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  • 1
    Electronic Resource
    Electronic Resource
    Transport mechanism of hydrophobic ions through lipid bilayer membranes (1971)
    Springer
    The journal of membrane biology 5 (1971), S. 225-245 
    Details
    ISSN: 1432-1424
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology
    Notes: Summary Evidence is presented that the transport of lipid-soluble ions through bilayer membranes occurs in three distinct steps: (1) adsorption to the membranesolution interface; (2) passage over an activation barrier to the opposite interface; and (3) desorption into the aqueous solution. Support for this mechanism comes from a consideration of the potential energy of the ion, which has a minimum in the interface. The formal analysis of the model shows that the rate constants of the individual transport steps can be determined from the relaxation of the electric current after a sudden change in the voltage. Such relaxation experiments have been carried out with dipicrylamine and tetraphenylborate as permeable ions. In both cases the rate-determining step is the jump from the adsorption site into the aqueous phase. Furthermore, it has been found that with increasing ion concentration the membrane conductance goes through a maximum. In accordance with the model recently developed by L. J. Bruner, this behavior is explained by a saturation of the interface, which leads to a blocking of the conductance at high concentrations.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01870551
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  • 2
    Electronic Resource
    Electronic Resource
    Optical properties of artificial chlorophyll membranes (1971)
    Springer
    The journal of membrane biology 4 (1971), S. 284-294 
    Details
    ISSN: 1432-1424
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology
    Notes: Summary The composition and structure of lipid bilayer membranes containing chlorophylla have been studied with photometric and fluorometric methods. A sensitive double-beam spectrophotometer is described by which the pigment concentration in the bilayer can be determined. Up to 3×1013 chlorophyll molecules per cm2 can be incorporated into the membrane, corresponding to a mean distance of 20 Å between the porphyrin rings. At high chlorophyll concentrations, the absorption peaks are shifted toward longer wavelengths, indicating an interaction between porphyrin rings in the film. Parallel to the spectral shifts, a large decrease in the fluorescence quantum yield and a depolarization of the fluorescence are observed. These findings suggest that transfer of excitation energy takes place between neighboring chlorophyll molecules in the membrane. When an oxidating agent (K2S2O8) is added toone external phase, exactly half of the chlorophyll in the film is destroyed. This observation suggests that the chlorophyll molecules are localized in the membrane surfaces with the phytyl chains inserted into the hydrocarbon core of the membrane and the porphyrin rings facing the aqueous solution.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF02431976
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  • 3
    Electronic Resource
    Electronic Resource
    Optical study of active ion transport in lipid vesicles containing reconstituted Na,K-ATPase (1985)
    Springer
    The journal of membrane biology 85 (1985), S. 49-63 
    Details
    ISSN: 1432-1424
    Keywords: Na,K-ATPase ; reconstitution ; potential sensitive dye ; ion fluxes ; transport kinetics ; activation energy
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology
    Notes: Summary A fluorescence method is described for the measurement of ATP-driven ion fluxes in lipid vesicles containing purified Na,K-ATPase. The membrane voltage of enzyme containing vesicles was measured by using a voltage-sensitive indocyanine dye. By addition of valinomycin the vesicle membrane is made selectively permeable to K+ so that the membrane voltage approaches the Nernst potential for K+. With constant external K+ concentration, the time course of internal K+ concentration can be continuously measured as change of the fluorescence signal after activation of the pump. The optical method has a higher time resolution than tracer-flux experiments and allows an accurate determination of initial flux rates. From the temperature dependence of active K+ transport its activation energy was determined to be 115 kJ/mol. ATP-stimulated electrogenic pumping can be measured as a fast fluorescence change when the membrane conductance is low (i.e., at low or zero valinomycin concentration). In accordance with expectation, the amplitude of the fast signal change increases with decreasing passive ion permeability of the vesicle membrane. The resolution of the charge movement is so high that a few pump turnovers can be easily detected.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01872005
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  • 4
    Electronic Resource
    Electronic Resource
    Electrogenic properties of the sodium-alanine cotransporter in pancreatic acinar cells: I. Tight-seal whole-cell recordings (1986)
    Springer
    The journal of membrane biology 94 (1986), S. 99-115 
    Details
    ISSN: 1432-1424
    Keywords: cotransport ; electrogenic transport ; sodium-coupled amino-acid transport ; pancreatic acinar cells ; whole-cell recording
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology
    Notes: Summary Electrical currents associated with sodium-coupled alanine transport in mouse pancreatic acinar cells were studied using the method of whole-cell recording with patch pipettes. Single cells or small clusters of (electrically coupled) cells were isolated by collagenase treatment. The composition of the intracellular solution could be controlled by internal perfusion of the patch pipette. In this way both inward and outward currents could be measured under “zero-trans” conditions, i.e., with finite concentrations of sodium andl-alanine on one side and zero concentrations on the other. Inward andoutward currents for equal but opposite concentration gradients were found to be of similar magnitude, meaning that the cotransporter is functionally nearly symmetric. The dependence of current on the concentrations of sodium andl-alanine exhibited a Michaelis-Menten behavior. From the sodium-concentration dependence of current as well as from the reversal potential of the current in the presence of an alanine-concentration, gradient, a sodium/alanine stoichiometric ratio of 1:1 can be inferred. The finding that N-methylated amino acids may substitute, forl-alanine, as well as the observed pH dependence of currents indicate that the pancreatic alanine transport system is similar to (or identical with) the “A-system” which is widespread in animal cells. The transport system is tightly coupled with respect to Na+; alanine-coupled inward flow of Na+ is at least 30 times higher than uncoupled Na+ flow mediated by the cotransporter. The current-voltage characteristic of the cotransporter could be (approximately) determined from the difference of transmembrane current in the presence and in the absence ofl-alanine. The sodium-concentration dependence of the current-voltage characteristic indicates that a Na+ ion approaching the binding site from the extracellular medium has to cross part of the transmembrane electric field.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01871191
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  • 5
    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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  • 6
    Electronic Resource
    Electronic Resource
    Fast charge translocations associated with partial reactions of the Na,K-pump: I. Current and voltage transients after photochemical release of ATP (1987)
    Springer
    The journal of membrane biology 97 (1987), S. 161-178 
    Details
    ISSN: 1432-1424
    Keywords: Na,K-ATPase ; ion pumps ; electrogenic transport ; concentration jump ; “caged” ATP
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology
    Notes: Summary Nonstationary electric currents are described which are generated by the Na,K-pump. Flat membrane sheets 0.2–1 μm in diameter containing a high density of oriented N,K-ATPase molecules are bound to a planar lipid bilayer acting as a capacitive electrode. In the aqueous phase adjacent to the bound membrane sheets, ATP is released within milliseconds from an inactive, photolabile precursor (“caged” ATP) by an intense flash of light. After the ATP-concentration jump, transient current and voltage signals can be recorded in the external circuit corresponding to a translocation of positive charge across the pump protein from the cytoplasmic to the extracellular side. These electrical signals which can be suppressed by inhibitors of the Na,K-ATPase require the presence of Na+ but not of K+ in the aqueous medium. The intrinsic pump currentI p (t) can be evaluated from the recorded current signal, using estimated values of the circuit parameters of the compound membrane system.I p (t) exhibits a biphasic behavior with a fast rising period, followed by a slower decline towards a small quasistationary current. The time constant of the rising phase ofI p (t) is found to depend on the rate of photochemical ATP release. Further information on the microscopic orgin of the current transient can be obtained by double-flash experiments and by chymotrypsin modification of the protein. These and other experiments indicate that the observed charge-translocation is associated with early events in the normal transport cycle. After activation by ATP, the pump goes through the first steps of the cycle and then enters a long-lived state from which return to the initial state is slow.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01869220
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  • 7
    Electronic Resource
    Electronic Resource
    Fast charge translocations associated with partial reactions of the Na,K-pump: II. Microscopic analysis of transient currents (1987)
    Springer
    The journal of membrane biology 97 (1987), S. 179-191 
    Details
    ISSN: 1432-1424
    Keywords: Na,K-ATPase ; ion pumps ; electrogenic transport ; Albers-Post cycle ; partial reactions
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology
    Notes: Summary Nonstationary pump currents which have been observed in K+-free Na+ media after activation of the Na,K-ATPase by an ATP-concentration jump (see the preceding paper) are analyzed on the basis of microscopic reaction models. It is shown that the behavior of the current signal at short times is governed by electrically silent reactions preceding phosphorylation of the protein; accordingly, the main information on charge-translocating processes is contained in the declining phase of the pump current. The experimental results support the Albers-Post reaction scheme of the Na,K-pump, in which the translocation of Na+ precedes translocation of K+. The transient pump current is represented as the sum of contributions of the individual transitions in the reaction cycle. Each term in the sum is the product of a net transition rate times a “dielectric coefficient” describing the amount of charge translocated in a given reaction step. Charge translocation may result from the motion of ion-binding sites in the course of conformational changes, as well as from movement of ions in access channels connecting the binding sites to the aqueous media. A likely interpretation of the observed nonstationary currents consists in the assumption that the principal electrogenic step is the E1-P/P-E2 conformational transition of the protein, followed by a release of Na+ to the extracellular side. This conclusion is supported by kinetic data from the literature, as well as on the finding that chymotrypsin treatment which is known to block the E1-P/P-E2 transition abolishes the current transient. By numerical simulation of the Albers-Post reaction cycle, the proposed mechanism of charge translocation has been shown to reproduce the experimentally observed time behavior of pump currents.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01869221
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  • 8
    Electronic Resource
    Electronic Resource
    Valinomycin-mediated ion transport through neutral lipid membranes: Influence of hydrocarbon chain length and temperature (1973)
    Springer
    The journal of membrane biology 14 (1973), S. 339-364 
    Details
    ISSN: 1432-1424
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology
    Notes: Summary Stationary electrical conductance experiments together with nonstationary relaxation experiments allow a quantitative determination of rate constants describing carrier-mediated ion transport. Valinomycin-induced ion transport across neutral lipid membranes was studied. The dependence of the transport parameters on the chain length of the lipid molecules, on the kind of alkali ion, and on the temperature was determined. The relaxation time τ the current following a voltage jump shows a marked increase with decreasing temperature or with increasing chain length of the lipid molecules. This variation of τ is interpreted on the basis of a varying membrane fluidity. It is shown that under favorable circumstances the equilibrium constant of complex formation in the aqueous phase may be obtained from membrane experiments. Furthermore, the kinetics of exchange of valinomycin between membrane and water was studied. We found a marked influence of the totus surrounding the black film on the kinetics as well as on the total amount of valinomycin molecules in the membrane. The problem of location of the free carrier molecules inside the membrane is discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01868084
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  • 9
    Electronic Resource
    Electronic Resource
    Kinetic analysis of carrier-mediated ion transport by the charge-pulse technique (1976)
    Springer
    The journal of membrane biology 27 (1976), S. 171-191 
    Details
    ISSN: 1432-1424
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology
    Notes: Summary The charge-pulse technique has been used previously for the study of quasistationary processes in membranes which required only a moderate time resolution. It is shown here that a time resolution of about 400 nsec may be achieved with this technique and that it may be applied to the kinetic analysis of carrier-mediated ion transport. By this method we have studied the transport of alkali ions through optically black monoolein membranes in the presence of the ion carrier valinomycin. All three relaxation processes that are predicted by theory have been resolved. From the relaxation times and the relaxation amplitudes the rate constants for the association (k R ) and the dissociation (k D ) of the ioncarrier complex, as well as the translocation rate constants of the complex (k MS ) and the free carrier (k S ) could be obtained. For 1m Rb+ at 25° C the values arek R =3×105 m −1 sec−1,k D =2×105 sec−1,k MS =3×105 sec−1,k S =4×104 sec−1. The activation energies of the single rate constants which have been estimated from experiments at two different temperatures range between 50 and 90 kJ/mol.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01869135
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  • 10
    Electronic Resource
    Electronic Resource
    Blocking of the gramicidin channel by divalent cations (1977)
    Springer
    The journal of membrane biology 35 (1977), S. 351-375 
    Details
    ISSN: 1432-1424
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology
    Notes: Summary The conductance of the gramicidin channel in the presence of alkali ions is strongly reduced when divalent cations such as Ca++ or Ba++ are added to the aqueous solutions in concentrations between 0.1 and 1m. Under the same conditions, carrier-mediated alkali ion transport is not affected by Ca++ and Ba++. Different divalent cations, differ considerably in their blocking action on the gramicidin channel; the effect of Mg++ or Zn++ is much smaller than that of Ca++ and Ba++. Besides reducing the single-channel conductance, the blocking ions also change the current-voltage characteristic of the channel from a nearly, linear to a strongly saturating behavior. These observations suggest that Ca++ or Ba++ (which are not permeable themselves) bind to a site at or near the channel mouth, thereby reducing the rate by which permeable ions enter and leave the channel. The blocking effect is analyzed in terms of the potential energy profile of the permeable ion in the channel. The saturating current-voltage characteristic may be explained by the assumption that in the presence of the blocking ion the passage over the entrance barrier is rate-limiting and, at the same time, only weakly voltage-dependent.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01869959
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