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  • current-voltage characteristic  (3)
  • Na, K-ATPase  (1)
  • 1
    Electronic Resource
    Electronic Resource
    Charge translocation by the Na,K-pump: I. Kinetics of local field changes studied by time-resolved fluorescence measurements (1991)
    Springer
    The journal of membrane biology 121 (1991), S. 141-161 
    Details
    ISSN: 1432-1424
    Keywords: Na, K-ATPase ; ion pumps ; electrogenic transport ; voltage-sensitive dyes ; electrochromic effects
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology
    Notes: Summary Membrane fragments containing a high density of Na, K-ATPase can be noncovalently labeled with amphiphilic styryl dyes (e.g., RH 421). Phosphorylation of the Na,K-ATPase by ATP in the presence of Na+ and in the absence of K+ leads to a large increase of the fluorescence of RH 421 (up to 100%). In this paper evidence is presented that the styryl dye mainly responds to changes of the electric field strength in the membrane, resulting from charge movements during the pumping cycle: (i) The spectral characteristic of the ATP-induced dye response essentially agrees with the predictions for an electrochromic shift of the absorption peak. (ii) Adsorption of lipophilic anions to Na, K-ATPase membranes leads to an increase, adsorption of lipophilic cations to the decrease of dye fluorescence. These ions are known to bind to the hydrophobic interior of the membrane and to change the electric field strength in the boundary layer close to the interface. (iii) The fluorescence change that is normally observed upon phosphorylation by ATP is abolished at high concentrations of lipophilic ions. Lipophilic ions are thought to redistribute between the adsorption sites and water and to neutralize in this way the change of field strength caused by ion translocation in the pump protein. (iv) Changes of the fluorescence of RH 421 correlate with known electrogenic transitions in the pumping cycle, whereas transitions that are known to be electrically silent do not lead to fluorescence changes. The information obtained from experiments with amphiphilic styryl dyes is complementary to the results of electrophysiological investigations in which pump currents are measured as a function of transmembrane voltage. In particular, electrochromic dyes can be used for studying electrogenic processes in microsomal membrane preparations which are not amenable to electrophysiological techniques.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01870529
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    Paper (German National Licenses)
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  • 2
    Electronic Resource
    Electronic Resource
    Voltage dependence of sodium-calcium exchange: Predictions from kinetic models (1987)
    Springer
    The journal of membrane biology 99 (1987), S. 1-11 
    Details
    ISSN: 1432-1424
    Keywords: Na−Ca exchange ; electrogenic transport ; current-voltage characteristic ; membrane potential ; transport models
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology
    Notes: Summary Voltage effects on the Na−Ca exchange system are analyzed on the basis of two kinetic models, a “consecutive” and a “simultaneous” reaction scheme. The voltage dependence of a given rate constant is directly related to the amount of charge which is translocated in the corresponding reaction step. Charge translocation may result from movement of an ion along the transport pathway, from displacement of charged ligand groups of the ion-binding site, or from reorientation of polar residues of the protein in the course of a conformational transition. The voltage dependence of ion fluxes is described by a set of coefficients reflecting the dielectric distances over which charge is translocated in the individual reaction steps. Depending on the charge of the ligand system and on the values of the dielectric coefficients, the flux-voltage curve can assume a variety of different shapes. When part of the transmembrane voltage drops between aqueous solution and binding site, the equilibrium constant of ion binding becomes a function of membrane potential. By studying the voltage dependence of ion fluxes in a wide range of sodium and calcium concentrations, detailed information on the microscopic properties of the transport system may be obtained.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01870617
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  • 3
    Electronic Resource
    Electronic Resource
    Microscopic description of voltage effects on ion-driven cotransport systems (1986)
    Springer
    The journal of membrane biology 91 (1986), S. 275-284 
    Details
    ISSN: 1432-1424
    Keywords: cotransport systems ; electrogenic transport ; voltage dependence ; charge movement ; current-voltage characteristic
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology
    Notes: Summary A microscopic model for the analysis of voltage effects on ion-driven cotransport systems is described. The model is based on the notion that the voltage dependence of a given rate constant is directly related to the amount of charge which is translocated in the corresponding reaction step. Charge translocation may result from the movement of an ion along the transport pathway, from the displacement of charged ligand groups of the ion-binding site, or from reorientation of polar residues of the protein in the course of a conformational transition. The voltage dependence of overall transport rate is described by a set of dimensionless coefficients reflecting the dielectric distances over which charge is displaced in the elementary reaction steps. The dielectric coefficients may be evaluated from the shape of the experimental flux-voltage curve if sufficient information on the rate constants of the reaction cycle is available. Examples of flux-voltage curves which are obtained by numerical simulation of the transport model are given for a number of limiting cases.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01868820
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  • 4
    Electronic Resource
    Electronic Resource
    Electrogenic properties of the sodium-alanine cotransporter in pancreatic acinar cells: II. Comparison with transport models (1986)
    Springer
    The journal of membrane biology 94 (1986), S. 117-127 
    Details
    ISSN: 1432-1424
    Keywords: cotransport ; electrogenic transport ; sodiumcoupled amino-acid transport ; current-voltage characteristic
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology
    Notes: Summary In this paper, the results of the preceding electrophysiological study of sodium-alanine cotransport in pancreatic acinar cells are compared with kinetic models. Two different types of transport mechanisms are considered. In the “simultaneous” mechanism the cotransporterC forms a ternary complexNCS with Na+ and the substrateS; coupled transport of Na+ andS involves a conformational transition between statesNC′S andNC″S with inward- and outward-facing binding sites. In the “consecutive” (or “ping-pong”) mechanism, formation of a ternary complex is not required; coupled transport occurs by an alternating sequence of association-dissociation steps and conformational transitions. It is shown that the experimentally observed alanine- and sodium-concentration dependence of transport rates is consistent with the predictions of the “simultaneous” model, but incompatible with the “consecutive” mechanism. Assuming that the association-dissociation reactions are not rate-limiting, a number of kinetic parameters of the “simultaneous” model can be estimated from the experimental results. The equilibrium dissociation constants of Na+ and alanine at the extracellular side are determined to beK N ″ 〈-64mm andK S ″ 〈-18mm. Furthermore, the ratioK N ″ /K N S″ of the dissociation constants of Na+ from the binary (NC) and the ternary complex (NCS) at the extracellular side is estimated to be 〈-6. This indicates that the binding sequence of Na+ andS to the transporter is not ordered. The current-voltage behavior of the transporter is analyzed in terms of charge translocations associated with the single-reaction steps. The observed voltage-dependence of the half-saturation concentration of sodium is consistent with the assumption that a Na+ ion that migrates from the extracellular medium to the binding site has to traverse part of the transmembrane voltage.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01871192
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    Paper (German National Licenses)
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