Sodium-Potassium-activated Adenosine Triphosphatase

Osmotic stress and viscous retardation of the Na,K-ATPase ion pump

The transport function of the Na pump (Na,K-ATPase) in cellular ion homeostasis involves both nucleotide binding reactions in the cytoplasm and alternating aqueous exposure of inward- and outward-facing ion binding sites. An osmotically active, nonpenetrating polymer (poly(ethyleneglycol); PEG) and a modifier of the aqueous viscosity (glycerol) were used to probe the overall and partial enzymatic reactions of membranous Na,K-ATPase from shark salt glands. Both inhibit the steady-state Na,K-ATPase as well as Na-ATPase activity, whereas the K(+)-dependent phosphatase activity is little affected by up to 50% of either. Both Na,K-ATPase and Na-ATPase activities are inversely proportional to the viscosity of glycerol solutions in which the membranes are suspended, in accordance with Kramers' theory for strong coupling of fluctuations at the active site to solvent mobility in the aqueous environment. PEG decreases the affinity for Tl(+) (a congener for K(+)), whereas glycerol increases that for the nucleotides ATP and ADP in the presence of NaCl but has little effect on the affinity for Tl(+). From the dependence on osmotic stress induced by PEG, the aqueous activation volume for the Na,K-ATPase reaction is estimated to be approximately 5-6 nm(3) (i.e., approximately 180 water molecules), approximately half this for Na-ATPase, and essentially zero for p-nitrophenol phosphatase. The change in aqueous hydrated volume associated with the binding of Tl(+) is in the region of 9 nm(3). Analysis of 15 crystal structures of the homologous Ca-ATPase reveals an increase in PEG-inaccessible water space of approximately 22 nm(3) between the E(1)-nucleotide bound forms and the E(2)-thapsigargin forms, showing that the experimental activation volumes for Na,K-ATPase are of a magnitude comparable to the overall change in hydration between the major E(1) and E(2) conformations of the Ca-ATPase.

Selectivity of pyruvate kinase for Na+ and K+ in water/dimethylsulfoxide mixtures

In aqueous media, muscle pyruvate kinase is highly selective for K+ over Na+. We now studied the selectivity of pyruvate kinase in water/dimethylsulfoxide mixtures by measuring the activation and inhibition constants of K+ and Na+, i.e. their binding to the monovalent and divalent cation binding sites of pyruvate kinase, respectively [Melchoir J.B. (1965) Biochemistry 4, 1518-1525]. In 40% dimethylsulfoxide the K0.5 app for K+ and Na+ were 190 and 64-fold lower than in water. Ki app for K+ and Na+ decreased 116 and 135-fold between 20 and 40% dimethylsulfoxide. The ratios of Ki app/K0.5 app for K+ and Na+ were 34-3.5 and 3.3-0.2, respectively. Therefore, dimethylsulfoxide favored the partition of K+ and Na+ into the monovalent and divalent cation binding sites of the enzyme. The kinetics of the enzyme at subsaturating concentrations of activators show that K+ and Mg2+ exhibit high selectivity for their respective cation binding sites, whereas when Na+ substitutes K+, Na+ and Mg2+ bind with high affinity to their incorrect sites. This is evident by the ratio of the affinities of Mg2+ and K+ for the monovalent cation binding site, which is close to 200. For Na+ and Mg2+ this ratio is approximately 20. Therefore, the data suggest that K+ induces conformational changes that prevent the binding of Mg2+ to the monovalent cation binding site. Circular dichroism spectra of the enzyme and the magnitude of the transfer and apparent binding energies of K+ and Na+ indicate that structural arrangements of the enzyme induced by dimethylsulfoxide determine the affinities of pyruvate kinase for K+ and Na+.

Effects of extracts from the cyanobacterium Microcystis aeruginosa on ion regulation and gill Na+,K+-ATPase and K+-dependent phosphatase activities of the estuarine crab Chasmagnathus granulata (Decapoda, Grapsidae)

Recent discoveries indicate that microcystins affect enzymes, such as Na(+),K(+)-ATPase, involved in ion regulation of aquatic animals, through K(+)-dependent phosphatase inhibition. In vitro studies showed the inhibitory effect of Microcystis aeruginosa extracts on Na(+),K(+)-ATPase and K(+)-dependent phosphatase activities in gills of Chasmagnathus granulata (Decapoda, Grapsidae). Extracts of M. aeruginosa were prepared from lyophilized or cultures cells of the cyanobacterium. For lyophilized cells, IC(50) values were estimated as 0.46 microg/L (95% confidence interval [CI]=0.40-0.52 microg/L) and 1.31 microg/L (95% CI=1.14-1.51 microg/L) for Na(+),K(+)-ATPase and K(+)-dependent phosphatase, respectively. However, extracts prepared from cultured cells presented a much lower inhibitory potency against both enzymes. Gas chromatography revealed long-chain fatty acids in the lyophilized cell extracts, indicating that they are in part responsible for the enzyme inhibition. In vivo studies showed that the toxin inhibited Na(+),K(+)-ATPase activity in anterior gills, whereas an increased augmented activity of glutathione-S-transferase was observed in both kind of gills, indicating that the crab has increased its ability to conjugate the toxin. No significant differences in hemolymph sodium or chloride concentration were detected. This result is in agreement with the lack of effects of microcystin on Na(+),K(+)-ATPase activity of posterior (osmoregulating) gills.

In vitro dose dependent inverse effect of nantenine on synaptosomal membrane K+-p-NPPase activity

The effect of nantenine, an aporphine alkaloid, on ATPase K+-dependent dephosphorylation was evaluated using p-nitrophenylphosphate (p-NPP) as substrate. Basal K+-p-NPPase activity was significantly increased with 3 x 10(-4) M, remained unchanged with 3 x 10(-6) M, 3 x 10(-5) M but was reduced with 7.5 x 10(-4) M and 1 x 10(-3) M nantenine, whereas Mg2+-p-NPPase activity was not modified. Kinetic studies showed that K+-p-NPPase inhibition by nantenine is competitive to KCl but non-competitive to substrate p-NPP, whereas K+-p-NPPase stimulation by nantenine is non-competitive to KCl but competitive to p-NPP. These data suggest that there may be two acceptor sites for nantenine in p-NPPase, one eliciting stimulation and the other inhibition of K+-dependent p-NPP hydrolysis. Considering the biphasic action of nantenine on seizures and the correlation between decreased ATPase activity and seizure development, alkaloid anticonvulsant effect observed at low nantenine doses is attributable to the stimulation of phosphatase activity whereas the convulsant effect at high alkaloid doses seems related to Na+, K+-ATPase inhibition.

Effect of methyl isocyanate on rabbit cardiac Na+, K(+)-ATPase

The present study describes the effect of methyl isocyanate (MIC) on rabbit cardiac microsomal Na+, K(+)-ATPase. Addition of MIC in vitro resulted in dose-dependent inhibition of Na+, K(+)-ATPase, Mg(2+)-ATPase and K(+)-activated p-nitrophenyl phosphatase (K(+)-PNPPase). Activation of Na+, K(+)-ATPase by ATP in the presence of MIC showed a decrease in Vmax with no change in Km. Similarly, activation of K+ PNPPase by PNPP in the presence of MIC showed a decrease in Vmax with no change in Km. The circular dichroism spectral studies revealed that MIC interaction with Na+, K(+)-ATPase led to a conformation of the protein wherein the substrates Na+ and K+ were no longer able to bind at the Na(+)- and K(+)-activation sites. The data suggest that the inhibition of Na+, K(+)-ATPase was non-competitive and occurred by interference with the dephosphorylation of the enzyme-phosphoryl complex.

Control of the Na+,K(+)-ATPase under normal and pathological conditions

The Na+,K(+)-ATPase is an important enzyme in determining the ionic milieu of the cerebromicrovasculature and neurons. The effect of hypertension or aging on this enzyme, as well as its susceptibility to regulation by fatty acids or aluminum, is the focus of this study. A significant increase (34%) in the apparent affinity constant (KD) but no change in the maximum binding capacity (Bmax) for [3H]ouabain binding to the cerebromicrovascular Na+,K(+)-ATPase occurs after induction of acute hypertension. In addition, long chain unsaturated fatty acids stimulate the binding of [3H]ouabain to the enzyme in microvessels from normotensive and hypertensive rats. The synaptosomal Na+,K(+)-ATPase is sensitive to aluminum. AlCl3 (1-100 microM) inhibits the K(+)-dependent-p-nitrophenylphosphatase (K(+)-NPPase) activity of the Na+,K(+)-ATPase in a dose-dependent manner. AlCl3 (100 microM) decreases the Vmax by 14% but does not alter the KM, suggestive of non-competitive inhibition. The enzyme from aged brain displays a greater Vmax, but shows the same susceptibility to AlCl3 as the enzyme from younger brain. In summary, disruption of the Na+,K(+)-ATPase may underlie, at least in part, abnormalities of nerve and vascular cell function in disorders where elevated concentrations of fatty acids or metal ions are involved.

Alterations of cerebromicrovascular Na+,K(+)-ATPase activity due to fatty acids and acute hypertension

Acute hypertension, induced in rats by intravenous injection of angiotensin II, previously has been shown to increase cerebrovascular permeability to macromolecules. The purpose of this study was to examine the effect of acute hypertension on Na+,K(+)-ATPase, the enzyme responsible for controlling ionic permeability of the cerebromicrovascular endothelium. The K(+)-dependent p-nitrophenylphosphatase activity of the cerebromicrovascular Na+,K(+)-ATPase was determined using microvessels prepared from hypertensive and normotensive rats. When compared to controls, a 70% decrease (P < 0.02) in the maximum rate (Vmax) of the Na+,K(+)-ATPase from hypertensive rats was evident with no change in the Michaelis constant (KM). In contrast, gamma-glutamyltranspeptidase, a marker enzyme for cerebral endothelial cells, was not significantly affected. Sodium arachidonate (1-100 microM) inhibited the phosphatase activity of the Na+,K(+)-ATPase in microvessels isolated from both normotensive and hypertensive rats in a dose-dependent manner. Furthermore, poly-unsaturated fatty acids (sodium linoleate and arachidonate) evoked the greatest inhibition of the enzyme, while sodium oleate and sodium palmitate inhibited the Na+,K(+)-ATPase to lesser extents. This regulation of enzyme activity by fatty acids was comparable in control and hypertensive groups. In summary, the data indicate that the cerebromicrovascular Na+,K(+)-ATPase was altered as a consequence of acute hypertension and that poly-unsaturated fatty acids can modulate this enzyme in microvessels derived from hypertensive or control rats.

Triorganotin inhibition of rat cardiac adenosine triphosphatases and catecholamine binding

Triorganotins have been reported to affect heme metabolism as well as the cardiovascular system. Our recent studies indicated that these organotins inhibit cardiac sarcoplasmic reticulum Ca(2+)-transport and cAMP-stimulated phosphorylation of specific proteins involved in Ca2+ transport, suggesting their interference with cardiac adrenergic function. The present study determines the effect of three organotins--tributyltin bromide (TBT), triethyltin bromide (TET) and trimethyltin chloride (TMT)--on rat cardiac ATPases and catecholamine binding, since these phenomena are involved in cardiac function. Cardiac membrane fraction was prepared from heart ventricles of male Sprague-Dawley rats. All three organotins inhibited cardiac Na+,K(+)-ATPase, [3H]ouabain binding, K(+)-activated p-nitrophenyl phosphatase (K(+)-PNPPase) and oligomycin-sensitive (OS) and oligomycin-insensitive (OI) Mg(2+)-ATPase in a concentration-dependent manner. K(+)-PNPPase was less sensitive to these triorganotins when compared to Na+K(+)-ATPase, suggesting that triorganotins affect the Na(+)-pump activity by acting on the Na(+)-dependent phosphorylation process. OS Mg(2+)-ATPase was more sensitive to these organotins when compared to OI Mg(2+)-ATPase, confirming their potent effect on the enzymes of oxidative phosphorylation. The order of potency is TBT greater than TET greater than TMT. TET and TMT, but not TBT, inhibited [3H]norepinephrine and [3H]dopamine binding to cardiac membranes in a concentration-dependent manner, the effect being more with TET. These results suggest that triorganotins inhibit sodium pump activity as well as ATP synthesis. Since Na+,K(+)-ATPase is involved in the active transport of catecholamines, triorganotins not only inhibited the catecholamine transport but also to some extent affected catecholamine binding, thus interfering with cardiac function.

Low level lead inhibits the human brain cation pump

The impact of low level lead exposure on human central nervous system function is a major public health concern. This study addresses the inhibition of the cation pump enzyme Na, K-ATPase by low level lead. Human brain tissue was obtained at autopsy and frozen until use. Brain homogenates were preincubated with PbCl2 for 20 min at 0 degrees C. Inhibition of K-paranitrophenylphosphatase (pNPPase), a measure of the dephosphorylation step of Na,K-ATPase, reached steady state within 10 min. K-pNPPase activity, expressed (mean +/- SEM) as a percentage of control (45.2 +/- 2.7 nmol/mg/min), fell to 96.3 +/- 0.9% at 0.25 uM [PbCl2] to 82.0 +/- 1.6% at 2.5 uM [PbCl2] in homogenates prepared from normal brain. Similar results were obtained with homogenates prepared from brains of patients with a history of alcohol abuse and of those with other miscellaneous conditions. Since the mean blood level of lead in the United States has ranged recently from 9.2 to 16.0 ug/dl (0.44 to 0.77 uM), these results indicate that current in vivo levels of lead exposure may impair important human brain function.

Effect of mercuric chloride on the kinetics of cationic and substrate activation of the rat brain microsomal ATPase system

Mercuric chloride (HgCl2), a neurotoxic compound, inhibited the adenosine triphosphatase (ATPase) system in a concentration-dependent manner. Hydrolysis of ATP was linear with time with or without HgCl2 in the reaction mixtures. Higher inhibition of (Na(+)-K+)ATPase activity by HgCl2 was observed in alkaline (8.0 to 9.0) pH and at lower temperatures (17 to 32 degrees). Activation energy values were increased slightly in the presence of HgCl2. Activation of (Na(+)-K+)ATPase by ATP in the presence of HgCl2 showed a decrease in Vmax from 15.29 to 5.0 mumol of inorganic phosphate (Pi)/mg protein/hr with no change in Km. Similarly, activation of K(+)-stimulated p-nitrophenyl phosphatase (K(+)-PNPPase) in the presence of HgCl2 showed a decrease in Vmax from 3.26 to 1.35 mumols of p-nitrophenol (PNP)/mg protein/hr with no change in Km. K(+)-activation kinetic studies indicated that HgCl2 decreased Vmax from 14.01 to 4.30 mumols Pi/mg protein/hr in the case of (Na(+)-K+)ATPase and from 3.45 to 2.40 mumols PNP/mg protein/hr in the case of K(+)-PNPPase with no changes in Km. Na(+)-activation of (Na(+)-K+)ATPase in the presence of HgCl2 showed a decrease in Vmax from 11.06 to 3.23 mumols Pi/mg protein/hr and an increase in Km from 1.06 to 2.08 mM. Preincubation of microsomes with sulfhydryl (SH) agents dithiothreitol, cysteine and glutathione protected HgCl2-inhibition of (Na(+)-K+)ATPase. The data suggest that HgCl2 inhibited (Na(+)-K+)ATPase by interfering with the dephosphorylation of the enzyme-phosphoryl complex.

Phosphorylation of Na+, K(+)-ATPase by ATP in the presence of K+ and dimethylsulfoxide but in the absence of Na+

Purified Na+, K(+)-ATPase was phosphorylated by [gamma-32P]ATP in a medium containing dimethylsulfoxide and 5 mM Mg2+ in the absence of Na+ and K+. Addition of K+ increased the phosphorylation levels from 0.4 nmol phosphoenzyme/mg of protein in the absence of K+ to 1.0 nmol phosphoenzyme/mg of protein in the presence of 0.5 mM K+. Higher velocities of enzyme phosphorylation were observed in the presence of 0.5 mM K+. Increasing K+ concentrations up to 100 mM lead to a progressive decrease in the phosphoenzyme (EP) levels. Control experiments, that were performed to determine the contribution to EP formation from the Pi inevitably present in the assays, showed that this contribution was of minor importance except at high (20-100 mM) KCl concentrations. The pattern of EP formation and its KCl dependence is thus characteristic for the phosphorylation of the enzyme by ATP. In the absence of Na+ and with 0.5 mM K+, optimal levels (1.0 nmol EP/mg of protein) were observed at 20-40% dimethylsulfoxide and pH 6.0 to 7.5. Addition of Na+ up to 5 mM has no effect on the phosphoenzyme level under these conditions. At 100 mM Na+ or higher the full capacity of enzyme phosphorylation (2.2 nmol EP/mg of protein) was reached. Phosphoenzyme formed from ATP in the absence of Na+ is an acylphosphate-type compound as shown by its hydroxylamine sensitivity. The phosphate radioactivity was incorporated into the alpha-subunit of the Na+, K(+)-ATPase as demonstrated by acid polyacrylamide gel electrophoresis followed by autoradiography.

Cytochemical demonstration of NPPase activity for detecting proton-translocating ATPase of Golgi complex in rat pancreatic acinar cells

An attempt at cytochemical demonstration of acidification proton-translocating ATPase (H(+)-ATPase) of Golgi complex in rat pancreatic acinar cells has been made by using p-nitrophenylphosphatase (NPPase) cytochemistry which is used for detecting of Na(+)-K(+)-ATPase (Mayahara et al. 1980) and gastric H(+)-K(+)-ATPase (Fujimoto et al. 1986). K(+)-independent NPPase activity was observed on the membrane of the trans cisternae of Golgi complex, but not inside of cisternae. The localization of NPPase activity is different from that of acid phosphatase activity where reaction products were seen on the inside of the trans Golgi cisternae. Since this activity was insensitive to vanadate, ouabain and independent of potassium ions, it was distinct from plasma membranous ATPases such as Na(+)-K(+)-ATPase and Ca2(+)-ATPase. The K(+)-independent NPPase activity was diminished by the inhibitors of H(+)-ATPase such as N-ethylmaleimide (NEM) and 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS). The NPPase reaction products were also seen on the membranes of other acidic organelles, i.e., lysosomes, endosomes, autophagosomes and coated vesicles. These results suggest that NPPase activity on the membrane of the Golgi complex and other acidic organelles corresponds with H(+)-ATPase which plays a role in acidification.

Inhibition of rat brain microsomal Na+/K(+)-ATPase and ouabain binding by mercuric chloride

This study concerned the effects of mercuric chloride on Na+/K(+)-ATPase and [3H]ouabain binding in rat brain microsomes in vitro. The data showed that HgCl2 inhibited Na+/K(+)-ATPase effectively at micromolar concentrations. The degree of inhibition was decreased with increases in enzyme concentration and incubation time. Variations in the ionic strength of Na+ and K+ did not alter the percent inhibition of Na+/K(+)-ATPase activity by HgCl2. Repeated washings partially restored enzyme activity. The binding of [3H]ouabain to microsomal membranes was inhibited by HgCl2 in a concentration-dependent manner. Cumulative inhibition studies with HgCl2 and ouabain indicated that these inhibitors did not act concurrently and independently on Na+/K(+)-ATPase.

Ultracytochemical characteristic of ouabain-sensitive, potassium-dependent p-nitrophenylphosphatase (Na(+)-K(+)-ATPase) of rat myocardium

The ultracytochemical localization of ouabain-sensitive, potassium-dependent p-nitrophenylphosphatase within a rat heart has been studied. Our cytochemical procedure for the detection of enzyme activity is based on an incubation medium consisting of tris-maleate buffer, p-nitrophenylphosphate (p-NPP) as a substrate, lead nitrate as a trapping agent, MgCl2, KCl, levamisole, final pH = 7.3. The physiological pH of the incubation medium highly increases the authenticity of the enzymatic reaction and is the main advantage of the technique. Postincubation processing of specimens by 0.1 mol/l tris-maleate buffer pH = 6.0 confirms the specificity of the reaction and removes nonspecific precipitation.

Modification of ligand binding to the Na+/K+-activated ATPase

Interactions between the ligands Mg2+, K+, and substrate and the Na+/K+-activated ATPase were examined in terms of a rapid-equilibrium, random-order, terreactant kinetic scheme for the K+-nitrophenyl phosphatase reaction that is catalyzed by this enzyme. At 37 degrees C and pH 7.5 the derived values for the dissociation constants from the free enzyme were 0.2, 0.08, and 1.4 mM for Mg2+, K+, and substrate, respectively. For Mg2+ interactions, the presence of 20% (v/v) dimethyl sulfoxide (Me2SO) increased the calculated affinity 25-fold; higher concentrations increased affinity still further. Neither reducing the temperature to 20 degrees C nor altering the pH from 6.5 to 8.3 appreciably changed the affinity for Mg2+ in the absence or presence of Me2SO. The Mg2+ sites are thus characterized by an absence of functional groups ionizable in the pH range 6.5-8.3, with binding driven by entropy changes, and with Me2SO, probably through solvation effects on the protein, increasing affinity for Mg2+ close to that for Ca2+ and Mn2+. By contrast, for K+ interactions, the presence of 20% Me2SO increased the calculated affinity only by half; moreover, reducing the temperature to 20 degrees C and the pH to 6.5 both increased affinity and diminished the response to Me2SO. The K+ sites are thus characterized by a marked sensitivity to pH and temperature, presumably through alterations in enzyme conformational equilibria that in turn are modifiable by Me2SO. Inhibition by higher concentrations of Mg2+, which varies inversely with the K+ concentration, was decreased by Me2SO. Finally, for substrate interactions, the presence of 20% Me2SO increased the calculated affinity 4-fold, and, as for Mg2+-binding, neither reducing the temperature nor varying the pH over the range 6.5-8.3 appreciably altered the affinity in the absence or presence of Me2SO. Thus, the substrate sites, like the Mg2+ sites, are characterized by an absence of functional groups ionizable in this range, with binding driven by entropy changes, and with Me2SO increasing affinity for substrate, in this case probably through favoring the partitioning of substrate from the medium into the hydrophobic active site.

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.

Inhibition by p-nitrophenylphosphate of acetylcholine release induced by Na+-deprivation

The effect of p-nitrophenylphosphate (p-NPP) on the release of acetylcholine evoked by drugs and ionic environments known to inhibit Na+, K+-ATPase was studied in isolated cortical slices of rat brain and longitudinal muscle strip of guinea-pig ileum. p-NPP inhibited the release of acetylcholine induced by sodium deprivation provided that the circumstances were in favour of the function of the K+-activated part of ATPase. However, it failed to antagonize the increase in the acetylcholine release elicited by omission of K+ or by administration of ouabain. Therefore it is concluded that the K+-stimulated phosphatase moiety of the Na+, K+-ATPase might be involved in the release of acetylcholine.

Effects of tricyclohexylhydroxytin on the kinetics of adenosine triphosphatase system and protection by thiol reagents

Tricyclohexylhydroxytin, commonly known as Plictran, inhibited Na+, K+-ATPase activity of rat brain synaptosomes in a concentration-dependent manner with median inhibitory concentration (IC-50) of 2 microM. Both K+-stimulated para-nitrophenylphosphatase and [3-H]-ouabain binding to synaptosomes were also inhibited by Plictran with IC-50 values of 11 and 30 microM, respectively. Altered pH and Na+, K+-ATPase activity curves demonstrated comparable inhibition in buffered neutral and alkaline pH ranges, and no inhibition was observed in acidic pH. The inhibition of Na+, K+-ATPase was independent of temperature. Kinetic studies of substrate (ATP) activation of Na+, K+-ATPase indicated uncompetitive inhibition. Results also showed noncompetitive inhibition for p-nitrophenylphosphate and uncompetitive inhibition for K+ activations of p-nitrophenylphosphatase. Preincubation of synaptosomes with dithiothreitol, a sulfhydryl (SH) agent, resulted in the complete protection of Plictran inhibition of Na+, K+-ATPase, K+-para-nitrophenylphosphatase, and [3-H]-ouabain binding. The protection was specific and concentration dependent since cysteine and glutathione did not afford protection. These results indicate that Plictran inhibited Na+, K+-ATPase by interacting with dephosphorylation of the enzyme-phosphoryl complex and exerted a similar effect to that of SH-blocking agents.

Ultracytochemistry of ouabain-sensitive K+-dependent p-nitrophenyl phosphatase in rat incisor enamel organ

Sprague-Dawley strain rats of 4-5 weeks old were perfusion-fixed with either a mixture containing 0.1 or 0.25% glutaraldehyde and 2% formaldehyde, or a 2% formaldehyde in 0.1 M sodium cacodylate buffer for 10 minutes. Non-decalcified 30-50-micron sections of the enamel organ taken from lower incisors were then processed for ultracytochemical demonstration of ouabain-sensitive, K+-dependent, p-nitrophenyl phosphatase, by use of the one-step lead method, representing the second dephosphorylative step of Na+-K+-ATPase. Throughout the secretory, transition, and maturation stages of amelogenesis, the enzymatic activity was demonstrated along the cytoplasmic side of the plasma membranes of the stratum intermedium and the papillary layer cells, especially along their numerous microvilli. The plasma membranes forming gap junctions and desmosomes were free of reaction or showed slight focal precipitates of reaction products. The stellate reticulum and the outer enamel epithelium exhibited either a weak reaction or were reaction negative. Secretory ameloblasts showed a weak trace-like reaction along the basal and lateral cell surfaces; however, the latter surfaces were sometimes completely free of reaction. Tomes' processes were usually reaction negative. Ameloblasts in the transition and maturation stages were devoid of enzymatic activity, except for a slight reaction along the plasma membranes of the basal cell surfaces of transition ameloblasts facing the papillary layer. The enzymatic activity described above was completely dependent on the presence of potassium and substrate in the incubation media and was almost completely inhibited by an addition of 10 mM ouabain to the incubation media.

Effect of amiodarone on Na+-, K+-ATPase and Mg2+-ATPase activities in rat brain synaptosomes

Amiodarone hydrochloride is a diiodinated antiarrhythmic agent widely used in the treatment of cardiac disorders. With the increasing use of amiodarone, several untoward effects have been recognized and neuropathy following amiodarone therapy has recently been reported. The present studies were carried out to study the effect of amiodarone on rat brain synaptosomal ATPases in an effort to understand its mechanism of action. Na+, K+-ATPase and oligomycin sensitive Mg2+ ATPase activities were inhibited by amiodarone in a concentration dependent manner with IC50 values of 50 microM and 10 microM respectively. [3H]ouabain binding was also decreased in a concentration dependent manner with an IC50 value of 12 microM, and 50 microM amiodarone totally inhibited [3H]ouabain binding. Kinetics of [3H]ouabain binding studies revealed that amiodarone inhibition of [3H]ouabain binding is competitive. K+-activated p-nitrophenyl phosphatase activity showed a maximum inhibition of 32 per cent at 200 microM amiodarone. Synaptosomal ATPase activities did not show any change in rats treated with amiodarone (20 mg kg-1 day-1) for 6 weeks, when compared to controls. The treatment period may be short, since the reported neurological abnormalities in patients were observed during 3-5 years of treatment. The present results suggest that amiodarone induced neuropathy may be due to its interference with sodium dependent phosphorylation of Na+, K+-ATPase reaction, thereby affecting active ion transport phenomenon and oxidative phosphorylation resulting in low turnover of ATP in the nervous system.

Solubilization and further chromatographic purification of highly purified, membrane-bound Na,K-ATPase

Highly purified membrane-bound Na,K-ATPase from pig kidney outer medulla was dissolved in the non-ionic detergent C12E8. Chromatography of the dissolved material on a DEAE matrix yielded enzymatical material having a ouabain-binding capacity of 6.9 nmoles per mg protein (measured according to Lowry et al., with bovine serum albumin as standard). This material, which after addition of lipids had the same K+-phosphatase turnover as the membrane-bound enzyme, could consist entirely of live molecules with a molecular weight of 145 kDa, a value close to that expected for alpha beta-promoters of Na,K-ATPase.

Gastric K+-stimulated p-nitrophenylphosphatase cytochemistry

A cytochemical study of gastric K+-stimulated p-nitrophenylphosphatase (K-NPPase) activity, corresponding to a K+-stimulated phosphoprotein phosphatase of H-K-ATPase system, has been made by a new cytochemical method. Sections of fixed guinea pig gastric mucosa in a mixture of 2% paraformaldehyde and 0.25% glutaraldehyde, were incubated with the incubation medium (1.0 M glycine-0.1 M KOH buffer, pH 9.0, 2.5 ml; 1.1 M KCl, 0.5 ml; 10 mM lead citrate dissolved in 50 mM KOH, 4 ml; levamisole, 6.0 mg; dimethyl sulfoxide, 2.0 ml; 0.1 M p-nitrophenylphosphate (Mg-salt), 1.0 ml; ouabain, 73.0 mg) for 30 min at room temperature. Under a light microscope the specific gastric K-NPPase reaction was distributed only in the parietal cells of the fundic glands. The electron microscopic cytochemistry showed that the gastric K-NPPase activity was localized on the membrane lining the apical surfaces, secretory canaliculi and tubulovesicles. On the other hand, ouabain-sensitive K-NPPase activity (Na-K-ATPase) was demonstrated to localize only in the basolateral membrane of parietal cells with Mayahara's method. These findings support the interrelationships between the apical surface membrane, secretory canalicular membrane and tubulovesicles, and the functional differentiation of the membrane between the secretory membrane and basolateral membrane.

Neuroendocrine control of secretion in pancreatic and parotid gland acini and the role of Na+,K+-ATPase activity

The results of our investigations into the localization of Na+,K+-pump activity in pancreatic and parotid acinar cells and the effects of hormones and neurotransmitters on pump turnover can be integrated with data on other aspects of stimulus-response coupling to construct models of the neurohumoral control of protein, fluid, and electrolyte secretion (Fig. 23). In both tissues, Ca2+ and cyclic AMP serve as intracellular messengers. In pancreatic acinar cells, the Ca2+-dependent pathway activated by the occupation of CCK or cholinergic receptors provides the primary stimulus for digestive enzyme secretion. Cyclic AMP plays a comparatively minor role; VIP and secretin are much less effective stimulators of protein secretion. Conversely, cyclic AMP levels in parotid acinar cells, which are modulated primarily through occupation of beta-adrenergic receptors, are a major determinant of enzyme secretion. Activation of the Ca2+-dependent pathway by cholinergic or alpha-adrenergic agonists or substance P is less important. The presence of dual control processes in each gland suggests that the observed differences in effectiveness of cyclic AMP- versus Ca2+-dependent secretagogues may reflect not different mechanisms, but rather a shift in the relative emphasis placed on each pathway. This emphasis could conceivably result from subtle variations in the interaction between cellular protein kinases and phosphatases and their phosphoprotein substrates. Electrolyte secretion, on the other hand, appears to involve both discrete and common entities. In pancreatic acinar cells from rodent species, cholinergic or CCK receptor occupancy elicits a Ca2+-dependent increase in the open-state probability of nonselective cation channels in the basolateral plasma membrane. The resultant influx of Na+ and efflux of K+ is most probably the factor which activates Na+, K+-pumps. Based on electron probe studies of the effects of cholinergic agonists on acinar cell Na+ and K+ contents discussed earlier, a transient reduction in the intracellular K+/Na+ ratio of up to 4-fold may occur. A shift of this magnitude in the cytoplasmic microenvironment of the Na+, K+-pump clearly would have a stimulatory influence (see discussion by Jorgensen, 1980). In addition, Ca2+ itself may have direct effects on Na+,K+-pump activity. Calcium at levels much above 1 microM progressively inhibits Na+,K+-ATPase activity (Tobin et al., 1973; Yingst and Polasek, 1985). In unstimulated guinea pig pancreatic acinar cells, Ca2+i measured by quin-2 fluorescence was 161 +/- 13 nM (Hootman et al., 1985a) which increased to a maximal concentration of 803 +/- 122 nM following CCh stimulation.(ABSTRACT TRUNCATED AT 400 WORDS)

Multiple ion-dependent and substrate-dependent Na+/K+-ATPase conformational states. Transient and steady-state kinetic studies

The hydrolysis of beta-(2-furyl)acryloyl phosphate (FAP), catalyzed by the Na+/K+-ATPase, is faster than the catalyzed hydrolysis of ATP. This is due to catalyzed hydrolysis of the pseudosubstrate by K+-dependent states of the enzyme, thus bypassing the Na+-dependent enzyme states that are required and are rate limiting in ATP hydrolysis. Unlike ATP, FAP is a positive effector of the E2 state. A study of FAP hydrolysis permits a detailed analysis of later steps in the overall ion translocation-ATP hydrolysis pathway. During the steady state of FAP hydrolysis in the presence of K+, substantial phosphoryl-enzyme is formed, as is indicated by the covalent incorporation of 32P from [32P]FAP. A comparison of the phosphoryl-enzyme yield with the rate of overall hydrolysis reveals that at 25 degrees C the phosphoryl-enzyme formed is all kinetically competent. Both the yield of phosphoryl-enzyme and the rate of overall hydrolysis of FAP are [K+] dependent. The transition E1 in equilibrium E2 is also [K+] dependent, but the rate of transition is differently affected by [K+] than are the above-mentioned two processes. Two distinct roles for K+ are indicated, as an effector of the E1-E2 equilibrium and as a "catalyst" in the hydrolysis of the E2-P. In contrast to the results at 25 degrees C, a virtually stoichiometric yield of phosphoryl-enzyme occurs at 0 degree C in the presence of Na+ and the absence of K+. At lower concentrations of K+ and in the presence of Na+, the hydrolysis of FAP at 0 degree C proceeds substantially through the E1-E2 pathway characteristic of ATP hydrolysis. The selectivity of FAP for the E2-K+-dependent pathway is due to the thermal inactivation of E1 at 25 degrees C in the absence of ATP or ATP analogues, even at high concentrations of Na+. These results emphasize the existence of multiple functional "E1" and "E2" states in the overall ATPase-ion translocation pathway.

Acute effect of glycerol on net cerebrospinal fluid production in dogs

The effect of glycerol administration on cerebrospinal fluid (CSF) formation in dogs was studied by means of a ventriculocisternal perfusion technique. Net CSF production rate decreased after oral administration of glycerol (3 gm/kg) from a baseline level of 42.33 +/- 6.68 microliter/min (mean +/- standard error) to a trough of 10.33 +/- 4.88 microliter/min at 90 minutes after administration (p less than 0.025). Serum osmolality concomitantly increased from a baseline value of 296 +/- 2.83 to 309 +/- 4.7 mOsm/kg H2O at 90 minutes. The mean percentage change in CSF production inversely correlated to the mean percentage change in serum osmolality, r = -0.85. Thus, glycerol administration decreases net CSF formation, and this effect may be related in part to the rise in serum osmolality.

Myometrial (Na+ + K+)-activated ATPase and its Ca2+ sensitivity

Ouabain-sensitive (Na+ + K+)-ATPase activity in the rat myometrial microsome fraction could only be determined following detergent treatment. The (Na+ + K+)-ATPase activity manifested by detergent treatment proved very stable even to high concentrations of NaN3, in contrast Mg+-ATPase activity was reduced to about 30 percent of the control. The major part of the Mg2+-ATPase in the myometrial membrane preparation was found to be identical with the NaN3-sensitive ATP diphosphohydrolase capable of ATP and ADP hydrolysis. This monovalent-cation-insensitive ATP hydrolysis could be extensively reduced by DMSO. Furthermore DMSO prevented the inactivation of the (Na+ + K+)-ATPase activity. 10-100 microM Ca2+ inhibited the (Na+ + K+)-ATPase activity obtained in the presence of SDS by 15-50 percent. The Ca2+ sensitivity of the enzyme was considerably decreased if the proteins solubilized by the detergent had been separated from the membrane fragments by ultracentrifugation. The inhibitory effect could be regained by combining the supernatant with the pellet. Ca2+ sensitivity of the (Na+ + K+)-ATPase activity was preserved even after removal of the solubilized proteins provided that DMSO had been applied. It appears that a factor in the plasma membrane solubilized by SDS may be responsible for the loss of Ca2+ sensitivity of the (Na+ + K+)-ATPase activity, the solubilization of which can be prevented by DMSO.

Divalent cations and the phosphatase activity of the (Na + K)-dependent ATPase

Phosphatase activity of a kidney (Na + K)-ATPase preparation was optimally active with Mg2+ plus K+. Mn2+ was less effective and Ca2+ could not substitute for Mg2+. However, adding Ca2+ with Mg2+ or substituting Mn2+ for Mg2+ activated it appreciably in the absence of added K+, and all three divalent cations decreased apparent affinity for K+. Inhibition by Na+ decreased with higher Mg2+ concentrations, when Ca2+ was added, and when Mn2+ was substituted for Mg2+. Dimethyl sulfoxide, which favors E2 conformations of the enzyme, increased apparent affinity for K+, whereas oligomycin, which favors E1 conformations, decreased it. These observations are interpretable in terms of activation through two cases of cation sites. (i) At divalent cation sites, Mg2+ and Mn2+, favoring (under these conditions) E2 conformations, are effective, whereas Ca2+, favoring E1, is not, and monovalent cations complete. (ii) At monovalent cation sites divalent cations compete with K+, while Na+ at these sites favors E1 conformations. K+ increases the Km for substrate, but both Ca2+ and Mn2+ decrease it, perhaps by competing with K+. On the other hand, phosphatase activity in the presence of Na+ plus K+ is stimulated by dimethyl sulfoxide, by higher concentrations of Mg2+ and Mn2+, but not by adding Ca2+; this is consistent with stimulation occurring through facilitation of an E1 to E2 transition, perhaps an E1-P to E2-P step like that in the (Na + K)-ATPase reaction sequence. However, oligomycin stimulates phosphatase activity with Mg2+ plus Na+ alone or Mg2+ plus low K+: this effect of oligomycin may reflect acceleration, in the absence of adequate K+, of an alternative E2-P to E1 pathway bypassing the monovalent cation-activated steps in the hydrolytic sequence.

Na+-like effect of imidazole on the phosphorylation of (Na+ + K+)-ATPase

A high basal level of phosphorylation (approx. 70% of the optimal Na+-dependent phosphorylation level) is observed in 50 mM imidazole-HCl (pH 7.0), in the absence of added Na+ and K+ and the presence of 10-100 microM Mg2+. In 50 mM Tris-HCl (pH 7.0) the basal level is only 5%, irrespective of the Mg2+ concentration. Nevertheless, imidazole is a less effective activator of phosphorylation than Na+ (Km imidazole-H+ 5.9 mM, Km Na+ 2 mM under comparable conditions). Imidazole-activated phosphorylation is strongly pH dependent, being optimal at pH less than or equal to 7 and minimal at pH greater than or equal to 8, while Na+-activated phosphorylation is optimal at pH 7.4. This suggests that imidazole-H+ is the activating species. Imidazole facilitates Na+-stimulated phosphorylation. The Km for Na+ decreases from 0.63 mM at 5 mM imidazole-HCl to 0.21 mM at 50 mM imidazole-HCl (pH 7; 0.1 mM Mg2+ in all cases). Imidazole-activated phosphorylation is more sensitive to inhibition by K+ (I50 = 12.5 microM) than Na+-activated phosphorylation (I50 = 180 microM). Mg2+ antagonizes activation by imidazole-H+ and also inhibition by K+. The Ki value for Mg2+ (approx. 0.3 mM) is the same for the two antagonistic effects. Tris buffer (pH 7.0) inhibits imidazole-activated phosphorylation with an I50 value of 30 mM in 50 mM imidazole-HCl (pH 7.0) plus 0.1 mM Mg2+. We conclude that imidazole-H+, but not Tris-H+, can replace Na+ as an activator of ATP-dependent phosphorylation, primarily by shifting the E2----E1 transition to the right, leading to a phosphorylating E1 conformation which is different from that in Tris buffer.

Redistribution of gastric K+-NPPase in vertebrate oxyntic cells in relation to hydrochloric acid secretion: a cytochemical study

Gastric K+-NPPase represents a partial reaction of the (K+-H+)ATPase system, which is considered to be the proton pump in mammalian parietal cells. In the present paper, K+-NPPase activity was cytochemically studied by the method of Mayahara et al. (1980) in gastric glands of birds, amphibia, and mammals, either in the resting state induced by cimetidine or after stimulation of HCl secretion by histamine. The gastric K+-NPPase cytochemical reaction was localized only in oxyntic cells of the gastric mucosa in the three species tested. The subcellular distribution of the K+-NPPase reaction product drastically changes with the secretory state of HCl. In resting cells, the K+-NPPase staining is associated with the membranes of the endocellular tubular system while in HCl-secreting cells, it is associated with the plasma membrane of the elaborate secretory surface characteristic of this functional state. The above results demonstrate that the same enzymatic activity, which is associated with the gastric proton pump, is present in both membranous systems of the oxyntic cell secretory pole. This fact supports the proposal that the tubular system represents a membrane reserve that inserts the proton pump into the luminal plasma membrane in vertebrate oxyntic cells under the action of HCl secretagogues.

Damaging effect of subzero temperature (-4 degrees C) on rabbit renal function

The effect of exposing rabbit kidneys to -4 degrees C for 1 hr in the unfrozen state was evaluated by means of measurement of tissue slice K/Na ratio and whole organ creatinine clearance. Freezing was prevented in one series (groups SC1-SC3) by supercooling with temperature monitoring and in a second series by a 2 M mixture of propylene glycol and glycerol. The latter agent was introduced prior to storage and later removed before the viability testing using a perfusion method (groups CPA1-CPA4). The results indicated a significant loss of slice and whole organ function during this short period of supercooling. The injury did not appear to result from either the rapidity of cooling or the formation of ice. There was some loss of function resulting from perfusion itself. Since this injury was evident in the whole organ but not in the tissue slice it may be ascribed to a vascular affect. When this damage was taken into account the data indicated that cryoprotective agents appeared to protect against any additional damage resulting from 1 hr storage at -4 degrees C.

Dimethyl sulfoxide decreases specific EGF binding

Dimethyl sulfoxide (DMSO) stimulates tyrosine phosphorylation of the hepatic EGF receptor in isolated membrane preparations. To determine whether DMSO affects EGF binding, primary cultures of rat hepatocytes were incubated with 1-10% DMSO for 30 min prior to the addition of 125I-EGF. DMSO (1-2%) reduced specific 125I-EGF binding; the effect was maximal (a 40-60% reduction) at 5-7.5% DMSO and was reversed by removing the DMSO. Scatchard analysis showed that the reduction in binding was due to a change in receptor affinity. The decrease in binding was not seen when other, slightly less polar, solvents (eg, acetone and ethanol) were tested. DMSO also reduced 125I-EGF binding to purified rat liver plasma membranes. This reduction was seen in the absence of added ATP and in membranes that had been pretreated with TLCK, a tyrosine kinase inhibitor. Thus, completion of the receptor autophosphorylation reaction was not necessary to effect the change. The data are consistent with a DMSO-induced alteration of receptor conformation that reversibly reduces receptor affinity.

A model for the reaction pathways of the K+-dependent phosphatase activity of the (Na+ + K+)-dependent ATPase

(Na+ + K+)-dependent ATPase preparations from rat brain, dog kidney, and human red blood cells also catalyze a K+ -dependent phosphatase reaction. K+ activation and Na+ inhibition of this reaction are described quantitatively by a model featuring isomerization between E1 and E2 enzyme conformations with activity proportional to E2K concentration: (formula; see text) Differences between the three preparations in K0.5 for K+ activation can then be accounted for by differences in equilibria between E1K and E2K with dissociation constants identical. Similarly, reductions in K0.5 produced by dimethyl sulfoxide are attributable to shifts in equilibria toward E2 conformations. Na+ stimulation of K+ -dependent phosphatase activity of brain and red blood cell preparations, demonstrable with KCl under 1 mM, can be accounted for by including a supplementary pathway proportional to E1Na but dependent also on K+ activation through high-affinity sites. With inside-out red blood cell vesicles, K+ activation in the absence of Na+ is mediated through sites oriented toward the cytoplasm, while in the presence of Na+ high-affinity K+ -sites are oriented extracellularly, as are those of the (Na+ + K+)-dependent ATPase reaction. Dimethyl sulfoxide accentuated Na+ -stimulated K+ -dependent phosphatase activity in all three preparations, attributable to shifts from the E1P to E2P conformation, with the latter bearing the high-affinity, extracellularly oriented K+ -sites of the Na+ -stimulated pathway.

DMSO increases tyrosine residue phosphorylation in membranes from murine erythroleukemia cells

Phosphorylation of membranes from murine erythroleukemia cells was performed in the presence and absence of the polar solvent dimethyl sulfoxide. Quantitation of the phosphoamino acid content revealed that DMSO stimulated phosphotyrosine accumulation by three-fold; serine and threonine phosphorylation decreased significantly. We had previously shown that DMSO stimulated tyrosine residue phosphorylation of the hepatic epidermal growth factor receptor. EGF had little effect in MEL membranes; therefore, DMSO results in accumulation of phosphotyrosine in cell membranes that do not exhibit significant EGF-dependent phosphorylation.

(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.

Inhibition of rat brain microsomal (Na+ + K+)-ATPase and K+-p-nitrophenylphosphatase by periodic acid

The effects of mild periodate exposure on the kinetics of (Na+ + K+)-ATPase and K+-p-nitrophenylphosphatase were studied using rat cerebral microsome preparations. Fifty percent inhibition of both enzyme activities was attained near 3 microM periodate concentrations. This inhibition was biphasic with time. Mg2+-ATPase and Mg2+-p-nitrophenylphosphatase activities were much less inhibited by periodate. Periodate inhibition was partially reversed by dimercaprol and dithiothreitol but not by diffusion. The possible reaction products formic acid, formaldehyde, glyceraldehyde, and acetaldehyde had no inhibitory effects in similar concentrations. Periodate exposure produced no detectable changes in the activation of (Na+ + K+)-ATPase by Na+, K+, Mg2+, or ATP. Residues shared by both (Na+ + K+)-ATPase and K+-p-nitrophenylphosphatase are both critical to hydrolytic function and sensitive to mild oxidation by periodate.

Oral glycerol for the treatment of traumatic intracranial hypertension

Hyperosmolar agents are a primary therapeutic modality employed in the treatment of traumatic intracranial hypertension. Profound hyperosmolarity accompanied by systemic dehydration is a potentially serious problem when these drugs are used repeatedly for control of intracranial pressure. Because glycerol, a water-soluble alcohol, is metabolized in the liver, its dehydrating capacity may be reduced in comparison to other agents. A series of 15 patients were treated with oral glycerol (0.5 to 1.0 gm/kg) with only minor changes in serum electrolytes, glucose, and urea nitrogen. Serum osmolarity rose from a baseline of 305 mOsm/liter to 355 mOsm/liter after 10 days of therapy. Glycerol was found to be effective and safe when employed in this protocol and proved to be a valuable adjunct to the standard methods available for control of intracranial hypertension.

Mode of inhibition of activity of Na+-K+-stimulated adenosine triphosphatase by indomethacin

1. The activity of the overall Na+-K+-ATPase reaction was inhibited by indomethacin in vitro. 2. The K+-NPPase activity was also inhibited by indomethacin. 3. The activity of Na+-dependent phosphorylation of Na+-K+-ATPase was activated by indomethacin. 4. Indomethacin required for 50% inhibition of K+-NPPase activity was 0.4 mM. 5. Inhibition mode of indomethacin for both the substrate and K+ in K+-NPPase reaction was competitive type. 6. The Ki values for indomethacin for the substrate and K+ in K+-NPPase reaction were 0.4 and 0.24 mM, respectively. 7. Inhibitory effect of indomethacin on K+-NPPase was reversible.

The cytochemical demonstration of NaK dependent adenosine triphosphatase at electrocyte level in Eigenmannia virescens (Gymnotidae)

ATPase activity (E.C. 3.6.1.3.) has been studied by electron microscopy with the help of several cytochemical techniques on Eigenmannia virescens electrocytes. Incubation was carried out with in two different media containing paranitrophenyl phosphate (p-NPP) or adenosine triphosphate (ATP) as substrate. With p-NPP the phosphate freed is captured at alkaline pH, either by strontium chloride or by lead citrate. With ATP the phosphate freed is captured at a pH close to neutrality by the lead nitrate. NaK ATPase activity was only demonstrated with the medium containing ATP; the positive results obtained with this technique were sensitive to ouabain. The enzyme is situated both on the membrane of the posterior face which is innervated and on that of the anterior face of the electrocytes. The cytoplasm of the anterior face is occupied by a strong concentration of tubules on whose membranes the enzyme is also present. The localisation of the enzyme on the tubules can explain biochemical results which indicate that 70% of the total NaK ATPase of the electrolytes is situated at the level of the anterior face.