Comparison of sources of a phosphorylated intermediate in transport ATPase

Sodium pump molecular activity and membrane lipid composition in two disparate ectotherms, and comparison with endotherms

Previous research has shown that the lower sodium pump molecular activity observed in tissues of ectotherms compared to endotherms, is largely related to the lower levels of polyunsaturates and higher levels of monounsaturates found in the cell membranes of ectotherms. Marine-based ectotherms, however, have very polyunsaturated membranes, and in the current study, we measured molecular activity and membrane lipid composition in tissues of two disparate ectothermic species, the octopus (Octopus vulgaris) and the bearded dragon lizard (Pogona vitticeps), to determine whether the high level of membrane polyunsaturation generally observed in marine-based ectotherms is associated with an increased sodium pump molecular activity relative to other ectotherms. Phospholipids from all tissues of the octopus were highly polyunsaturated and contained high concentrations of the omega-3 polyunsaturate, docosahexaenoic acid (22:6 (n-3)). In contrast, phospholipids from bearded dragon tissues contained higher proportions of monounsaturates and lower proportions of polyunsaturates. Sodium pump molecular activity was only moderately elevated in tissues of the octopus compared to the bearded dragon, despite the much greater level of polyunsaturation in octopus membranes. When the current data were combined with data for the ectothermic cane toad, a significant (P = 0.003) correlation was observed between sodium pump molecular activity and the content of 22:6 (n-3) in the surrounding membrane. These results are discussed in relation to recent work which shows a similar relationship in endotherms.

Relationship between body size, Na+-K+-ATPase activity, and membrane lipid composition in mammal and bird kidney

We investigated the relationship between body size, Na(+)-K(+)-ATPase molecular activity, and membrane lipid composition in the kidney of five mammalian and eight avian species ranging from 30-g mice to 280-kg cattle and 13-g zebra finches to 35-kg emus, respectively. Na(+)-K(+)-ATPase activity was found to be higher in the smaller species of both groups. In small mammals, the higher Na(+)-K(+)-ATPase activity was primarily the result of an increase in the molecular activity (turnover rate) of individual enzymes, whereas in small birds the higher Na(+)-K(+)-ATPase activity was the result of an increased enzyme concentration. Phospholipids from both mammals and birds contained a relatively constant percentage of unsaturated fatty acids; however, phospholipids from the smaller species were generally more polyunsaturated, and a complementary significant allometric increase in monounsaturate content was observed in the larger species. In particular, the relative content of the highly polyunsaturated docosahexaenoic acid [22:6(n-3)] displayed the greatest variation with body mass, scaling with allometric exponents of -0.21 and -0.26 in the mammals and birds, respectively. This allometric variation in fatty acid composition was correlated with Na(+)-K(+)-ATPase molecular activity in mammals, whereas in birds molecular activity only correlated with membrane cholesterol content. These relationships are discussed with respect to the metabolic intensity of different-sized animals.

The prognostic value of the brain sodium-potassium ATPase enzyme concentration in head injury

Sodium-potassium adenosine triphosphatase (ATPase) enzyme was determined in the brain tissue of 11 patients with head injury and 6 control patients. Patients with head injury included in this study were selected from two categories: (a) patients in deep coma due to severe head injury [Glasgow Coma Scale (GCS) less than 8; 6 cases]; (b) patients with depressed skull fractures with dural tears who were conscious and able to give an adequate verbal response (GCS greater than 10; 5 cases). The level of the enzyme was significantly reduced in comatose patients with severe head injury as compared to the controls (P less than 0.001) or to conscious patients with depressed fractures (P less than 0.001). In the group of conscious patients with depressed fractures, the enzyme level was no different from that of the controls (P = 0.4215). All comatose patients with severely reduced enzyme levels subsequently died, whereas those with depressed fractures with normal enzyme levels survived. The relationship between a low enzyme level and brain edema in severe head injury is discussed.

Photoaffinity labeling of erythrocyte membrane (Na+ + K+)-ATPase with high specific activity [125I]iodoazidogalactosyl digitoxigenin

Photoaffinity labeling of (Na+ + K+)-ATPase in erythrocyte membranes with cardiotonic steroid derivatives, followed by gel electrophoresis, requires a radiolabel of very high specific activity, since the enzyme represents less than 0.05% of the total membrane protein. We report the synthesis of a radioiodinated, photosensitive derivative of the cardiac glycoside, 3-beta-O-(4-amino-4,6-dideoxy-beta-D-galactosyl)digitoxigenin, with very high specific activity. The product, [125I]iodoazidogalactosyl digitoxigenin ([125I]IAGD), is carrier-free with a specific activity of 2200 Ci/mmol. Incubation of [125I]IAGD (1.8 nM) with human erythrocyte membranes (300 micrograms protein), followed by photolysis and analysis by SDS-PAGE, showed specific radiolabeling of a polypeptide that had the same molecular weight as catalytic alpha subunit (100,000 Mr) of (Na+ + K+)-ATPase in eel electroplax microsomes. Photoaffinity labeling of erythrocyte and electroplax membranes by [125I]IAGD was specific for the cardiac glycoside binding site of (Na+ + K+)-ATPase since radiolabeling of the alpha subunit was inhibited when ouabain was included in the pre-photolysis incubation. [125I]IAGD can, therefore, be used as a probe in structural studies of human erythrocyte membrane (Na+ + K+)-ATPase.

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

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

The steady-state kinetic mechanism of ATP hydrolysis catalyzed by membrane-bound (Na+ + K+)-ATPase from ox brain. II. Kinetic characterization of phosphointermediates

(1) The kinetics of the phosphorylated enzymic intermediates of (Na+ + K+)-ATPase from ox brain, which are formed by incubation of the enzyme with 25 microM AT32P, 150 mM Na+ and 1 mM Mg2+, have been studied in dephosphorylation experiments at 1 degree C. The dephosphorylation of the 32P-labelled enzyme was initiated by addition of either 1 mM unlabelled ATP, 2.5 mM ADP or 1 mM unlabelled ATP + ADP in concentrations from 25 to 1000 microM. (2) In the absence of ADP the dephosphorylation curve was linear in a semilogarithmic plot almost from t = 0, whereas by addition of ADP a biphasic behaviour was obtained. The slope of the slow phase of dephosphorylation was virtually independent of the ADP concentration. (3) The results were analysed by the mathematical equation corresponding to the simplest possible model for the interconversion and breakdown of the phosphointermediates: (formula: see text) where alpha, beta, H and G are functions of all the rate constants and H and G furthermore are functions of the initial values for [E1P] and [E2P]. (4) The analysis confirmed the model and enabled the determination of all the rate constants. (5) k-1 was found to be equal to k'-1 + k"-1 . [ADP] indicating an ADP-independent 'spontaneous' dephosphorylation of E1P. The rate constant for this process was close to that for dephosphorylation of E2P, i.e., k'-1 congruent to k3. Also the value of k"-1 was determined. (6) k3 was found to be at least 10 . k-2. The implication of this for the role of the E1P to E2P transition in the Na+ + K+)-stimulated ATP hydrolysis will be discussed in detail in the following paper (Plesner, I.W., Plesner, L., Nørby, J.G. and Klodos, I. (1981) Biochim. Biophys. Acta 643, 483--494). (7) A refinement of the model, accounting for the effect of Na+ on the steady-state ratio between [E1P] and [E2P] is proposed: (formula: see text). At [Na+] = 150 mM as used here, E1P(Na) and E'1P are assumed to be in rapid equilibrium. (8) Comparison of our results with those of others underlines the general validity of the conclusions of the present paper.

Calcium ion-dependent phosphorylation of human erythrocyte membranes

Calcium ions promote the rapid transfer of the terminal phosphate of ATP to a protein of human erythrocyte membranes. The concentration of Ca2+ for half-maximal effect is 7 muM. At nonlimiting ATP concentrations the level of 32P incorporated by the membranes is independent of the presence or absence of Mg2+. The number of phosphorylating sites in a single erythrocyte membrane is about 700. The influence of pH on the rate of hydrolysis of the bound phosphate and its rapid release on exposure to hydroxylamine are both consistent with an acylphosphate bond. The phosphate in the protein undergoes rapid turnover. Enzymatic splitting of the phosphate is stimulated by Mg2+ but not by Ca2+. It is proposed that Mg2+ accelerates the splitting of the phosphate by favoring the conversion of the phosphoprotein from a state of low reactivity to a state of high reactivity towards water. The reactions described probably are intermediate steps in the hydrolysis of ATP catalyzed by the Ca2+-dependent ATPase of human erythrocyte membranes.

The effect of Mg2+ and chelating agents on intermediary steps of the reaction ofNa+,K+-activated ATPase

(1) It has been investigated how varying concentrations of free magnesium with and without EDTA influence the properties of the phospho-enzyme formed in the presence of sodium by the (Na+ plus K+)-activated enzyme system. (2) The phospho-enzyme formed in the presence of sodium and a high concentration of free magnesium has the same rate of (a) spontaneous dephosphorylation, (b) dephosphorylation after addition of potassium, and (c) dephosphorylation after addition of ADP, as a phospho-enzyme formed in the presence of sodium and a low concentration of magnesium. (3) With sodium and a given concentration of free magnesium, high or low, EDTA present during formation of the phospho-enzyme leads to a decrease in the rate of (a) spontaneous dephosphorylation, and (b) dephosphorylation after addition of potassium to the phospho-enzyme. (4) The rate of dephosphorylation after addition of ADP to phospho-enzyme formed without and with EDTA is the same. But as the rate of spontaneous dephosphorylation is lower with EDTA than without, ADP gives a higher increase in the rate of dephosphorylation of phospho-enzyme formed with EDTA than without. (5) The experiments thus show that the reported different sensitivity towards potassium and ADP of phospho-enzyme formed in the presence of a low and high concentration of free magnesium, respectively, is due to the EDTA used to decrease the free magnesium concentration and not to the decrease in the free magnesium as such.

Chemical characterization and pronase susceptibility of the Na:K pump-associated phosphoprotein of human red blood cells

The phosphoproteins formed by incubation of red cell ghosts with [gamma-(32)P]ATP in the presence of Mg and Na + Mg have been characterized by sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis. The (32)P-labeled phosphoprotein was seen as a single peak confined to the region of the diffuse 90,000 dalton polypeptide band; labeling with Na + Mg considerably increased the quantity of (32)P-phosphoprotein contained in this band relative to labeling with Mg alone. Treatment of intact cells with Pronase known to partially hydrolyze the glycoproteins and the 90,000 daltons polypeptide did not change either the amount or the position of the (32)P-phosphoprotein present in the gels. The molecular weight of the (32)P-phosphoprotein is estimated to be 103,000. Pronase treatment of intact cells also did not significantly alter any of the transport parameters of the membrane such as the K pump flux, ouabain binding, or Na,K-ATPase. In contrast, treatment of ghosts with Pronase not only resulted in drastic alteration of the transport parameters but also inhibited the formation of the phosphoprotein under all conditions. Thus, while the Na:K pump is not intrinsically resistant to Pronase, those elements of the pump which are susceptible are not accessible from the outside of the cell. Further, SDS-polyacrylamide gel electrophoresis after Pronase treatment of intact cells results in a substantial increase in the purification of the phosphoprotein relative to that which was previously possible in ghosts.

Demonstration of a phosphopeptide intermediate in the Mg ++ -dependent, Na + - and K + -stimulated adenosine triphosphatase reaction of the erythrocyte membrane

Human erythrocyte membranes are phosphorylated by [gamma-(32)P]ATP in association with the Mg(++)-dependent, Na(+) and K(+)-stimulated ATPase (EC 3.1.6.3) reaction. To delineate the membrane species involved, phosphorylated membranes were analyzed by polyacrylamide gel electrophoresis in sodium dodecyl sulfate, under conditions that minimize hydrolysis of acyl phosphate linkages. Three radioactive components were detected, of which only one was a phosphopeptide, of apparent molecular weight 105,000. The phosphate bound to this peptide undergoes rapid turnover and is discharged by hydroxylamine. In the presence of Mg(++), the phosphorylation of this peptide is specifically stimulated by Na(+) and blocked by ethylene diamine tetraacetate; its dephosphorylation is stimulated by K(+) and blocked by ouabain. We conclude that this phosphopeptide is an intermediate in the Mg(++)-dependent, Na(+)- and K(+)-stimulated ATPase reaction of the erythrocyte membrane.

The involvement of phosphatidylserine in adenosine triphosphatase activity of the sodium pump

1. A study has been made to see whether the ATPase of the Na pump is activated by phospholipids.2. Solubilization of a membrane-containing (microsomal) fraction from ox brain cortex with deoxycholate removed most or all of the ouabain-inhibited ATPase and p-nitrophenylphosphatase (pNPPase) activities. Addition of sonicated dispersions of crude commercial samples of phosphatidic acid, phosphatidylinositol or phosphatidylserine stimulated both enzymic activities, but neither phosphatidylcholine nor phosphatidylethanolamine affected them. After partial purification by chromatography, however, definite activation was demonstrated only with that component of crude samples which migrated like phosphatidylserine.3. The ATPase activity dependent on phosphatidylserine was eliminated by removal of Na and K or by addition of ouabain and was substantially inhibited by oligomycin. The corresponding component of pNPPase activity was partially inhibited by ouabain or by removal of most of the K, but was little affected by oligomycin. The phosphatidylserine-dependent enzyme(s) therefore exhibited properties characteristic of the ATPase of the Na pump.4. Phosphatidylserine dispersions similarly activated both the ATPase and the pNPPase of the particulate enzyme preparations, untreated with deoxycholate.5. The results suggest that the system for active transport of Na and K involves a complex unit of phosphatidylserine and the protein of the ATPase.