(Na+ + K+)-adenosine triphosphatase of mammalian brain. Catalytic and regulatory K+ sites distinguishable by selectivity for Li+

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.

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

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

Inhibition of (Na+, K+)-ATPase by fluoride: evidence for a membrane adaptation to ethanol

Fluoride ions inhibit several membrane enzymes in a manner that is dependent on membrane fluidity. Inhibition of (Na+, K+)-ATPase by fluoride ions may be a model for membrane effects on (Na+, K+)-ATPase. Therefore, we have examined properties of fluoride inhibition relative to interactions with ethanol and to ligands that alter sensitivity of (Na+, K+)-ATPase to ethanol. Fluoride ion reduced the K0.5 and Hill coefficient for K+ activation of p-nitrophenylphosphatase. Ethanol decreased the Hill coefficient and apparent affinity for inhibition of phosphatase activity by fluoride ion while dimethylsulfoxide had the opposite effects. Chronic ethanol treatment in vivo, which produced behavioral tolerance, had effects on fluoride inhibition opposite to those of ethanol in vitro. Inhibition by fluoride therefore may provide a useful marker for physiologic or pharmacologic conditions that alter regulation of (Na+, K+)-ATPase by membrane properties.

Difference between neuronal and nonneuronal (Na+ + K+)-ATPases in their conformational equilibrium

Several experiments were carried out to study the difference between two isozymes (alpha(+) and alpha) of (Na+ + K+)-ATPase in the conformational equilibrium. Rat brain (Na+ + K+)-ATPase was much more thermolabile than the kidney enzyme. Both enzymes were protected from heat inactivation not only by Na+ and K+, but also by choline in varying degrees, though there was a difference between the two enzymes in the protection by the ligands. The brain enzyme was partially protected from N-ethylmaleimide (NEM) inactivation by both Na+ and K+, but the effects of the ligands on NEM inactivation of the kidney enzyme were more complex. Though ligands differentially affected the thermostability and NEM sensitivity of the two enzymes, the effects were not simply related to the conformational states. The sensitivity of phosphoenzyme (EP) formed in the presence of ATP, Na+, and Mg2+ to ADP or K+ and K+-p-nitrophenyl phosphatase (pNPPase) was then studied as a probe of the differences in the conformational equilibrium between the two isozymes. The EP of the brain enzyme was partially sensitive to ADP, while those of the heart and kidney enzymes were not. At physiological Na+ concentrations the percentages of E1P formed by the brain and kidney enzymes were determined to be about 40-50 and 10-20% of the total EP, respectively. The hydrolytic activity of pNPP in the presence of Li+, a selective activator at catalytic sites of the reaction, was much higher in the kidney enzyme than in the brain enzyme. The inhibition of K+-stimulated pNPPase by ATP and Na+ was greater in the latter enzyme than in the former. These results suggest that neuronal and nonneuronal (Na+ + K+)-ATPases differ in their conformational equilibrium: the E1 or E1P may be more stable in the alpha(+) than in the alpha during the turnover, and conversely the E2 or E2P may be more stable in the latter than in the former.

Depolarization of brain cortex slices and synaptosomes by lithium. Determination of K+-equilibrium potential in cortex slices

K+-equilibrium potential was determined in brain cortex slices of rat by measuring 86Rb+ distribution between the extra- and intracellular space. The ratio of internal to external Rb+ concentration was 39 +/- 1.8, corresponding to a resting membrane potential of 93.8 mV. Li+ (1-126 mM) decreased the membrane potential in both cortex slices and synaptosomes in a concentration-dependent manner. The presence of 1 mM Li+ was enough to cause a slight but distinct depolarization. During incubation in Li+-containing medium slices took up K+; however, for depolarization the presence of extracellular Li+ seemed to be necessary.

Substance P induces alterations on cerebral lipids involved in membrane fluidity

This study was undertaken to examine the variations in rat brain of cholesterol, phospholipid and phospholipid fatty acid composition induced by substance P. The cholesterol content was increased by substance P; concomitantly, an increase of the ratio cholesterol/phospholipid was observed. These changes do not appear to be responsible of the stimulation observed in Na+,K+-ATPase activity by substance P action. Phospholipid fatty acid analysis revealed that the peptide induced a decrease in both linoleic and arachidonic acids content.

Brain Na+,K+-ATPase: alteration of ligand affinities and conformation by chronic ethanol and noradrenergic stimulation in vivo

These experiments examined effects of chronic ethanol, repeated noradrenergic stimulation or inhibition, and ethanol combined with the noradrenergic treatments on regulation of Na+,K+-ATPase. Chronic treatment with ethanol reduced the sensitivity of K+-p-nitrophenyl-phosphatase to ethanol, increased affinity for K+, reduced the sensitivity of K+ affinity to ATP or ethanol, and reduced delta H and delta S for K+ activation and for the E1-E2 transition. These effects were all opposite to those of ethanol added in vitro. Treatment with yohimbine had the opposite effects on ethanol sensitivity, K+ affinity, K+ interactions with ethanol and ATP, and thermodynamic parameters for cation activation or conformational change. These effects were similar to those of norepinephrine in vitro. The effects of yohimbine treatment were eliminated or reduced in rats also treated with ethanol. Depletion of norepinephrine had effects opposite to those of yohimbine. These data are consistent with a reduction in membrane fluidity, at least in the vicinity of Na+,K+-ATPase, during ethanol tolerance. Exposure to norepinephrine, in vitro or in vivo, had effects on Na+,K+-ATPase that were similar to those of increased membrane fluidity.

Free fatty acids and (Na+,K+)-ATPase: effects on cation regulation, enzyme conformation, and interactions with ethanol

Effects of free fatty acids on parameters of (Na+,K+)-ATPase regulation related to enzyme conformation were examined. Sensitivity to inhibition by free fatty acid increased as the number of double bonds increased. Free fatty acids reduced affinity for K+ or Na+ at their regulatory sites without altering apparent K+ affinity at its high-affinity site, and increased apparent affinity for ATP. The apparent E2/E1 ratio and apparent delta H and delta S for the E1-E2 transition were reduced by fatty acid. High K+ or low temperature reduced the sensitivity of enzyme to inhibition by free fatty acid. In the presence of low K+, arachidonic acid potentiated inhibition of phosphatase activity by ethanol. Arachidonic acid alone had little effect on the rate of ouabain binding, but accelerated ouabain binding in the presence of K+. These data suggest that fatty acids alter (Na+,K+)-ATPase by preventing the univalent cation-mediated transition to E2, the K+-sensitive form of enzyme. (Na+,K+)-ATPase could potentially be influenced in vivo by free fatty acids released by phospholipases or during hypoxia, or by changes in membrane lipid saturation.

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

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

Brain (Na+,K+)-ATPase: biphasic interaction with erythrosin B

Brain (Na+,K+)-adenosine triphosphatase (EC 3.6.1.3) has both high and low affinity ouabain binding sites. It has been proposed that the high affinity ouabain binding sites characterize a nerve-specific form of the enzyme. Erythrosin B has been reported to inhibit high affinity ouabain binding selectively. The experiments in this paper were carried out in order to characterize the interactions of erythrosin B with (Na+,K+)-ATPase and to examine the specificity of erythrosin B for enzyme with high affinity for ouabain. Inhibition by erythrosin B was biphasic, with a rapid and a slow phase. The rapid phase appeared to be relatively specific for enzyme with high affinity for ouabain, while the slow phase was not. Inhibition by erythrosin B was accelerated by Mg2+ and was retarded by ATP, K+, or Na+ and ATP. Erythrosin B increased apparent affinity of the enzyme for K+ and decreased apparent affinity for Na+ and for ATP. These results indicate that erythrosin B interacts with an ATP site and has effects on cation affinities opposite to those of ATP. Erythrosin B inhibition is proportional to high affinity ouabain binding if brief incubation times and moderate concentrations are used.

Temperature effects on cation affinities of the (Na+, K+)-ATPase of mammalian brain

Effects of temperature on the Na+-dependent ADP-ATP exchange and the p-nitrophenylphosphatase reactions catalysed by (Na+, K+)-ATPase were examined. Apparent Mg2+ affinity decreased with decreasing temperature. Arrhenius plots of p-nitrophenylphosphatase in the presence of Na+ and ATP had discontinuities similar to those previously reported for (Na+ + K+)-ATPase, while those of p-nitrophenylphosphatase measured without Na+ or ATP did not. The apparent activation energy for p-nitrophenylphosphatase was a function of the physical characteristics of the cation acting at the K+ site.

Modification of the rate of ouabain binding to (Na+ + K+)ATPase by lithium ions

We report on the interactions of Li+, a congener of K+ with the (Na+ + K+)-ATPase from E Electricus as measured by their effects on the rate of [3H]-ouabain binding to this enzyme. Like K+, Li+ slows ouabain binding under both Type I (Na+ + ATP) and Type II (P1) conditions, but with lower affinity. In contrast to K+, the Li+ inhibition curve is hyperbolic, suggesting interaction at an uncoupled site. Also differing from the complete inhibition by high K+, a residual ouabain-binding rate persists at high Li+. The interactions of Li+ and K+ are synergistic: the apparent K+ affinity increases 3 to 4-fold in presence of Li+. These results are consistent with the conclusion that Li+ interacts with only one of the two K+ sites and may be of interest in interpreting lithium pharmacology.