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At least 19 recordsLinked to original sources
PubMed1976

Evidence of a novel role for monovalent cations in pyruvate kinase catalysis.

In the pyruvate kinase (EC 2.7.1.40) reaction, the complete enzyme-products complex consists of enzyme, pyruvate, ATP, a divalent cation, and a monovalent cation, usually K+. The dissociation of this complex can be slow permitting reversible enolization of pyruvate in this complex during the course of the forward reaction. High concentrations of each component in the enzyme-products complex inhibits the forward reaction, but only elevated concentrations of the monovalent cation decrease the net rate of product dissociation relative to that of pyruvate enolization. This result indicates that the monovalent cation can be the first component released from the enzyme-products complex and that the presence of an inhibiting monovalent cation concentration leads to re-formation of the complete complex, which is necessary for pyruvate enolization. The evidence suggests that the monovalent cation can bind and release with each turnover of the enzyme. While the data do not permit the conclusion that first release of monovalent cation is the exclusive pathway for dissociation of the enzyme-products complex, no other component once released can reassociate rapidly enough to form a complete complex during the forward reaction. Inhibition by these components must be attributed to the formation of abortive complexes.

Adenosine Triphosphate↗
PubMed1979

Activation of Thermus phosphofructokinase by monovalent cations.

The presence of the monovalent cations Tl+, NH+4, K+, Rb+ or Cs+, in decreasing order of potency, produce a marked equivalent increase in the specific enzyme activity of phosphofructokinase (ATP:D-fructose-6-phosphate 1-phosphotransferase, EC 2.7.1.11) purified from extreme thermophile, Thermus X-1. By contrast, the monovalent cations Li+, Na+ or CH3NH+3 produce no detectable catalyitic activation at concentrations up to 100 mM. The relative potency of these cations suggests that each polypeptide chain in the tetrameric enzyme possesses a cationbinding site having tetragonal symmetry and that the protein ligands are principally hydroxyl or carboxylate oxygens. Only the enzyme-cation complex and not the enzyme by itself exhibits cooperativity with respect to the dependence of catalytic rate on the concentration of the substrate, fructose 6-phosphate. In the presence of subsaturating but not saturating concentrations of substrate, the catalytic activation produced by monovalent cations is also cooperative. Exclusion chromatographic measurements indicate that the enzyme remains tetrameric at catalytic concentrations in the presence or absence of an activating monovalent cation.

Catalysis↗
PubMed1976

Size-dependent allosteric effects of monovalent cations on rabbit liver fructose-1,6-bisphosphatase.

Effects of monovalent cations on the neutral rabbit liver fructose-1,6-bisphosphatase are multifunctional and dependent on their nonhydrated ionic size. (a) The maximal velocity is increased by addition of monovalent cations with the optimum stimulation occurring with a nonhydrated ionic radius of 1.2 A in the presence of a chelating agent such as EDTA. (B) Activation curves are sigmoidal with n values varying from 1.5 to 2.3 as ionic radius of monovalent cation increases. The apparent Ka values from 16.0 to 180 mM, obtained for various monovalent cations, have a linear relationship to ionic radii of cations. (c) At lower concentrations of fructose 1,6-bisphosphate monovalent cations show the inhibitory effect and the apparent Km for fructose 1,6-bisphosphate is increased as the concentration of monovalent cation is increased. A linear relationship is obtained between the slopes of increase in the Km and the reciprocals of ionic volume of monovalent cations. (d) The apparent Ka for Mg2+ is also increased as the concentration of monovalent cation is increased, and a linear relationship is obtained again between the increases in Ka and the reciprocals of ionic volume of monovalent cations. The cooperative nature for Mg2+ saturation is decreased as the Ka increases. (e) The apparent Ki for AMP is also linearly altered as the concentration of monovalent cation is varied. However, the alteration of the Ki is unusual, that is, the smaller cations than K+ increase the Ki (Li+ greater than Na+ greater than NH4+), whereas the larger cations decrease the value ((CH2CH2OH)3N+ greater than Cs+ greater than Rb+). The effect of K+ is insignificant. Alterations in the Ki are also linearly related to the reciprocals of ionic volume of monovalent cations. The cooperative nature for AMP inhibition is decreased or increased as the Ki increased or decreased. (f) In the absence of the chelating agent, the curves for Mg2+ saturation and AMP inhibition were hyperbolic without monovalent cations. By addition of monovalent cation the Ka for Mg+2+ or Ki for AMP is increased and cooperative natures for binding of both ligands are induced. For nonspherical monovalent cations, the application of "functional ionic radius" is proposed. Functional ionic radii of NH4+, (CH2OH)3CNH3+, and (CH2CH2OH)3N+ are estimated to be 1.17, 2.55, and 2.87 A, respectively. The presence of two distinct sites for the actions of monovalent cations is suggested.

Allosteric Regulation↗
PubMed1977

Monovalent cation activation of tryptophanase.

The interaction of monovalent cations with holotryptophanase has been examined by spectral and kinetic methods. Using S-orthonitrophenyl-L-cysteine as a substrate, activation by the following monovalent cations was demonstrated; values of KA (mM, in italics) and Vmax (mumol min-1 mg) aare given in parentheses: Li+ (54 +/- 11.6, 4.3 +/- 0.28), Na+ (40 +/- 0.03, 18) K+ (1.44 +/- 0.06, 41.1 +/- 3.5), Tl+ (0.95 +/- 0.1, 39 +/- 4.4), NH4+ (0.23 +/- 0.01, 57.9 +/- 2.6), Rb+ (3.5 +/- 0.3, 33.5 +/- 1.8), Cs+ (14.6 +/- 2.6, 21 +/- 2.3). It was demonstrated by circular dichroic spectra that the competitive inhibitor, ethionine, interacts with the holoenzyme in the absence of activating monovalent cations, although it does not undergo labilization of the alpha proton. On addition of monovalent cation to the holoenzyme-ethionine complex, a marked increase occurs in absorption of 508 nm resulting from labilization of the alpha proton with formation of the quinoid form of the pyridoxal phosphate moiety of the enzyme-substrate complex at the catalytic center (Morino, Y., and Snell, E.E. (1967) J. Biol. Chem; 242, 2800-2809. The extent of formation of this quinoid intermediate was linearly related to the maximum velocity observed with each cation except NH4+, which was anomalously active. When measured at 500 nm, the change in absorption ranged from deltaA = 0.45 mg-1 of tryptophanase for NH4+ to 0.06 mg-1 for Li+. Two moles of thallium (I) were bound per mole of subunit. The data are most consistent with the interaction of monovalent cation at or near the catalytic center in such a way that it either participates directly in the reaction or is required for the critical alignment of one or more functional groups necessary for catalysis.

Binding Sites↗
PubMed1977

Interaction of phosphate with monovalent cation uptake in yeast.

The uptake of monovalent cations by yeast via the monovalent cation uptake mechanism is inhibited by phosphate. The inhibition of Rb+ uptake shows saturation kinetics and the phosphate concentration at which half-maximal inhibition is observed is equal to the Km of phosphate for the sodium-independent phosphate uptake mechanism. The kinetic coefficients of Rb+ and TI+ uptake are affected by phosphate: the maximal rate of uptake is decreased and the apparent affinity constants for the translocation sites are increased. In the case of Na+ uptake, the inhibition by phosphate may be partly or completely compensated by stimulation of Na+ uptake via a sodium-phosphate cotransport mechanism. Phosphate effects a transient stimulation of the efflux of the lipophilic cation dibenzyldimethylammonium from preloaded yeast cells and a transient inhibition of dibenzyldimethylammonium uptake. Possibly, the inhibition of monovalent cation uptake in yeast can be explained by a transient depolarization of the cell membrane by phosphate.

Anaerobiosis↗
PubMed1976

Ionophore A23187: the effect of H+ concentration on complex formation with divalent and monovalent cations and the demonstration of K+ transport in mitochondria mediated by A23187.

The two-phase extraction technique has been used to study the equilibrium between A23187, metal cations, and H+. Under these conditions the ionophore forms charge neutral isostoichiometric complexes with divalent cations in which both carboxylate groups of the 2:1 A23187:M2+ complexes are deprotonated. In ethanol, however, the methyl ester of A23187 also binds divalent cations indicating that protonated complexes between A23187 and cations should also exist. With monovalent cations, A23187 forms two charge-neutral complexes of stoichiometries and relative stabilities: A2HM greater than AM. Examination of energy utilization K+ and H+ movements, and light scattering capacity of mitochondria in the presence of divalent cation chelators, A23187, and valinomycin demonstrates that A23187 can act as a nigericin type K+ ionophore under appropriate conditions. Formation constants for the A2HM complexes with monovalent cations indicate that with appropriate conditions transport of Li+ and Na+ mediated by A23187 would also be expected. The binding constant data and associated free energies of complex formation are compared as a function of ionic radius and of cation charge. The data indicate that lack of conformational mobility in A23187 is responsible for the high cation size selectivity of this compound. To explain the transport selectivity of A23187 for divalent cations, it is proposed that this ionophore forms a family of five complexes, isostoichiometric between cations of different valence but of which only charge-neutral species are permeant to membranes. The charge of a given complex is in turn determined by that of the cation. The concept is consistent with the divalent cation transport specificity of A23187, explains the observed monovalent cation transport, and is useful in rationalizing the differences in charge selectivity between A23187 and X-537A.

Animals↗
PubMed1979

Effects of inotropic and arrhythmogenic digoxin doses and of digoxin-specific antibody on myocardial monovalent cation transport in the dog.

The effects of digoxin on monovalent cation active transport were determined in cardiac tissue obtained from dogs given inotropic, toxic, or lethal doses of digoxin. In hemodynamically monitored dogs, active uptake of the K+ analogue Rb+ was determined in vitro in a control myocardial biopsy, and then in serial biopsies from the same dog after the infusion of [3H]digoxin in doses sufficient to cause a sustained positive inotropic effect in the absence of toxicity, and finally after additional doses to induce overt toxicity. Nontoxic digoxin doses producing a mean increase of 20% in left ventricular (LV) dP/dt significantly reduced Rb+ active transport by 25% below control values. At the onset of digoxin-induced arrhythmias, maximal LV dP/dt was 53% above control whereas active Rb+ transport was reduced by 60% below baseline values (P less than 0.001). Control dogs given vehicle alone showed no significant change in contractility or in monovalent cation active transport. In another group of dogs given a lethal dose of digoxin, Rb+ active transport was reduced 59% below control levels at the onset of overt toxicity and was further reduced 80% below control at the time of onset of a fatal rhythm disturbance. When dogs were given high affinity digoxin-specific IgG or Fab fragments at the onset of overt toxicity, toxicity was rapidly reversed, and monovalent cation active transport increased to 51% of control at the time of restoration of sinus rhythm. Twenty-four hours after antibody reversal of arrhythmias, monovalent cation transport values approximated normal control levels. These data provide quantitative estimates of the extent of inhibition of monovalent cation transport by digoxin at inotropic, toxic, and lethal endpoints. Similar degrees of transport inhibition were present at the time of onset of digoxin-induced arrhythmias and at the time or arrhythmia reversal by digoxin-specific antibodies.

Animals↗
PubMed1979

Effects of monovalent cations on AMP nucleosidase from Azotobacter vinelandii.

The effect of monovalent cations on the purified AMP nucleosidase (AMP phosphoribohydrolase, EC 3.2.2.4) from Azotobacter vinelandii was investigated. All the monovalent cations were activators of the enzyme: Rb+ and Cs+ were the most effective, followed by K+, Na+, NH4+ and Li+ in that order. The apparent Ka for MgATP and nH values (Hill's interaction coefficient) decreased from 0.9 to 0.1 mM, and from 4 to 1, respectively, with the increase in K+ concentration, suggesting that the cation effects are on MgATP binding rather than catalysis. Gel filtration studies have revealed that the enzyme forms a non-dissociable enzyme species with a Stokes radius of 6.0--6.2 nm in the presence of saturating concentrations of monovalent cations, which can be distinguished from the 5.5-nm enzyme species showing temperature-dependent dissociation of the molecule in sulfate or phosphate. These results suggest that these ligands affect the association of the subunits through changes in the environment of the hydrophobic side chains of the enzyme molecules.

Adenosine Monophosphate↗
PubMed1979

Effects of monovalent cations on (Na+ + K+)-ATPase in rat brain slices.

The influence of monovalent cations on membrane (Na + K+)-ATPase was estimated in vitro in intact cells from the oxygen consumption of rat brain cortical slices. High concentrations of K+, Rb+ or Cs+ stimulated the respiration in the presence of Na+. This stimulation was antagonized by ouabain in a concentration- and time-dependent manner. Additionally, only combinations of monovalent cations, that stimulate (Na+ + K+)-ATPase, increased oxygen consumption, indicating that the stimulated portion of respiration is realted to the (Na+ + K+)-ATPase activity. Low concentrations of Rb+ and Cs+, however, failed to affect oxygen consumption. Li+ slightly and transiently stimulated oxygen uptake at low concentrations and inhibited it at higher concentrations. Low concentrations of Tl+ also stimulated respiration in a K+-free medium. However, the inhibitory effects of Tl+ were predominant at higher concentrations or in the presence of K+. Thus, monovalent cations can alter (Na+ + K+)-ATPase activity. While Rb+ and Li+ produce opposite effects on this enzyme system under certain conditions, these actions do not seem to be related to the antidepressant action of Rb+ and the antimanic action of Li+.

Animals↗
PubMed1978

Insulin effects on monovalent cation transport and Na-K-ATPase activity.

The effects of insulin on monovalent cation transport and on Na-K-ATPase activity from intact cells, tissue homogenates, and purified enzyme of the avian salt gland were studied. Monovalent cation active transport, measured by ouabain-inhibitable 86Rb+ uptake, was significantly increased (21.9 +/- 7.3% SE) in tissue slices exposed to insulin (100 mU/ml) for 15 min. A small but significant (12.2 +/- 1.9%) increase in Na-K-ATPase activity was similarly observed after salt gland tissue slices were exposed to insulin. This increase in enzymatic activity did not occur when broken-cell homogenates were exposed to insulin. Purified preparations of Na-K-ATPase showed no insulin enhancement of activity either in the presence of optimal or less than fully activating Na+ and ATP concentrations. Na-K-ATPase activity was the same in detergent-activated homogenates of both control and insulin-treated slices, consistent with insulin activation of existing enzyme sites. These data support the hypothesis that at least part of the increase in monovalent cation active transport produced by insulin is related to enhanced Na-K-ATPase activity and indicate that the latter phenomenon is dependent on some components or properties of the intact cell.

Adenosine Triphosphatases↗
PubMed1977

The interaction of monovalent cations with the sodium pump of low-potassium goat erythrocytes.

1. The activation by Na ions and the effect of the anti-L antibody on the sodium pump of low-potassium type (LK) erythrocytes, have been studied by measuring ouabain-sensitive ATPase activity of red cell membranes of LK goats. The experimental data were first corrected for incomplete occupation of the external K sites of the pump, using a saturation function obtained from influx experiments.2. Double-reciprocal plots of the corrected rates against Na concentration at various fixed K concentrations, yield a pattern of competitive K inhibition when it is assumed that three equivalent sodium sites take part in the internal activation of LK-(Na+K)-ATPase. The dissociation constant of Na at each site (K(m)) lies between 10 and 20 mM and that of K as competitive inhibitor (K(i)), between 1.5 and 4.5 mM.3. The maximal rate of hydrolysis of LK goat (Na + K)-ATPase is not different from those usually obtained with the high-potassium type (HK) red cell enzyme. Then, the low pumping rate of LK erythrocytes in physiological conditions is only reflecting the poor Na affinity, both absolute and relative, at the internal Na sites of their sodium pumps.4. The stimulation of the ouabain-sensitive ATPase activity by sensitization of the membranes with anti-L serum, is mediated by a threefold reduction of the K(m)/K(i) ratio at each site. K(m) decreases by a factor of 10, but there is also a smaller diminution of K(i). The maximal rate of hydrolysis, however, is unchanged by the anti-L treatment. The least-squares fitting of the pooled data by the rate equation, converges better with less than three and more than two equivalent sodium sites.5. The affinity sequence at two external K sites of the LK goat erythrocyte sodium pump, determined in the presence of 100 mM external Na, is Rb > K > Cs. It is obtained from the concentration dependence in influx experiments, and is the same as reported for human red cells.6. Cubic-root Dixon plots of the corrected ouabain-sensitive ATPase activity against the concentration of K and its congeners, show the sequence Tl > K > Rb > Na > Cs for the affinities at the internal cation sites of the LK sodium pump. Anti-L treatment decreases the relative magnitude of Na and Cs selectivities, it being not certain whether a Rb-Na transition then occurs.7. The results are discussed in terms of possible mechanisms whereby the sodium pump of LK and HK red cells may adjust the properties of their cation sites upon translocation of monovalent cations.

Adenosine Triphosphatases↗
PubMed1977

Dependence of tRNA structure in solution upon ionic condition of the solvent. Fluorescence studies of monovalent cation binding to tRNAPhe from barley embryos.

Dependence of barley phenylalanine tRNA (tRNAPhe) fluorescence intensity at 430 nm upon LiCl, NaCl, KCl, CsCl or NH4Cl concentration was measured in 0.01 M Tris-HCl, pH 7.5, 0.001 M Na2EDTA solutions. Increase of monovalent cation concentration in the solvent from 0 to 2 M induced about 3-fold fluorescence intensity enhancement. The fractional fluorescence change was used as a measure of bound ligand concentration. Fluorescence Scatchard plots revealed three classes of monovalent cation binding sites on the tRNA molecule: interacting (strong) and independent (weak and very weak) sites. Calculated from Scatchard plots binding constants (K), for strong and weak binding of monovalent cations (in the case of Na+ binding: Ks = 26 M-1 and Kw = 4.3 M-1 respectively) exhibit linear dependence upon ionic radius (r). Two limiting values obtained from the plot of K versus r: K(max) at r = 0 r(max) at K = 0, characterize additionally strong and weak monovalent cation binding sites (Ks(max) = 42 M-1 Kw(max) = 8.5 M-1, rs(max) = 0.23 nm and rw(max) = 0.22 nm). A model of the relationship between weak Mg2+ binding sites and monovalent cation binding sites as well as of monovalent cations binding to tRNA is proposed.

Cations, Monovalent↗
PubMed1975

Effects of monovalent cations on cardiac Na+, K+-ATPase activity and on contractile force.

The relationship between Na+, K+-ATPase inhibition by monovalent cations and their inotropic effect was studied in guinea pig hearts. The activity of partially purified cardiac enzyme was assayed in the presence of 5.8 mM KC1 and either 20 or 150 mM NaCl. Rb+ and Tl+ inhibited Na+, K+-ATPase activity, the magnitude of the inhibition by these cations being greater in the assay media containing lower Na+ concentrations. Tl+ produced a dose-dependent inhibition of Na+, K+-ATPase activity in the presence of 20 mM Na+ and 75 mM K+, a cationic condition similar to that of intracellular fluid. Other monovalent cations such as K+, Cs+, NH4+, Na or Li+ produced essentially no effect on the Na+, K+-ATPase activity or slightly stimulated it. In left atrial strips stimulated with field electrodes and bathed in Krebs-Henseleit solution (5.8 mM K+ and 145 mM Na+), addition of Cs+ failed to alter the isometric contractile force significantly. NH4+ and K+ caused a transient positive inotropic effect which was partially blocked by propranolol. The positive inotropic response to K+ was followed by a negative inotropic response. Rb+ produced a sustained, dose-dependent inotropic response reaching a plateau at 1-2 min, whereas Tl+ produced a dose=dependent positive inotropic effect which developed slowly over a 30-min period. The positive inotropic effects produced by Rb+ and Tl+ were insensitive to propranolol pretreatment. Concentrations of Tl+ and cardiac glycosides which produce similar inotropic effects appear to cause the same degree of Na+-pump inhibition. The onset of the positive inotropic response to Rb+ or Tl+ was not dependent on the number of contractions which is in contrast to the cardiac glycoside-induced inotropic response. Substitution of 20 mM LiCl for an equimolar amount of NaCl in Krebs-Henseleit solution produced a significantly greater inotropic response than that observed when sucrose was substituted for NaCl. It appears that, among monovalent cations, only sodium pump inhibitors produce a sustained positive inotropic response.

Adenosine Triphosphatases↗
PubMed1975

Functional consequences of modifying highly reactive arginyl residues of fructose 1,6-bisphosphatase. Loss of monovalent cation activation.

Modification of pig kidney fructose 1,6-bisphosphatase with 2,3-butanedione (in the presence of AMP) results in the loss of activation of the enzyme by monovalent cations. Under these conditions about 8 arginyl residues per mole of enzyme were modified. No other residues were modified. No loss of monovalent cation activation occurs when modification with 2,3-butanedione is carried out in the presence of AMP plus the substrate fructose 1,6-bisphosphate and 3.2 less arginyl residues were modified. Since fructose 1,6-bisphosphatase contains 4 subunits, it is suggested that one arginyl residue per subunit plays an essential role in monovalent cation activation of the enzyme. Studies on sulfhydryl group reactivity toward 5,5'-dithiobis(2-nitrobenzoic acid) explain the protection exerted by fructose 1,6-bisphosphate against the loss of monovalent cation activation in terms of an enzyme conformational change induced by substrate, which makes unreactive the essential arginyl residue. The results of the present paper, as well as previous evidence, are discussed in terms of the mechanism of monovalent cation activation of fructose 1,6-biphosphatase.

Animals↗
PubMed1977

Thyroid-induced alterations in myocardial sodium-potassium-activated adenosine triphosphatase, monovalent cation active transport, and cardiac glycoside binding.

The effects of thyroid hormone on guinea pig myocardial NaK-ATPase activity, transmembrane monovalent cation active transport, and cardiac glycoside binding were were examined. NaK-ATPase activities of left atrial and left ventricular homogenates of control and triiodothyronine (T3)-treated animals were determined, and compared to activities of skeletal muscle and liver. T3 administration was associated with a significant increase of 18% in left atrial and left ventricular NaK-ATPase specific activities. This increment was less than that noted in skeletal muscle (+42%) and liver (+30%). To determine if enhanced NaK-ATPase activity was accompanied by increased monovalent cation active transport, in vitro 86Rb+ uptake by left atrial strips and hemidiaphragms was measured. Transition from the euthyroid to the hyperthyroid state resulted in a 68% increase in active 86Rb+ uptake by left atrium, and a 62% increase in active uptake by diaphragm. Passive 86Rb+ uptake was not affected in either tissue. Ouabain binding by atrial and ventricular homogenates of T3-treated animals was increased by 19 and 17%, respectively, compared to controls, in close agreement with thyroid-induced increments in NaK-ATPase activiey. Taken together, these results are consistent with enhanced myocardial NaK-ATPase activity and monovalent cation activt transport due to an increase in the number of functional enzyme complexes.

Adenosine Triphosphatases↗
PubMed1979

Adsorption of monovalent cations to bilayer membranes containing negative phospholipids.

The electrophoretic mobilities of multilamellar phosphatidylserine vesicles were measured in solutions containing monovalent cations, and the xi potentials, the electrostatic potentials at the hydrodynamic plane of shear, were calculated from the Helmholtz--Smoluchowski equation. In the presence of 0.1 M lithium, sodium, ammonium, potassium, rubidium, cesium, tetraethylammonium, and tetramethylammonium chloride, the xi potentials were -60, -62, -72, -73, -77, -80, -82, and -91 mV, respectively. Similar results were obtained with phosphatidylglycerol vesicles; different results were obtained with cardiolipin, phosphatidylinositol, and phosphatidic acid vesicles. The phosphatidylserine results are interpreted in terms of the Stern equation, a combination of the Gouy equation from the theory of the diffuse double layer, the Boltzmann relation, and the Langmuir adsorption isotherm. Evidence is presented that suggests the hydrodynamic plane of shear is 2 A from the surface of the membrane in solutions containing the alkali metal cations. With this assumption, the intrinsic association constants of the above monovalent cations with phosphatidylserine are 0.8, 0.6, 0.17, 0.15, 0.08, 0.05, 0.03, and 0 M-1, respectively. The validity of this approach was tested in two ways. First, the xi potentials of vesicles formed from mixtures of phosphatidylserine and a zwitterionic lipid, phosphatidylcholine, were measured in solutions containing different concentrations of sodium. All the data could be described by the Stern equation if the "relaxation" of the ionic atmosphere, which is predicted by classic electrostatic and hydrodynamic theory to occur at low salt concentrations and high potentials, was circumvented by using only large (diameter greater than 13 micrometers) vesicles for these measurements. Second, the fluorescent probe 2-(p-toluidinyl)naphthalene-6-sulfonate was used to estimate the potential at the surface of phosphatidylserine and phosphatidylglycerol vesicles sonicated in 0.1 M NaCl. Reasonable agreement with the predicted values of the surface potential was obtained.

Cations, Monovalent↗