide (20%, v/v) on the Ca2(+)-ATPase are compared. Dimethyl sulfoxide also elicits the appearance of the low-affinity binding site, which in this enzyme is strictly dependent on calmodulin. Dimethyl sulfoxide increases the Ca2+ affinity of the enzyme in a manner similar to that observed with the use of calmodulin and of acidic phospholipids. This was tested using both native and partially trypsinized ATPase. When activated by calmodulin the enzyme is inhibited by compound 48/80, trifluoperazine and calmidazolium. When activated by dimethyl sulfoxide the enzyme is still inhibited by calmidazolium but is no longer inhibited by either compound 48/80 or trifluoperazine. Activation of the ATPase promoted by either calmodulin or dimethyl sulfoxide is abolished when the Ca2+ concentration is raised from 10 microM to 2 mM. The effect of dimethyl sulfoxide is also abolished by 20 mM Pi. In the presence of 1 to 10 mM Ca2+ the ATPase catalyzes an ATP in equilibrium Pi exchange. The rate of exchange increases several fold when dimethyl sulfoxide is included in the assay medium.

Relationships between the neuronal sodium/potassium pump and energy metabolism. Effects of K+, Na+, and adenosine triphosphate in isolated brain synaptosomes

The relationships between Na/K pump activity and adenosine triphosphate (ATP) production were determined in isolated rat brain synaptosomes. The activity of the enzyme was modulated by altering [K+]e, [Na+]i, and [ATP]i while synaptosomal oxygen uptake and lactate production were measured simultaneously. KCl increased respiration and glycolysis with an apparent Km of about 1 mM which suggests that, at the [K+]e normally present in brain, 3.3-4 mM, the pump is near saturation with this cation. Depolarization with 6-40 mM KCl had negligible effect on ouabain-sensitive O2 uptake indicating that at the voltages involved the activity of the Na/K ATPase is largely independent of membrane potential. Increases in [Na+]i by addition of veratridine markedly enhanced glycoside-inhibitable respiration and lactate production. Calculations of the rates of ATP synthesis necessary to support the operation of the pump showed that greater than 90% of the energy was derived from oxidative phosphorylation. Consistent with this: (a) the ouabain-sensitive Rb/O2 ratio was close to 12 (i.e., Rb/ATP ratio of 2); (b) inhibition of mitochondrial ATP synthesis by Amytal resulted in a decrease in the glycoside-dependent rate of 86Rb uptake. Analyses of the mechanisms responsible for activation of the energy-producing pathways during enhanced Na and K movements indicate that glycolysis is predominantly stimulated by increase in activity of phosphofructokinase mediated via a rise in the concentrations of adenosine monophosphate [AMP] and inorganic phosphate [Pi] and a fall in the concentration of phosphocreatine [PCr]; the main moving force for the elevation in mitochondrial ATP generation is the decline in [ATP]/[ADP] [Pi] (or equivalent) and consequent readjustments in the ratio of the intramitochondrial pyridine nucleotides [( NAD]m/[NADH]m). Direct stimulation of pyruvate dehydrogenase by calcium appears to be of secondary importance. It is concluded that synaptosomal Na/K pump is fueled primarily by oxidative phosphorylation and that a fall in [ATP]/[ADP][Pi] is the chief factor responsible for increased energy production.

Differential effects of substrates on three transport modes of the Na+/K(+)-ATPase

With a purified Na+/K(+)-ATPase preparation reconstituted into phospholipid vesicles, Na+/K+, Na+/Na+, and uncoupled Na+ transport were studied using three nucleotides and five substrates of the K(+)-phosphatase reaction that this enzyme also catalyzes. For Na+/K+ exchange, CTP was half as effective as ATP and GTP one-twentieth; of the phosphatase substrates only carbamyl phosphate and 3-O-methylfluorescein phosphate produced significant transport and at merely 1% of the rate with ATP. For Na+/Na+ exchange, comparable rates of transport were produced by ATP, CTP, carbamyl phosphate and acetyl phosphate, although the actual rate of transport with ATP was only 2.4% of that for Na+/K+ exchange; slower rates occurred with GTP (69%), 3-O-methylfluorescein phosphate (51%), and nitrophenyl phosphate (33%). Only umbelliferone phosphate was ineffective. For uncoupled Na+ transport results similar to those for Na+/Na+ exchange were obtained, but the actual rate of transport was still slower, 1.4% of that for Na+/K+ exchange. Thus, not only nucleotides but a variety of phosphatase substrates (which are phosphoric acid mixed anhydrides) can phosphorylate the enzyme at the high-affinity substrate site to form the E1P intermediate of the reaction sequence. Oligomycin inhibited Na+/K+ exchange with ATP by half, but with carbamyl phosphate not at all; with CTP the inhibition was intermediate, one-fourth. By contrast, oligomycin inhibited Na+/Na+ exchange by one-fifth with all three substrates. A quantitative, steady-state kinetic model accounts for the relative magnitudes of Na+/K+ and Na+/Na+ exchanges with ATP, CTP, and carbamyl phosphate as substrates, as well as the extents of inhibition by oligomycin. The model requires that even when Na+ substitutes for K+ a slow step in the reaction sequence is the E2 to E1 conformational transition.

Solvent effects on substrate and phosphate interactions with the (Na+ + K+)-ATPase

(Na+ + K+)-ATPase activity of a dog kidney enzyme preparation was markedly inhibited by 10-30% (v/v) dimethyl sulfoxide (Me2SO) and ethylene glycol (Et(OH)2); moreover, Me2SO produced a pattern of uncompetitive inhibition toward ATP. However, K+-nitrophenylphosphatase activity was stimulated by 10-20% Me2SO and Et(OH)2 but was inhibited by 30-50%. Me2SO decreased the Km for this substrate but had little effect on the Vmax below 30% (at which concentration Vmax was then reduced). Me2SO also reduced the Ki for Pi and acetyl phosphate as competitors toward nitrophenyl phosphate but increased the Ki for ATP, CTP and 2-O-methylfluorescein phosphate as competitors. Me2SO inhibited K+-acetylphosphatase activity, although it also reduced the Km for that substrate. Finally, Me2SO increased the rate of enzyme inactivation by fluoride and beryllium. These observations are interpreted in terms of the E1P to E2P transition of the reaction sequence being associated with an increased hydrophobicity of the active site, and of Me2SO mimicking such effects by decreasing water activity: (i) primarily to stabilize the covalent E2P intermediate, through differential solvation of reactants and products, and thereby inhibiting the (Na+ + K+)-ATPase reaction and acting as a dead-end inhibitor to produce the pattern of uncompetitive inhibition; inhibiting the K+-acetylphosphatase reaction that also passes through an E2P intermediate; but not inhibiting (at lower Me2SO concentrations) the K+-nitrophenylphosphatase reaction that does not pass through such an intermediate; and (ii) secondarily to favor partitioning of Pi and non-nucleotide phosphates into the hydrophobic active site, thereby decreasing the Km for nitrophenyl phosphate and acetyl phosphate, the Ki for Pi and acetyl phosphate in the K+-nitrophenylphosphatase reaction, accelerating inactivation by fluoride and beryllium acting as phosphate analogs, and, at higher concentrations, inhibiting the K+-nitrophenylphosphatase reaction by stabilizing the non-covalent E2.P intermediate of that reaction. In addition, Me2SO may decrease binding at the adenine pocket of the low-affinity substrate site, represented as an increased Ki for ATP, CTP and 3-O-methylfluorescein phosphate.

Characteristics of 3-O-methylfluorescein phosphate hydrolysis by the (Na+ + K+)-ATPase

With 3-O-methylfluorescein phosphate (3-OMFP) as substrate for the phosphatase reaction catalyzed by the (Na+ + K+)-ATPase, a number of properties of that reaction differ from those with the common substrate p-nitrophenyl phosphate (NPP): the Km is 2 orders of magnitude less and the Vmax is two times greater, and dimethyl sulfoxide (Me2SO) inhibits rather than stimulates. In addition, reducing the incubation pH decreases both the Km and Vmax for K+-activated 3-OMFP hydrolysis as well as the K0.5 for K+ activation. However, reducing the incubation pH increases inhibition by Pi and the Vmax for 3-OMFP hydrolysis in the absence of K+. When choline chloride is varied reciprocally with NaCl to maintain the ionic strength constant, NaCl inhibits K+-activated 3-OMFP hydrolysis modestly with 10 mM KCl, but stimulates (in the range 5-30 mM NaCl) with suboptimal (0.35 mM) KCl. In the absence of K+, however, NaCl stimulates increasingly over the range 5-100 mM when the ionic strength is held constant. These observations are interpreted in terms of (a) differential effects of the ligands on enzyme conformations; (b) alternative reaction pathways in the absence of Na+, with a faster, phosphorylating pathway more readily available to 3-OMFP than to NPP; and (c) a (Na+ + K+)-phosphatase pathway, most apparent at suboptimal K+ concentrations, that is also more readily available to 3-OMFP.

Substrate sites of the (Na+ + K+)-ATPase: pertinence of the adenine and fluorescein binding sites

The (Na+ + K+)-activated ATPase catalyzes the K+-activated hydrolysis of 3-O-methylfluorescein phosphate (3OMFP) with a Km of 50 microM, nearly two orders of magnitude lower than the Km for nitrophenyl phosphate, 3 mM. Both ATP and nitrophenyl phosphate are competitors toward 3OMFP with Ki values corresponding to their Km values (for ATP that at the low-affinity sites of the E2 conformation). Enzyme treated with fluorescein isothiocyanate (FITC) such that 60% of the (Na+ + K+)-ATPase activity is lost still hydrolyzes both 3OMFP and nitrophenyl phosphate: the apparent Km values are increased less than 2-fold and the Vmax is unaffected. ATP still inhibits these K+-phosphatase reactions of the FITC-treated enzyme, and this inhibition can exceed the 40% of residual (Na+ + K+)-ATPase activity. Evaluation of a kinetic model indicates that the Ki for ATP is increased about an order of magnitude by FITC-binding. Similar results obtain with trinitrophenyl-ATP (TNP-ATP) as inhibitor, in this case with Ki values in the micromolar range. Finally, FITC treatment increases K+-activated ADPase activity. These observations are interpreted as the fluorescein ring of 3OMFP binding to the adenine pocket of the substrate site, thereby conferring high affinity, just as the fluorescein ring of FITC binding to the adenine pocket in the E1 conformation permits specific linkage of the isothiocyanate chain to a particular lysine, Lys-501. Then, coincident with the transition to the E2 conformation, which bears the low-affinity site for ATP and which catalyzes the K+-phosphatase reaction, the FITC molecule tethered to Lys-501 is pulled from the adenine pocket: allowing 3OMFP and ADP to bind as substrates and ATP and TNP-ATP as inhibitors, albeit in altered conformation. The E1 to E2 transition thus involves not only a change from high to low affinity for ATP, but also a distortion of the adenine pocket and the orientation between Lys-501 and Asp-369, the residue associated with catalysis.

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+.

The sided action of Na+ and of K+ on reconstituted shark (Na+ + K+)-ATPase engaged in Na+-Na+ exchange accompanied by ATP hydrolysis. I. The ATP activation curve

The ATP hydrolysis dependent Na+-Na+ exchange of reconstituted shark (Na+ + K+)-ATPase is electrogenic with a transport stoichiometry as for the Na+-K+ exchange, suggesting that translocation of extracellular Na+ is taking place via the same route as extracellular K+. The preparation thus offers an opportunity to compare the sided action of Na+ and K+ on the affinity for ATP in a reaction in which the intermediary steps in the overall reaction seems to be the same without and with K+. With Na+ but no K+ on the two sides of the enzyme, the ATP-activation curve is hyperbolic and the affinity for ATP is high. Extracellular K+ in concentrations of 50 microM (the lowest tested) and up gives biphasic ATP activation curves, with both a high- and a low-affinity component for ATP. Cytoplasmic K+ also gives biphasic ATP-activation curves, however, only when the K+ concentration is 50 mM or higher (Na+ + K+ = 130 mM). The different ATP-activation curves are explained from the Albers-Post scheme, in which there is an ATP-dependent and an ATP-independent deocclusion of E2(Na2+) and E2(K2+), respectively, and in which the dephosphorylation of E2-P is rate limiting in the presence of Na+ (but no K+) extracellular, whereas in the presence of extracellular K+ it is the deocclusion of E2(K2+) which is rate limiting.

Membrane biochemistry of the ouabain-resistant potassium transport system

A 6.5-kilobase fragment of genomic DNA from mutant mouse cells under ouabain selection pressure conferred ouabain resistance when transfected into ouabain-sensitive CV1 green monkey fibroblasts. Ouabain resistance was induced in the presence of 10 microM ouabain. Amiloride (500 microM) completely blocked ouabain-insensitive 86Rb+ uptake into these cells. Plasma membranes from these cells demonstrated little sodium-dependent adenosine triphosphatase (ATPase) activity but had potassium-dependent and ouabain-resistant p-nitrophenylphosphatase activity. Like Na+,K+-ATPase this activity was vanadate- and sodium-inhibitable. Also, like the Na+,K+-ATPase, sodium inhibition of the p-nitrophenylphosphatase was reversed by 10 microM adenosine 5'-triphosphate.

Demonstration of cooperating alpha subunits in working (Na+ + K+)-ATPase by the use of the MgATP complex analogue cobalt tetrammine ATP

The MgATP complex analogue cobalt-tetrammine-ATP [Co(NH3)4ATP] inactivates (Na+ + K+)-ATPase at 37 degrees C slowly in the absence of univalent cations. This inactivation occurs concomitantly with incorporation of radioactivity from [alpha-32P]Co(NH3)4ATP and from [gamma-32P]Co(NH3)4ATP into the alpha subunit. The kinetics of inactivation are consistent with the formation of a dissociable complex of Co(NH3)4ATP with the enzyme (E) followed by the phosphorylation of the enzyme: (Formula: see text). The dissociation constant of the enzyme-MgATP analogue complex at 37 degrees C is Kd = 500 microM, the inactivation rate constant k2 = 0.05 min-1. ATP protects the enzyme against the inactivation by Co(NH3)4ATP due to binding at a site from which it dissociates with a Kd of 360 microM. It is concluded, therefore, that Co(NH3)4ATP binds to the low-affinity ATP binding site of the E2 conformational state. K+, Na+ and Mg2+ protect the enzyme against the inactivation by Co(NH3)4ATP. Whilst Na+ or Mg2+ decrease the inactivation rate constant k2, K+ exerts its protective effect by increasing the dissociation constant of the enzyme.Co(NH3)4ATP complex. The Co(NH3)4ATP-inactivated (Na+ + K+)-ATPase, in contrast to the non-inactivated enzyme, incorporates [3H]ouabain. This indicates that the Co(NH3)4ATP-inactivated enzyme is stabilized in the E2 conformational state. Despite the inactivation of (Na+ + K+)-ATPase by Co(NH3)4ATP from the low-affinity ATP binding site, there is no change in the capacity of the high-affinity ATP binding site (Kd = 0.9 microM) nor of its capability to phosphorylate the enzyme Na+-dependently. Since (Na+ + K+)-ATPase is phosphorylated Na+-dependently from the high-affinity ATP binding site although the catalytic cycle is arrested in the E2 conformational state by specific modification of the low-affinity ATP binding site, it is concluded that both ATP binding sites coexist at the same time in the working sodium pump. This demonstration of interacting catalytic subunits in the E1 and E2 conformational states excludes the proposal that a single catalytic subunit catalyzes (Na+ + K+)-transport.

Lectin inhibition and kinetics of microsomal K+-dependent p-nitrophenyl phosphatase of frog epidermis

The specific activity of K+-dependent p-NPPase (paranitrophenylphosphatase) from frog (Rana ridibunda) epidermis microsomal preparation was determined. The activity was proportional to time of incubation and protein concentrations under our assays conditions. Optimal phosphatase activity was at pH from 8 to 9 and over 35 degrees C. 10(-3) M ouabain inhibited 100% of the activity and the Ki was estimated about 5 X 10(-5) M. The Km for p-NPP was 3.8 mM and 2.1 for K+. The lectins GSI and GSII produced 80-90% of non-competitive inhibition of the activity. 50% of inhibition by GSI was obtained at 2 micrograms/ml. The Km for p-NPP did not change but the Vmax of activity was clearly reduced for both GSI and GSII lectins.

Reaction sequences for (Na+ + K+)-dependent ATPase hydrolytic activities: new quantitative kinetic models

To delineate better the reaction sequence of the (Na+ + K+)-ATPase and illuminate properties of the active site, kinetic data were fitted to specific quantitative models. For the (Na+ + K+)-ATPase reaction, double-reciprocal plots of velocity against ATP (in the millimolar range), with a series of fixed KCl concentrations, are nearly parallel, in accord with the ping pong kinetics of ATP binding at the low-affinity sites only after Pi release. However, contrary to requirements of usual formulations, Pi is not a competitor toward ATP. A new steady-state kinetic model accommodates these data quantitatively, requiring that under usual assay conditions most of the enzyme activity follows a sequence in which ATP adds after Pi release, but also requiring a minor alternative pathway with ATP adding after K+ binds but before Pi release. The fit to the data also reveals that Pi binds nearly as rapidly to E2 X K X ATP as to E2 X K, whereas ATP binds quite slowly to E2 X P X K: the site resembles a cul-de-sac with distal ATP and proximal Pi sites. For the K+-nitrophenyl phosphatase reaction also catalyzed by this enzyme, the apparent affinities for both substrate and Pi (as inhibitor) decrease with higher KCl concentrations, and both Pi and TNP-ATP appear to be competitive inhibitors toward substrate with 10 mM KCl but noncompetitive inhibitors with 1 mM KCl. These data are accommodated quantitatively by a steady-state model allowing cyclic hydrolytic activity without obligatory release of K+, and with exclusive binding of substrate vs. either Pi or TNP-ATP. The greater sensitivity of the phosphatase reaction to both Pi and arsenate is attributable to the weaker binding by the occluded-K+ enzyme form occurring in the (Na+ + K+)-ATPase reaction sequence. The steady-state models are consistent with cyclical interconversion of high- and low-affinity substrate sites accompanying E1/E2 transitions, with distortion to low-affinity sites altering not only affinity and route of access but also separating the adenine- and phosphate-binding regions, the latter serving in the E2 conformation as the active site for the phosphatase reaction.

Effects of the ATP, ADP and inorganic phosphate on the transport rate of the Na+,K+-pump

(Na+ + K+)-ATPase from kidney outer medulla was incorporated into artificial dioleoylphosphatidylcholine vesicles. In the reconstituted system the pump can be activated by adding ATP to the external medium. ATP-driven potassium extrusion by the Na+,K+-pump was studied using a voltage-sensitive dye in the presence of valinomycin. ADP strongly reduced the turnover rate of the pump with a concentration for half-maximal inhibition of cD,1/2 = 0.1 mM. cD,1/2 was found to be virtually independent of ATP concentration, indicating that the inhibition is non-competitive with respect to ATP. The non-competitive inhibition by ADP can be explained on the basis of the Post-Albers reaction cycle of the Na+,K+-pump, assuming that the main action of ADP is the reversal of the phosphorylation step. A similar 'product inhibition' was observed with inorganic phosphate, but at much higher concentrations (cP,1/2 = 14 mM).

Cation sites, spermine, and the reaction sequence of the (Na+ + K+)-dependent ATPase

Spermine, at 0.3 mM, inhibits the K+-nitrophenyl phosphatase activity of a dog kidney (Na+ + K+)-ATPase preparation, increasing the K0.5 for K+, reducing the Km for substrate, and affecting little the inhibition by Na+. These actions can be attributed, in a model of the phosphatase reaction, to parallel decreases in affinity for K+ and Na+ at their cytoplasmically accessible sites. In the (Na+ + K+)-ATPase reaction, spermine increases the K0.5 for Na+ and, to a lesser degree, the K0.5 for K+ as activators. With spermine, the double-reciprocal plots of velocity vs. ATP concentration (in the range 0.3-3 mM), at fixed levels of K+ (from 1 to 10 mM), remain parallel but are rotated clockwise and spread somewhat, reflecting stimulation at low ATP concentrations and inhibition at high ATP but low KCl concentrations. These actions can be attributed, in a steady-state ping-pong model of the ATPase reaction, solely to decreased rates of binding of Na+ and K+ to their sites, with major effects at the cytoplasmically accessible sites for Na+ (acceptance) and K+ (discharge), and with a lesser effect at the extracellularly accessible sites for K+ (acceptance). On these grounds, spermine is a highly specific and potentially valuable reagent for studying the reaction. Furthermore, the model for K+-ATP interactions not only supports a specific reaction sequence (K+ addition, Pi release, ATP addition, K+ release) but also argues against the availability of low-affinity substrate sites except during sharply restricted segments of the reaction sequence, thereby favoring proposals that the low-affinity substrate sites are transformed into high-affinity substrate sites with the E2 to E1 conformational change.

Effects of altering the ATP/ADP ratio on pump-mediated Na/K and Na/Na exchanges in resealed human red blood cell ghosts

Resealed human red blood cell ghosts were prepared to contain a range of ADP concentrations at fixed ATP concentrations and vice versa. ATP/ADP ratios ranging from approximately 0.2 to 50 were set and maintained (for up to 45 min) in this system. ATP and ADP concentrations were controlled by the addition of either a phosphoarginine- or phosphocreatine-based regenerating system. Ouabain-sensitive unidirectional Na efflux was determined in the presence and absence of 15 mM external K as a function of the nucleotide composition. Na/K exchange was found to increase to saturation with ATP (K 1/2 approximately equal to 250 microM), whereas Na/Na exchange (measured in K-free solutions) was a saturating function of ADP (K 1/2 approximately equal to 350 microM). The elevation of ATP from approximately 100 to 1,800 microM did not appreciably affect Na/Na exchange. In the presence of external Na and a saturating concentration of external K, increasing the ADP concentration at constant ATP was found to decrease ouabain-sensitive Na/K exchange. The decreased Na/K exchange that still remained when the ADP/ATP ratio was high was stimulated by removal of external Na. Assuming that under normal substrate conditions the reaction cycle of the Na/K pump is rate-limited by the conformational change associated with the release of occluded K [E2 X (K) X ATP----E1 X ATP + K], increasing ADP inhibits the rate of these transformations by competition with ATP for the E2(K) form. A less likely alternative is that inhibition is due to competition with ATP at the high-affinity site (E1). The acceleration of the Na/K pump that occurs upon removing external Na at high levels of ADP evidently results from a shift in the forward direction of the transformation of the intermediates involved with the release of occluded Na from E1P X (Na). Thus, the nucleotide composition and the Na gradient can modulate the rate at which the Na/K pump operates.

Delta endotoxin inhibits Rb+ uptake, lowers cytoplasmic pH and inhibits a K+-ATPase in Manduca sexta CHE cells

Delta endotoxin, a 68 kilodalton protein isolated from Bacillus thuringiensis spp. Kurstaki, is a potent entomocidal agent that alters a K+ current across midgut tissue of many phytophagous insects. This toxin completely inhibited the vanadate-sensitive 86Rb+ uptake and mimicked the vanadate-induced decrease in cytosolic pH in a cell line (CHE) originating from Manduca sexta embryonic tissue. The toxin also inhibited a K+-sensitive-ATPase in the plasma membranes isolated from these cells. Using the K+-sensitive-ATPase substrate p-nitrophenyl phosphate, delta endotoxin was found to have a Ki of 0.4 microM. These data suggest that the toxin inhibits a K+-ATPase responsible for 86Rb+ uptake in the CHE cells. The relationship between the toxin inhibition of K+-ATPase and toxin-altered K+ current is discussed.