Temperature-dependence of activation and inhibition of rat-brain adenosine triphosphatase activated by sodium and potassium ions

The Effects of Temperature on Cellular Physiology

Temperature impacts biological systems across all length and timescales. Cells and the enzymes that comprise them respond to temperature fluctuations on short timescales, and temperature can affect protein folding, the molecular composition of cells, and volume expansion. Entire ecosystems exhibit temperature-dependent behaviors, and global warming threatens to disrupt thermal homeostasis in microbes that are important for human and planetary health. Intriguingly, the growth rate of most species follows the Arrhenius law of equilibrium thermodynamics, with an activation energy similar to that of individual enzymes but with maximal growth rates and over temperature ranges that are species specific. In this review, we discuss how the temperature dependence of critical cellular processes, such as the central dogma and membrane fluidity, contributes to the temperature dependence of growth. We conclude with a discussion of adaptation to temperature shifts and the effects of temperature on evolution and on the properties of microbial ecosystems.

Effects of Dissolved Potassium on Growth Performance, Body Composition, and Welfare of Juvenile African Catfish (Clarias gariepinus)

Optimal crop production in aquaponics is influenced by water pH and potassium concentrations. The addition of potassium hydroxide (KOH) into the recirculating aquaculture system (RAS) may benefit aquaponics by increasing the water pH for better biofilter activity and supplementing K for better plant growth and quality. We investigated the growth, feed conversion, body composition and welfare indicators of juvenile African catfish (Clarias gariepinus) treated with four concentrations of K (K0 = 2, K200 = 218, K400 = 418, and K600 = 671 mg L−1). While growth, feed conversion and final body composition were unaffected, the feeding time and individual resting significantly increased with increasing K+. The swimming activity and agonistic behavior were reduced significantly under increased concentrations of K+. Leftover feed and the highest number of skin lesions were observed under K600. We suggest that K+ concentrations between 200 and 400 mg L−1 can improve the welfare status of juvenile African catfish. This enables the application of KOH in RAS to supply alkalinity to achieve optimum nitrification at minimum water exchange and improve the nutritional profile of the process water with benefits for the welfare status of African catfish and aquaponics plant production and quality.

A new method of the electrolyte-free long-term preservation of human sperm at 4 degrees C

OBJECTIVES

To develop a new method for the long-term preservation of human sperm.

SETTING

Andrology laboratory of our hospital.

PATIENTS

Thirty-one normal and 19 asthenozoospermic semen samples obtained from patients attending our infertility clinic. The average sperm motility was 70.2% and 36.0% in the normal and asthenozoospermic groups, respectively.

INTERVENTIONS

Ejaculated sperm were centrifuged and washed using the electrolyte-free Percoll gradient and then were preserved at 4 degrees C.

MAIN OUTCOME MEASURES

The motility of the preserved sperm was analyzed using computer-assisted semen analyzer after the addition of Ham's modified F-10 (GIBCO, Grand Island, NY).

RESULTS

In the normal group, motility rate after the addition of Ham's F-10 for 1, 2, and 4 weeks of preservation was 65.4%, 40.4%, and 5.5%, respectively. In the asthenozoospermic group, motility rate after 1 and 2 weeks of preservation was 31.3% and 18.1%, respectively. Preservation solutions containing sodium or potassium decreased motility after preservation. Restoration of preserved sperm was not achieved by incubation alone; however, reinitiation was induced by incubation together with Ham's F-10.

CONCLUSIONS

Human sperm in the electrolyte-free solution survived for a long period of time at 4 degrees C and reinitiation of sperm motility after preservation required the addition of Ham's F-10.

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)

Hemoglobin degradation, lipid peroxidation, and inhibition of Na+/K(+)-ATPase in rat erythrocytes exposed to acrylonitrile

The effect of acrylonitrile (VCN) on erythrocyte lipid metabolism was investigated in vitro in metabolically active red cells from male Sprague-Dawley rats containing three types of hemoglobins: oxyhemoglobin, methemoglobin, and carbon monoxyhemoglobin. VCN at the concentration of 10 mM rapidly depleted erythrocyte glutathione (GSH) (75% of control) and induced lipid peroxidation (274% of control). Degradation of oxy- and methemoglobin was directly proportional to the extent of lipid peroxidation (r = 0.89). Addition of glucose to the incubation medium decreased hemoglobin degradation while it slightly increased VCN-induced lipid peroxidation. The highest amount of lipid peroxidation occurred in erythrocytes containing carbon monoxyhemoglobin and glucose. In the isolated red cell membranes incubated with 10 mM VCN, the lipid peroxidation was 400% of controls. VCN (25 mM) noncompetitively inhibited erythrocyte membrane Na+/K(+)-ATPase activity and the degree of inhibition was inversely proportional to the reaction temperature (r = -0.88). These findings indicate that the VCN induced hemoglobin degradation and lipid peroxidation are two extremes of a spectrum of oxidative damage in red cells leading to a change in physical state of membrane structure causing inhibition of adenosine triphosphate (ATPase) activity.

Temperature-dependencies of various catalytic activities of membrane-bound Na+/K+-ATPase from ox brain, ox kidney and shark rectal gland and of C12E8-solubilized shark Na+/K+-ATPase

The temperature dependence of ouabain-sensitive ATPase and phosphatase activities of membrane fragments containing the Na+/K+-ATPase were investigated in tissue from ox kidney, ox brain and from shark rectal glands. The shark enzyme was also tested in solubilized form. Arrhenius plots of the Na+/K+-ATPase activity seem to be linear up to about 20 degrees C, and non-linear above this temperature. The Arrhenius plots of mammalian enzyme (ox brain and kidney) were steeper, especially at temperatures below 20-30 degrees C, than that of shark enzyme. The Na+-ATPase activity showed a weaker temperature-dependence than the Na+/K+-ATPase activity. The phosphatase reactions measured, K+-stimulated, Na+/K+-stimulated and Na+/K+/ATP-stimulated, also showed a weaker temperature-dependence than the overall Na+/K+-ATPase activity. Among the phosphatase reactions, the largest change in slope of the Arrhenius plot was observed with the Na+/K+/ATP)-stimulated phosphatase reaction. The Arrhenius plots of the partial reactions were all non-linear. Solubilization of shark enzyme in C12E8 did not change the curvature of Arrhenius plots of the Na+/K+-ATPase activity or the K+-phosphatase activity. Since solubilization involves a disruption of the membrane and an 80% delipidation, the observed curvature of the Arrhenius plot can not be attributed to a property of the membrane as such.

Active ion transport in the renal proximal tubule. I. Transport and metabolic studies

Various aspects of the interrelationship between ion transport and cellular metabolism were investigated using a suspension of rabbit cortical tubules that were mainly proximal in nature. Using the intact tubules, the compartmentation of K within the renal cell was studied by performing 42K uptake studies. The oxygen consumption (QO2) of the tubules was measured under similar conditions, as well as when the Na pump was stimulated by increasing Na+ entry with nystatin. In addition, the state 3 rate of respiration was measured when the mitochondria of digitonin-permeabilized tubules were stimulated by ADP. At 37 and 25 degrees C, a single-compartmental uptake of 42K was observed, which suggests that extracellular K+ communicates with a single compartment within the renal cell. Between 37 and 15 degrees C, the ouabain-sensitive QO2 and the initial 42K uptake rate were parallel in an Arrhenius-type plot, which indicated that active ion transport and oxidative phosphorylation remain tightly coupled within this temperature range. At all temperatures between 37 and 15 degrees C, nystatin stimulated the QO2, which demonstrates that the entry of Na+ into the renal cells was rate limiting for active Na+ transport throughout this temperature range. Between 37 and 20 degrees C, the nystatin-stimulated QO2 was nearly equal to the state 3 rate of respiration, which suggests that active ion transport may be limited by ATP availability under these conditions. At 15 degrees C, nystatin addition stimulated the QO2 well below the state 3 respiratory rate.

Substrate binding site of microsomal cytochrome P-450 directly faces membrane lipids

Cytochrome P-450, purified from liver microsomes of phenobarbital-treated rabbits, was incorporated into dimyristoylphosphatidylcholine liposomes. The binding of benzphetamine to the liposome-bound cytochrome P-450 was examined by measuring the benzphetamine-induced spectral change at various temperatures. The van't Hoff plot of the apparent spectral dissociation constant showed a distinct break at the temperature of phase transition of the synthetic lipid. On the other hand, no such break was observed for benzphetamine binding to microsomal bound cytochrome P-450. These results suggest that the substrate binding site of cytochrome P-450 is embedded in the apolar interior of phospholipid bilayer membranes.

Lipid-protein interactions of reconstituted membrane-associated adenosinetriphosphatases. Use of a gel-filtration procedure to examine phospholipid-activity relationships

A gel-filtration procedure is described for the reconstitution of partially delipidated membrane adenosinetriphosphatases (Mg2+-ATPase and (Na+ + K+)-ATPase) into liposomes of defined composition. After detergent solubilization of membrane enzyme preparations, reconstitution of these ATPases was achieved by the rapid removal of deoxycholate by Sephadex G-50 chromatography. Proteoliposomes were separated from unincorporated enzyme by chromatography on Sepharose CL-4B. Sedimentation characteristics in sucrose density gradients and electron microscopy confirmed that both Mg2+-ATPase and (Na+ + K+)-ATPase were reconstituted into liposomes of phosphatidylcholine and yielded preparations having high recoveries of enzyme activity by comparison with the control membrane preparations. Reconstitution of (Na+ + K+)-ATPase into synthetic phosphatidylcholines of defined fatty acid composition reveals an inverse relationship between enzyme activity and the chain length of the saturated fatty acids DMPC, DPPC and DSPC. Higher recoveries were obtained when one or more fatty acid chains was unsaturated. Full reactivation occurred with DOPC (18:1/18:1). There was a positive correlation between the specific activity of reconstituted (Na+ + K+)-ATPase and the temperature of the thermal phase transition of the synthetic phosphatidyl cholines studied. This was not seen with Mg2+-ATPase. It is suggested that 'membrane fluidity' influences the catalytic activity of (Na+ + K+)-ATPase but not that of Mg2+-ATPase.

Kinetics of the sodium pump in red cells of different temperature sensitivity

Ouabain-sensitive K influx into ground squirrel and guinea pig red cells was measured at 5 and 37 degrees C as a function of external K and internal Na. In both species the external K affinity increases on cooling, being three- and fivefold higher in guinea pig and ground squirrel, respectively, at 5 than at 37 degrees C. Internal Na affinity also increased on cooling, by about the same extent. The effect of internal Na on ouabain-sensitive K influx in guinea pig cells fits a cubic Michaelis-Menten-type equation, but in ground squirrel cells this was true only at high [Na]i. There was still significant ouabain-sensitive K influx at low [Na]i. Ouabain-binding experiments indicated around 800 sites/cell for guinea pig and Columbian ground squirrel erythrocytes, and 280 sites/cell for thirteen-lined ground squirrel cells. There was no significant difference in ouabain bound per cell at 37 and 5 degrees C. Calculated turnover numbers for Columbian and thirteen-lined ground squirrel and guinea pig red cell sodium pumps at 37 degrees C were about equal, being 77-100 and 100-129 s-1, respectively. At 5 degrees C red cells from ground squirrels performed significantly better, the turnover numbers being 1.0-2.3 s-1 compared with 0.42-0.47 s-1 for erythrocytes of guinea pig. The results do not accord with a hypothesis that cold-sensitive Na pumps are blocked in one predominant form.

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.

Neomycin inhibition of adenosine triphosphatase: evidence for a neomycin-phospholipid interaction

The inhibition of canine renal sodium potassium adenosine triphosphatase (ATPase) by neomycin was examined. Neomycin inhibited ATPase nearly maximally at 0.02 mM. The inhibition was temperature dependent with a decrease in inhibition occurring at temperatures below 21 degrees C, a temperature which corresponded to a change in activation energy of the ATPase as determined by Arrhenius plot. Preincubation of the ATPase with phosphoinositides was found to prevent the inhibition by neomycin. Other phospholipids were not found to prevent the inhibition. These results indicate a possible interaction between neomycin and the phosphoinositides of the ATPase complex.

Erythrocyte ghost Na+,K+-adenosine triphosphatase in Duchenne muscular dystrophy

Erythrocyte ghost membranes have been prepared by two different methods from patients with Duchenne muscular dystrophy (DMD), carriers of DMD, patients with other neuromuscular diseases, and normal individuals. The susceptibility of the membrane Na+,K+-adenosine triphosphatase (ATPase) to the cardiac glycoside, ouabain, has been investigated using various assay conditions. A stimulation of the enzyme has not been detected under any of the conditions employed. Using either a "high salt" (100 mM NaCl, 20 mM KCl) or a "low salt" (1 mM NaCl, 2 mM KCl) assay in the presence of EGTA a reduced susceptibility of the enzyme to ouabain was observed in preparations from patients with DMD compared with those from normal individuals. This behaviour was not manifest in preparations from NAD carriers or from patients with other neuromuscular diseases. The response of the erythrocyte membrane Na+,K+-ATPase activity to changes in temperature has also been investigated. The temperature response of the enzyme from DMD and DMD carrier preparations was indistinguishable from that of normal preparations. In all cases a break in the Arrhenius plot occurred at 21 degrees C.

Purification and characterization of (Na+ + K+)-ATPase from toad kidney

This report describes the partial purification and the characteristics of (Na+ + K+)-ATPase (ATP phosphohydrolase, EC 3.6.1.3) from an amphibian source. Toad kidney microsomes were solubilized with sodium deoxycholate and further purified by sodium dodecyl sulphate treatment and sucrose gradient centrifugation, according to the methods described by Lane et al. [(1973) J. Biol. Chem. 248, 7197--7200], Jørgensen [(1974) Biochim. Biophys. Acta 356, 36--52] and Hayashi et al. [(1977) Biochim. Biophys. Acta 482, 185--196]. (Na+ + K+)-ATPase preparations with specific activities up to 1000 mumol Pi/mg protein per h were obtained. Mg2+-ATPase only accounted for about 2% of the total ATPase activity. Sodium dodecyl sulphate-polyacrylamide gel electrophoresis revealed three major protein bands with molecular weights of 116 000, 62 000 and 26 000. The 116 000 dalton protein was phosphorylated by [gamma-32P]ATP in the presence of sodium but not in the presence of potassium. The 62 000 dalton component stained for glycoproteins. The Km for ATP was 0.40 mM, for Na+ 12.29 mM and for K+ 1.14 mM. The Ki for ouabain was 35 micron. Temperature activation curves showed two activity peaks at 37 degrees C and at 50 degrees C. The break in the Arrhenius plot of activity versus temperature appeared at 15 degrees C.

[Kidney preservation by hypothermic storage in Collins and Sacks solutions: the influence of 0-30 min of warm ischemia on the available preservation period (author's transl)]

123 kidneys of mongrel dogs were stored under hypothermic conditions in Collins or Sacks solutions for 12 up to 72 h and then transplanted. Before preservation the kidneys were subdued to 0-30 min warm ischemia. Kidney function after transplantation was measured by PAH-and inulin-clearances. Successful 24 h preservation was likewise possible with Collins and Sacks solutions if there was not any warm ischemia. Kidney function was reduced by the length of preservation, according to the formula; y = 166.02 . e-0,125x (y = PAH-clearance and x = preservation time). With both solutions the most attainable preservation time after 15 min warm ischemia was only 12 h, longer preservation time or ischemic periods were not tolerable. However, the function of the ischemically damaged organ was significantly better preserved by Sacks solution in comparison with Collins solution-although the preservation period could not be extended by this solution. Therefore, as far as human kidney preservation is concerned, Collins solution should be replaced by Sacks solution.

Activation of (Na+ + K+)-dependent ATPase by lipid vesicles of negative phospholipids

1. Kidney (Na+ + K+)-stimulated ATPase was depleted of phospholipids by extraction with lubrol and inserted in lipid structures of known composition. Both ouabain-sensitive ATPase and phosphatase reactions could be partially restored by lipid replacement. 2. Lipid vesicles of natural and synthetic negative phospholipids proved to be effective. The low activity of uncharged liposomes was increased when negative charges were included into the bilayer structure. 3. Reactivation by negative phospholipids was accompanied by spontaneous re-assembly of a stable lipid-protein complex. By contrast, the interaction of lipid deficient ATPase complex with uncharged lamellae was possible only after sonication of lipid-protein suspension. Reactivation did not ensue. 4. The ouabain-sensitive ATPase reactivated by synthetic dioleoylphosphatidylglycerol yielded curvilinear Arrhenius plots. The same pattern was seen with the original undepleted microsomal preparation. A discontinuity close to the temperature of fluid-order transition was found with dimyristoyl phosphatidylglycerol. 5. It is concluded that reassembly of lipid-deficient (Na+ + K+)-stimulated ATPase requires the addition of diacylphospholipids with fluid acyl-chains and negatively charged polar heads able to assemble in an expanded lamellar configuration.

Temperature dependence of membrane function. Disparity between active potassium transport and (Na+ & K+)ATPase activity

Ouabain-sensitive K+ influx in mammalian erythrocytes exhibits far less temperature sensitivity than the ((Na+ & K+)ATPase prepared by hypotonic lysis from the same population of cells. The results are not in accord with lipid phase change as the critical mechanism of cold inhibition of intact pumps.

Cultured cells from renal cortex of hibernators and nonhibernators. Regulation of cell K+ at low temperature

Cells were grown as primary monolayer cultures from kidney cortex of guinea pigs (nonhibernators), hamsters and ground squirrels (both hibernating species). When plates of cells were placed at 5 degrees C, cells of guinea pigs lost 37% of their K+ in 2 h and those of the hibernator lost about 10%. Uptake of 42K into the cells exhibited a simple, single exponential time course at both temperatures. Unidirectional efflux of K+ was equal to K+ influx in all cultures at 37 degrees C and, within limits of error, in hibernator cells at 5 degrees C. Efflux was 3-to 5-fold greater than influx in guinea pig cells at 5 degrees C. After 2 h in the cold the ouabain sensitive K+ influx remaining (7-15% of that at 37 degrees C) was about the same in the cells of the 3 species. Cells from active hamsters and from hibernating ground squirrels, however, exhibited significantly greater pump activity after 45 min in the cold (19 and 14%, respectively). The stimulation of K+ influx by increasing [K+] did not show an increase in Km+ at 5 degrees C in cells of guinea pigs and ground squirrels. Lowering [K+]c and/or raising [Na+]c by treatment in low- and high-K+ media caused only slight stimulation of K+ influx, except in cells of ground squirrels at 5 degrees C in which the stimulation was at least 11-times greater than at 37 degrees C or in cells of guinea pigs at either temperature. This altered kinetic response of K+ transport to cytoplasmic ion stimulation with cooling accounted for about one-third of the improved regulation of K+ at 5 degrees C in ground squirrel cells; the other two-thirds was attributable to a greater decrease in K+ leak with cooling. The inhibition of active transport by cold in all 3 species was much less severe than that previously seen in any (Na++K+)-ATPase of mammalian cells.

[Kinetics of Ca 2+ or Mg 2+ activated ATPase from lymphocyte plasma membranes]

The kinetic study of the C2+ ATPase activity of lymphocyte plasma memebranes allowed some properties of this enzyme to be evidenced. The Ca2+-activated hydrolysis of ATP is independent of a non-specific alkaline phosphatase. The substrate of the ATPase activity is the chelate Ca2+- ATP. Mg2+ may substitute for Ca2+ both as chelating ion and as activating ion. Several results suggest that we have only one ATPase, activated either by Ca2+-, or by Mg2+ with less efficiency; both chelates hve the same Km; pH values for maximum activity and transition temperatures are identical; the effects of free ions are also the same, activation at low concentration and inhibition at high concentration.

A linked active transport system for Na+ and K+ in a glial cell line

Ouabain (5 X 10(-4) M) induced a 6-fold increase in intracellular Na+ and a 65% loss of cellular K+ in C6 glial cells which was accompanied by a 12 mV decrease in the resting membrane potential. Following ouabain washout intracellular ion concentrations and the membrane potential returned to control levels suggesting that C6 is capable of active Na+ transport which is linked to uptake of K+. A portion of K+ uptake under steady-state conditions is also active since K+ influx was reduced 32% by ouabain. Five mM cyanide significantly increased cell Na+ and significantly decreased cell K+ and the membrane potential. The similarity in the ratio of Na+ gained/K+ lost (ouabain 1.24, cyanide 1.41) suggests that the two agents inhibit the same ion transport system. Decreased temperature had the paradoxical effect of increasing intracellular K+ while significantly decreasing both membrane potential and K+ influx. Part of this effect may be due to the marked reduction in K+ efflux at low temperature. At 6 degrees C cell loss of K+ was much less than loss of K+ with ouabain at 37 degrees C. The observation of a linked Na+-K+ transport system in C6 cell confirms the hypothesis that coupled active Na+-K+ exchange occurs in glial cells and suggests that ionic transport may regulate certain aspects of glial metabolism.

Protein-liposome interactions and their relevance to the structure and function of cell membranes

Recent studies on the interactions of soluble proteins, membrane proteins and enzymes with phospholipid model membranes are reviewed. Similarities between the properties of such systems and the behavior of biomembranes, such as alterations in the redox potential of cytochrome c after binding to membranes and effects of phospholipid fluidity on (Na+K) ATPase activity, are emphasized. The degree of correspondence between the behavior of model systems and natural membranes encourages the continuing use of model membranes in studies on protein-lipid interactions. However, some of the data on the increase of surface pressure of phospholipid monolayers by proteins and increases in the permeability of liposomes indicate that many soluble proteins also have a capability to interact hydrophobically with phospholipids. Thus a sharp distinction between both peripheral and integral membrane proteins and non-membrane proteins are not seen by these techniques. Cautious use of such studies, however, should lead to greater understanding of the molecular basis of cell membrane structure and function in normal and pathological states. Studies implicating protein-lipid interactions and (Na+K) ATPase activity in membrane alterations in disease states are also briefly discussed.

Ouabain-binding and phosphorylation of (Na+ + K+) ATPase treated with N-ethylmaleimide or oligomycin

Ouabain-binding and phosphorylation of (Na+ mk+)-ATPase (EC 3.6.1.3) of the plasma membranes from kidney were investigated after treatment with N-ethylmaleimide or oligomycin. Either of these inhibitors brought about the following changes: the phosphoenzyme, formed in the presence of Na+, Mg2+ and ATP became essentially insensitive to splitting by K+ but was split by ADP. One mole of this ADP-sensitive phosphoenzyme bound one mole of ouabain but the enzyme-ouabain complex was less stable than in the native enzyme primarily because the rate of its dissociation increased. Ouabain was bound to the ADP-sensitive phosphoenzyme in the presence of Mg2+ alone and addition of inorganic phosphate enhanced both the rate of formation and the steady-state level of the enzyme-ouabain complex. The inhibitors did not affect the properties of this second type of complex. Both in the native enzyme and in the enzyme treated with the two inhibitors inorganic phosphate enhanced ouabain binding by phosphorylating the active center of the enzyme as shown (a) by mapping the labeled peptides from the enzyme after peptic digestion, (b) by inhibition of this phosphorylation with Na+ and (c) by the 1:1 stoichiometric relation between this phosphorylation and the amount of bound ouabain. Unlike the phosphoenzyme, the binding of ouabain remained sensitive to K+ in the enzyme treated with the inhibitors. K+ slowed ouabain-binding either in the presence of Na+, Mg2+ and ATP or of Mg2+ and inorganic phosphate. A higher concentration of K+ was needed to slow ouabain-binding either in the presence of Na+, Mg2+ and ATP or of Mg2+ and inorganic phosphate. A higher concentration of K+ was needed to slow ouabain-binding than to stimulate dephosphorylation. This finding is interpreted as being an indication of separate sites for K+ on the enzyme: a site(s) with high K+-affinity which stimulates dephosphorylation, another site(s) with moderate K+-affinity which inhibits ouabain-binding. Inhibitors may enhance formation of the ADP-sensitive phosphoenzyme by blocking interaction between K+ and the site(s) with high affinity.

Studies on (Na+ + K+)-activated ATPase. XXXVIII. A 100 000 molecular weight protein as the low-energy phosphorylated intermediate of the enzyme

Phosphorylation of NaI-treated bovine brain cortex microsomes by inorganic phosphate in the presence of Mg2+ and ouabain has been studied at 0 degrees C (pH 7.4) and 20 degrees C (pH 7.0). Nearly maximal (90%) and half-maximal phosphorylation are achieved at 20 degrees C within 2 min with 50--155 and 5.6--17 muM 32Pi, respectively, and at 0 degrees C within 75 s with 300--600 and 33--66 muM 32Pi, respectively. Maximal phosphorylation yields 146 pmol 32P - mg-1 protein. Without ouabain (20 degrees C, pH 7.0) less than 25% of the incorporation observed in the presence of ouabain is reached. Preincubation of the native microsomes with Mg2+ and K+, in order to decompose possibly present high-energy phosphoryl-bonds prior to ouabain treatment, does not affect the maximal phosphate incorporation. This indicates that the inorganic phosphate incorporation is not due to an exchange with high-energy phosphoryl-bonds, which might have been preserved in the microsomal preparations. Phosphorylation of the native microsomes by ATP in the presence of Mg2+ and Na+ reaches 90 and 50% maximal levels within 15--30 s at 0 degrees C and pH 7.4 at concentrations of [gamma-32P]ATP of 5--32 and 0.5--3.5 muM, respectively. The maximal phosphorylation level is 149 pmol 32P-mg-1 protein, equal to that of ouabain-treated microsomes phosphorylated by inorganic phosphate. Both inorganic phosphate and ATP phosphorylate on site per active enzyme subunit of 135 000 molecular weight. From the equilibrium constants for the phosphorylation of ouabain-treated microsomes by inorganic phosphate at 0 degrees C and 20 degrees C standard free-energy changes of --5.4 and --6.8 kcal/mol, respectively, are calculated. These values yield a standard enthalpy change of 14 kcal/mol and an entropy change of 70 cal/mol - degree K. This characterizes the reaction as a process driven by an entropy change. The intermediate formed by phosphorylation with Pi has maximal stability at acidic pH, as is the case for the intermediate formed with ATP. Solubilization in sodium dodecyl sulfate stabilizes the phosphoryl-bond in the pH range of 4--7. The non-solubilized preparation has optimal stability at pH 2--4, the level of which is equal to that of detergent-solubilized intermediate. Sodium dodecyl sulfate gel electrophoresis of the microsomes at pH 3, following incorporation of 32Pi yields 11 protein bands, only one of which (mol. wt 100 000--106 000) carries the radioactive label. This protein has the same molecular weight as the protein, which is phosphorylated by ATP in the presence of Mg2+ and Na+.

Temperature-dependent changes in activation energies of the transport systems for nucleosides, choline and deoxyglucose of cultured Novikoff rat hepatoma cells and effects of cytochalasin B and lipid solvents

The initial rates of transport of uridine, thymidien, purines, choline and 2-deoxy-D-glucose by cultured Novikoff rat hepatoma cells were determined as a function of temperature between 5 and 41 degrees C. Arrhenius plots of all transport systems exhibited sharp breaks in slope; between 17 and 23 degrees for uridine, thymidine and hypoxanthine-guanine transport and between 29 and 32 degrees for choline and 2-deoxy-D-glucose transport. The activation energies for the transport systems changed from 15-26 kcal/mole below the transition temperatures to 4-9 kcal/mole above the transition temperatures. Propagation of the cells in the presence of cis-6-octadecenoic acid which results in marked changes in the lipid composition of cell membrane, had little effect on the temperature characteristics of the various transport systems. Similarly, propagation of the cells for 24 hr in media containing Tween 40 or nystatin had no effect on the capacity of the cells to transport the various substrates or on the temperature dependence of the transport systems. The presence of ethanol, phenethyl alcohol or Persantin at concentrations that inhibited thymidine and 2-deoxy-D-glucose transport between 40 and 70% also did not alter the transition temperatures or activation energies for the transport of these substrates. Cytochalasin B, on the other hand, shifted the transition temperature for 2-deoxy-D-glucose transport to higher temperatures in a concentration-dependent manner, whereas it had no effect on the temperature dependence of thymidien transport.

Alterations in phospholipid-dependent (Na+ +K+)-ATPase activity due to lipid fluidity. Effects of cholesterol and Mg2+

The (Na+ +K+)-activated, Mg2+-dependent ATPase from rabbit kidney outer medulla was prepared in a partially inactivated, soluble form depleted of endogenous phospholipids, using deoxycholate. This preparation was reactivated 10 to 50-fold by sonicated liposomes of phosphatidylserine, but not by non-sonicated phosphatidylserine liposomes or sonicated phosphatidylcholine liposomes. The reconstituted enzyme resembled native membrane preparations of (Na+ +K+)-ATPase in its pH optimum being around 7.0, showing optimal activity at Mg2+:ATP mol ratios of approximately 1 and a Km value for ATP of 0.4 mM. Arrhenius plots of this reactivated activity at a constant pH of 7.0 and an Mg2+: ATP mol ratio of 1:1 showed a discontinuity (sharp change of slope) at 17 degrees C, with activation energy (Ea) values of 13-15 kcal/mol above this temperature and 30-35 kcal below it. A further discontinuity was also found at 8.0 degrees C and the Ea below this was very high (greater than 100 kcal/mol). Increased Mg2+ concentrations at Mg2+:ATP ratios in excess of 1:1 inhibited the (Na+ +K+)-ATPase activity and also abolished the discontinuities in the Arrhenius plots. The addition of cholesterol to phosphatidylserine at a 1:1 mol ratio partially inhibited (Na+ +K+)-ATPase reactivation. Arrhenius plots under these conditions showed a single discontinuity at 20 degrees C and Ea values of 22 and 68 kcal/mol above and below this temperature respectively. The ouabain-insensitive Mg2+-ATPase normally showed a linear Arrhenius plot with an Ea of 8 kcal/mol. The cholesterol-phosphatidylserine mixed liposomes stimulated the Mg2+-ATPase activity, which now also showed a discontinuity at 20 degrees C with, however, an increased value of 14 kcal/mol above this temperature and 6 kcal/mol below. Kinetic studies showed that cholesterol had no significant effect on the Km values for ATP. Since both cholesterol and Mg2+ are known to alter the effects of temperature on the fluidity of phospholipids, the above results are discussed in this context.

Differential effects of temperature on a membrane adenosine triphosphatase and associated phosphatase

Arrhenius plots of a membrane (Na+ + K+)-dependent ATPase (adenosine triphosphatase) activity showed characteristic discontinuities, whereas those of the associated K+-dependent phosphatase activity did not. These findings support the contention that the phosphatase activity does not depend on phospholipid in the same way as does the ATPase activity.

Movements of labelled sodium ions in isolated rat superior cervical ganglia

1. Isolated rat superior cervical ganglia were incubated in Krebs solution containing (24)Na and carbachol for 4 min at 25 degrees C. They were then washed at 3 degrees C for 15 min to remove extracellular (24)Na and the efflux of residual intracellular (24)Na stimulated by warming to 25 degrees C.2. During the 15 min wash at 3 degrees C desaturation curves became exponential with a rate constant of 0.012 +/- 0.001 min(-1) (n = 24). This was assumed to represent loss of intracellular (24)Na, and initial uptake of (24)Na was calculated therefrom by back-extrapolation to zero wash-time. After 4 min in (24)Na + 180 muM carbachol intracellular [(24)Na] so calculated was 61.6 +/- 3.1 mM (n = 18), representing 83% labelling of intracellular Na. In the absence of carbachol intracellular [(24)Na] was 10.0 +/- 0.5 mM, representing 49% labelling. Extracellular Na was labelled by > 90% after 4 min in (24)Na. The apparent rate constant for washout of extracellular (24)Na was 0.6 min(-1) at 3 degrees C and 0.95 min(-1) at 25 degrees C.3. The loss of the residual intracellular (24)Na during temperature stimulation was interpreted quantitatively in terms of an exponential decline of the bulk of intracellular (24)Na with an extrusion rate constant of 0.39 +/- 0.1 min(-1) (n = 18), efflux being delayed by passage through the extracellular space with an effective rate constant of 0.8-1.2 min(-1).4. The peak rate constant (k(C)) for the desaturation curve at 25 degrees C was 0.35 +/- 0.01 min(-1). An Arrhenius plot of log k(C)/T degrees K(-1) yielded a two-stage linear regression with a transition at 20 degrees C. Activation energies of 8 and 31 kcal. mole(-1) were calculated above and below this transition respectively.5. Omission of K from the 25 degrees C temperature-stimulating solution reduced k(C) by 62%. The K-sensitive component of extrusion rate constant was a hyperbolic function of [K](e) with half-saturation at 5.6 mM-[K](e) and maximum k(C) of 0.58 min(-1).6. Cyanide (2 mM), 2,4-dinitrophenol (1 mM) and ouabain (1.4 mM) reduced k(C) by 50-90%. The half-maximally inhibiting concentration of ouabain was about 60 muM.7. Substitution of sucrose, Li or choline for external Na did not reduce the extrusion rate of (24)Na in either 6 mM-[K](e) or 0 mM-[K](e). Li stimulated (24)Na extrusion in Na-free, K-free solution.8. The properties of the ganglionic Na pump deduced from rates of temperature-stimulated (24)Na extrusion accord with the view that the ganglion hyperpolarization observed after Na loading by exposure to nicotinic depolarizing agents results from electrogenic Na extrusion. A comparable hyperpolarization is observed after temperature stimulation following Na loading.