Alkali ion transport through lipid bilayer membranes mediated by enniatin A and B and beauvericin

Summary Stationary conductance measurements with lipid bilayer membranes in the presence of enniatin A and B and beauvericin were performed. For comparison, some valinomycin systems were investigated. It was found that the conductance in the case of enniatin A and B is caused by a carrier ion complex with a 11 stoichiometry, whereas for beauvericin, a 31 carrier ion complex has to be assumed to explain the dependence of the conductance on carrier and ion concentration in the aqueous phase. The current-voltage curves measured with dioleoyl phosphatidylcholine membranes show a superlinear behavior for the three carriers in the presence of potassium. On the other hand, supralinear current-voltage curves were observed with membranes from different monoglycerides, except for beauvericin. The results obtained with enniatin A and B are in a satisfactory agreement with an earlier proposed carrier model assuming a complexation between carrier and ion at the membrane water interface.The discrimination between potassium and sodium ions is much smaller for the enniatins than for valinomycin. This smaller selectivity as well as the fact that potassium ions cause the highest conductance with lipid bilayer membranes may be due to the smaller size of the cyclic enniatin molecules, which contain 6 residues in the ringvs. 12 in the case of valinomycin. Charge-pulse relaxation studies were performed with enniatin A and B, beauvericin, and valinomycin. For monoolein membranes only in the case of valinomycin, all three relaxations predicted by the model could be resolved. In the case of the probably more fluid membranes from monolinolein (^{9, 12}-C_{18: 2}) and monolinolenin (^{9, 12, 15}-C_{18: 3}) for all carrier systems except for beauvericin, three relaxations were observed.The association rate constantk _R , the dissociation rate constantk _D , and the two translocation rate constantsk _MS andk _s for complexed and free carrier, respectively, could be calculated from the relaxation data. The carrier concentration in the aqueous phase had no influence on the rate constants in all cases, whereas a strong saturation of the association rate constantk _R with increasing ion concentration was found for the enniatins. Because of the saturation,k _R did not exceed a value of 4×10⁵ m ^–1 sec^–1 with 1m salt irrespective of carrier, ion, or membrane-forming lipid.A similar but less pronounced saturation behavior was also observed for the translocation rate constantk _S of the free carrier. The other two rate constants were independent of the ion concentration in the aqueous phase. In the case of the enniatins, the translocation rate constantk _MS was not independent from the kind of the transported ion. In the series K⁺, Rb⁺ and Cs⁺,k _MS increases about threefold. The turnover numbers for the carriers as calculated from the rate constants range between 10⁴ sec^–1 and 10⁵ sec^–1 and do not show a strong difference between the individual carriers. The conductance difference in the systems investigated here is therefore mainly caused by the partition coefficients, which are smaller for the enniatins than for valinomycin.     

Mechanism of ion transport through lipid bilayer-membranes mediated by peptide cyclo-(d-Val-l-Pro-l-Val-d-Pro)3

The cyclic dodecapeptide PV, cyclo-(d-Val-l-Pro-l-Val-d-Pro)₃, a structural analogue of the ion-carier valinomycin, increase the cation permeability of lipid bilayer membranes. This paper reports the results of two types of relaxation experiments, namely relaxation of the membrane current after a voltage jump and decay of the membrane voltage after a charge pulse in lipid bilayer membranes exposed to PV. From the relaxation data, the rate constant for the translocation of the ion carrier complex across the membrane, as well as the partition coefficient of the complex between water and membrane solution interface were computed and found to be about one order of magnitude less than the comparable values for valinomycin (Val). Furthermore, the dependence of the initial membrane conductivity on ion concentration was used to evaluate the equilibrium constant, K, of complexation between PV and some monovalent cations in water. The values of K yield the following selectivity sequence of PV: Na⁺ < NH₄⁺ < K⁺ < Cs⁺ < Rb⁺. These and earlier results are consistent with the idea that PV promotes cation movement across membranes by the solution complexation mechanism which involves complexation between ion and carrier in the aqueous phase and transport of the carrier across the membrane. In the particular form of the solution complexation mechanism operating here, the PV present in the PV-cation complex carrying charge across the membrane derives from the side from which the current is flowing (cis-mechanism). As shown previously, valinomycin, in contrast to PV, acts by an interfacial complexation mechanism in which the Val in the Val-cation complex derives from the side toward which current is flowing (trans-mechanims). The comparison of the kinetic properties of these two closely related compounds yields interesting insights into the relationship between chemical structure and function of ion carriers.     

Ion transport mediated by the valinomycin analogue cyclo(L-Lac-L-Val-D- Pro-D-Val)3 in lipid bilayer membranes

Cyclo(L-Lac-L-Val-D-Pro-D-Val)3 (PV-Lac) a structural analogue of the ion-carrier valinomycin, increases the cation permeability of lipid bilayer membranes by forming a 1:1 ion-carrier complex. The selectively sequence for PV-Lac is identical to that of valinomycin; i.e., Rb+ greater than K+ greater than Cs+ greater than or equal to NH+4 greater than Na+ greater than Li+. The steady-state zero-voltage conductance, G(0), is a saturating function of KCl concentration. A similar behavior was found for Rb+, Cs+, and NH+4. However, the ion concentration at which G(0) reaches a plateau strongly depends on membrane composition. The current-voltage curves present saturating characteristics, except at low ion concentrations of Rb+, K+, or Cs+. The ion concentration at which the saturating characteristics appear depends on membrane composition. These and other results presented in this paper agree with a model that assumes complexation between carrier and ion at the membrane-water interface. Current relaxation after voltage-jump studies were also performed for PV-Lac. Both the time constant and the amplitude of the current after a voltage jump strongly depend on ion concentration and membrane composition. These results, together with the stationary conductance data, were used to evaluate the rate constants of the PV-Lac-mediated K+ transport. In glycerolmonooleate they are: association rate constant, 2 x 10(6) M-1 s-1; dissociation rate constant, 4 x 10(5) s-1; translocation rate constant for complex, 5 x 10(4) s-1; and the rate of translocation of the free carrier (ks), 55 s-1. ks is much smaller for PV-Lac than for valinomycin and thus limits the efficiency with which the carrier is able to translocate cations across the membrane.     

Effects of variation of ion and methylation of carrier on the rate constants of macrotetralide-mediated ion transport in lipid bilayers

Summary The effects of methylation on the rate constants of carrier-mediated ion transport have been studied on monooleindecane bilayers with K⁺, Rb⁺, NH ₄ ⁺ , and TI⁺ ions, using the series of homologue carriers, nonactin, monactin, dinactin, trinactin, and tetranactin, each member of the series differing from the previous one by only one methyl group. Measurements of the amplitude and time constant of the current relaxation after a voltage jump over a large domain of voltage and permeant ion concentration, together with a computer curve-fitting procedure, have allowed us, without the help of steady-state current-voltage data, to deduce and compare the values of the various rate constants for ion transport: formation (k _Ri) and dissociation (k _Di) of the ion-carrier complex at the interface, translocation across the membrane interior of the carrier (k _s) and the complex (k _is). With the additional information from steady-state low-voltage conductance measurements, we have obtained the value of the aqueous phase-membrane and torus-membrane partition coefficient of the carrier ({ie191-1} and {ie191-2}). From nonactin to tetranactin with the NH ₄ ⁺ ion,k _is, and {ie191-3} are found to increase by factors of 5 and 3, respectively,k _Di and {ie191-4} to decrease respectively by factors 8 and 2, whilek _Ri andk _s are practically invariant. Nearly identical results are found for K⁺, Rb⁺, and Tl⁺ ions.k _Ri,k _s andk _is are quite invariant from one ion to the other except for Tl⁺ wherek _Ri is about five times larger. On the other hand,k _Di depends strongly on the ion, indicating that dissociation is the determining step of the ionic selectivity of a given carrier. The systematic variations in the values of the rate constants with increasing methylation are interpreted in terms of modifications of energy barriers induced by the carrier increasing size. Within this framework, we have been able to establish and verify a fundamental relationship between the variations ofk _is andk _Di with methylation.     

The Rate Constants of Valinomycin-Mediated Ion Transport through Thin Lipid Membranes

Electrical relaxation experiments have been performed with phosphatidylinositol bilayer membranes in the presence of the ion carrier valinomycin. After a sudden change of the voltage a relaxation of the membrane current with a time constant of about 20 μsec is observed. Together with previous stationary conductance data, the relaxation amplitude and the relaxation time are used to evaluate the rate constants of valinomycin-mediated potassium transport across the lipid membrane. It is found that the rate constants of translocation of the free carrier S and the carrier-ion complex MS⁺ are nearly equal (2·10⁴ sec^-1) and are of the same order as the dissociation rate constant of MS⁺ in the membrane-solution interface (5·10⁴ sec^-1). The equilibrium constant of the heterogeneous association reaction M⁺ (solution) + S (membrane) → MS⁺ (membrane) is found to be ~ 1 M^-1, about 10⁶ times smaller than the association constant in ethanolic solution.     

Chloride conductance of the Amphiuma red cell membrane

Summary Like most other red cells, the giant erythrocytes ofAmphiuma means possess a system for rapid exchange of chloride across the membrane. Also, there are indications that the net transport of chloride in these cells is slow. The size ofAmphiuma erythrocytes allows direct measurements of membrane potential with microelectrodes. The present work exploits the possibility that such measurements can be used to give a quantitative estimate of the chloride conductance (G _Cl) of the Amphiuma red cell membrane. The membrane potential was measured as a function of extracellular chloride concentration (5–120mM), using an impermeant anion (Para-amino-hippurate) as a substitute. Furthermore, the effect of different pH values (6.0–7.2) was studied. For each extracellular chloride concentration the membrane potential was determined at a pH at which hydroxyl, hydrogen, and bicarbonate ions were in electrochemical equilibrium. From these membrane potentials and the corresponding chloride concentrations in the medium (at constant intracellular ion concentrations), theG _Cl of the membrane was calculated to be 3.9×10^–7 {ie27-1} cm^–2. This value is some six orders of magnitude smaller than that calculated from the rate of tracer exchange under equilibrium conditions. The experimental strategy used gives the value for a partial transference number which takes into account only ions which arenot in electrochemical equilibrium. Whereas this approach gives a value forG _Cl, it does not permit calculation of the overall membrane conductance. From the calculated value ofG _Cl it is possible to estimate that the maximal value of the combined conductances of hydroxyl (or proton) and bicarbonate ions is 0.6×10^–7 {ie27-2} cm^–2. The large discrepancy between the rate of exchange of chloride and its conductance is in agreement with measurements on human and sheep red cells employing the ionophore valinomycin to increase the potassium conductance of the membrane. The results in the present study were, however, obtained without valinomycin and an accompanying assumption of a constant field in the membrane. Therefore, the present measurements give independent support to the above mentioned conclusions.     

A laser-T-jump study of the adsorption of dipolar molecules to planar lipid membranes

The adsorption of the herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) as well as of other dipolar molecules to the interface of artificial lipid membranes gives rise to a change of the dipole potential between the membrane interior and water. As a consequence of the adsorption of the neutral species, the conductance of planar membranes, observed in the presence of the macrocyclic ion carriers nonactin or valinomycin, may change by many orders of magnitude. Using this effect in combination with a laser-T-jump technique, the kinetics of the adsorption process were measured and were interpreted on the basis of a Langmuir-isotherm. A partition coefficient (at small concentrations) of _HA =4.7·10^–4 cm, a rate constant of desorption k _HA100 s^-1 and a maximum surface density N _D=7.7·10¹³/cm² were found. The concentration at half saturation is K _HA=2.7·10^-4 M.Using these values the membrane conductance induced by the ion carrier nonactin and the shape of the current-voltage relationship as a function of the ligand concentration in water was analyzed. A maxiumum dipole potential of V _D ^max =-239 m V and a contribution of b=3.1·10^-15V cm² per single adsorbed 2,4-D molecule was found. 74% of the dipole potential acts on the inner membrane barrier separating the two interfacial adsorption planes of nonactin. The remainder (26%) favours interfacial complex formation between nonactin and K⁺ from the aqueous phase. The data hold for membranes formed from dioleoyllecithin in n-decane.     

Kinetic analysis of carrier-mediated ion transport by the charge-pulse technique

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.     

Effect of cell membrane-active antimicrobial agents on membrane potential and viability of mycoplasmas

The membrane potential ofMycoplasma mycoides subsp.capri has been determined to beE _M=−48 mV±10%, inside negative. In this study we investigated the influence of cell membrane-active antimicrobial agents, viz., valinomycin, gramicidin, polymyxin, and clotrimazole, on membrane potential and viability ofM. mycoides subsp.capri. Valinomycin, an ionophore with extreme potassium selectivity, induced a membrane hyperpolarization,E _M=−110 mV. Valinomycin was not cidal, but static to mycoplasmas. Obviously the potassium drain induced by valinomycin can be compensated for by the organisms. Gramicidin is an antibiotic forming cation conduction channels across membranes. It induced a rapid depolarization,E _M=+23 mV, of mycoplasma membranes. At low concentrations, gramicidin had a static effect, whereas at high concentrations it was cidal to mycoplasmas. The rapid permeation of cations through the stationary ion channels formed by gramicidin obviously exerts an inhibitory or even lethal effect on mycoplasma metabolism and growth. Polymyxin B induced a depolarization,E _M=−35 mV, of mycoplasma membranes only when the organisms had been pretreated and hyperpolarized with valinomycin. After treatment with both valinomycin and polymyxin B, a slight inhibition of mycoplasma growth was observed. Clotrimazole, a synthetic imidazole antimycotic, hyperpolarized mycoplasma membranes (E _M=−80 mV). At high concentrations clotrimazole was cidal, whereas at low concentrations it was static to mycoplasmas.     

Flux ratio of valinomycin-mediated K+ fluxes across the human red cell membrane in the presence of the protonophore CCCP

Summary The ratio of valinomycin-mediated unidirectional K⁺ fluxes across the human red cell membrane, has been determined in the presence of the protonophore carbonylcyanidem-chlorophenylhydrazone, CCCP, using the K⁺ net efflux and⁴²K influx. The driving force for the net efflux (V _m –E _K ⁺) has been calculated from the membrane potential, estimated by the CCCP-mediated proton distribution and the Nernst potential for potassium ions across the membrane. An apparent driving potential for the K⁺ net efflux has been calculated from the K⁺ flux ratio, determined in experiments where the valinomycin and CCCP concentrations were varied systematically. This apparent driving force, in conjunction with the actual driving force calculated on basis of the CCCP estimated membrane potential, is used to calculate a flux ratio exponent, which represents an estimate of the deviation of valinomycin-mediated K⁺ transport from unrestricted electrodiffusion, when protonophore is present.In the present work, the flux ratio exponent is found to be 0.90 when the CCCP concentration is 5.0 m and above, while the exponent decreases to about 0.50 when no CCCP is present. The influence of CCCP upon the rate constants in the valinomycin transport cycle is discussed. The significance of this result is that red cell membrane potentials are overestimated, when calculated from valinomycin-mediated potassium isotope fluxes, using a constant field equation.     

Transport of alkali cations through thin lipid membranes by (222)C10-Cryptand,an ionizable mobile carrier

Summary The kinetics of K⁺ and Na⁺ transport across the membrane of large unilamellar vesicles (L.U.V.) were compared at two pH's, with two carriers: (222)C ₁₀-cryptand (diaza-1, 10-decyl-5-hexaoxa-4,7,13,16,21,24-bicyclo[8.8.8.]hexacosane) and valinomcyin, i.e. an ionizable macrobicyclic amino polyether and a neutral macrocyclic antibiotic. The rate of cation transport by (222)C₁₀ saturated as cation and carrier concentrations rose. The apparent affinity of (222)C₁₀ for K⁺ was higher and less pH dependent than that for Na⁺ but resembled the affinity of valinomycin for K⁺. The efficiency of (222)C₁₀ transport of K⁺ decreased as the pH fell and the carrier concentration rose, and was about ten times lower than that of valinomycin. Noncompetitive K⁺/Na⁺ transport selectivity of (222)C₁₀ decreased as pH, and cation and carrier concentrations rose, and was lower than that of valinomycin. Transport of alkali cations by (222)C₁₀ and valinomycin was noncooperative. Reaction orders in cationn(S) and carrierm(M) varied with the type of cation and carrier and were almost independent of pH;n(S) andm(M) were not respectively dependent on carrier or cation concentrations. The apparent estimated constants for cation translocation by (222)C₁₀ were higher in the presence of Na⁺ than of K⁺ due to higher carrier saturation by K⁺, and decreased as pH and carrier concentration increased. Equilibrium potential was independent of the nature of carrier and transported cation. Results are discussed in terms of the structural, physicochemical and electrical characteristics of carriers and complexes.     

Transport kinetics of hydrophobic ions in lipid bilayer membranes Charge-pulse relaxation studies

A modified version of the charge-pulse relaxation technique with improved time resolution was applied to the study of transport kinetics of hydrophobic ions (tetraphenylborate, dipicrylamine) through lipid bilayer membranes. Besides a better time resolution the charge-pulse method has the additional advantage that the perturbation of the membrane can be kept small (voltage amplitudes between 1 and 10 mV). The results of the analysis support the model proposed earlier, according to which the overall transport takes place in three consecutive steps, adsorption of the ion from water to the interface, translocation to the opposite interface, and desorption into the aqueous phase. The translocation rate constant K_i and the partition coefficient γ of the hydrophobic ion between water and the membrane were measured for lecithins with different mono-unsaturated fatty acid residues. Increasing the chain length of the fatty acid from C₁₆ to C₂₄ resulted in a decrease of k_i by a factor of about 9 in the case of tetraphenylborate and by a factor of about 17 in the case of dipicrylamine.     

Impedance analysis of valinomycin activity in nano-BLMs

Nano-black lipid membranes (nano-BLMs) were obtained by functionalization of highly ordered porous alumina substrates with an average pore diameter of 60 nm based on a self-assembled alkanethiol submonolayer followed by spreading of 1,2-diphytanoyl-sn-glycero-3-phosphocholine dissolved in n-decane on the hydrophobic substrate. By means of impedance spectroscopy, we analyzed the influence of the self-assembled alkanethiol submonolayer on the electrical properties of the nano-BLMs as well as their long-term stability. We were able to stably integrate nano-BLMs into a flow through system, which allowed us to readily exchange buffer solutions several times and accounts for mass transport phenomena. The ionophore valinomycin was successfully inserted into nano-BLMs and its transport activity monitored as a function of different potassium and sodium ion concentrations reflecting the specificity of valinomycin for potassium ions.     

CD and nmr studies on the interaction of lithium ion with valinomycin and gramicidin-S

The conformation of the valinomycin–lithium complex has been studied using CD and nmr techniques. The lithium ion induced significant changes in the chemical shifts of the NH and C^αH protons, as well as in the CD spectra of valinomycin. From the analysis of the lithium ion titration data, it is concluded that valinomycin forms a 1:1 type weak complex with lithium, having a stability constant of 48 L mol^?1 at 25°C. This conformation is different from the familiar valinomycin–potassium complex. The nature of the interaction at low and high concentrations of lithium ions with valinomycin (ionophore) and gramicidin-S (nonionophore) has been compared. At high salt concentrations, there was a further change in the CD and nmr spectra of valinomycin, giving a second plateau region at > 3M of the salt. In the case of gramicidin-S, no significant changes in the nmr or CD spectra were observed in the lower concentration range corresponding to where changes were observed in the case of valinomycin. However, the addition of lithium salt at concentrations greater than 3M induced changes in both the CD and nmr spectra of gramicidin-S, and the titration graph of molar ellipticity versus concentration of lithium perchlorate gave a plateau region at concentrations greater than this. These results indicate that the effects of lithium at low and high concentrations are independent of each other. The conformational transitions at very high salt concentrations (denaturation) are more likely due to solvent structural perturbations rather than to the consequences of ion binding.     

Isolation of the alanine carrier from the membranes of a thermophilic bacterium and its reconstitution into vesicles capable of transport

A carrier protein mediatine alanine transport was purified from the membranes of the thermophilic bacterium PS3, by ion exchange chromatography in the presence of both Triton X-100 and urea. The alanine carrier was recovered in the nonadsorbed fraction from either DEAE-or CM-cellulose columns, suggesting that its isoelectric point was in the neutral pH region. The final preparation contained virtually no electron transfer components, ATPase, or NADH dehydrogenase. Polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate revealed that the final preparation consisted of two major protein components with molecular weights of 36,000 and 9,400. Active transport of alanine after incorporation of the alanine carrier into reconstituted proteoliposomes was driven not only by an artificial membrane potential generated by potassium ion diffusion via valinomycin but also by mitochondrial cytochrome oxidase incorporated into the same liposomes and supplemented with both cytochrome c and ascorbic acid. The membrane-integrated portion (TF₀) of the ATPase complex uncoupled alanine transport by conducting protons across the membrane.     

Influence of membrane structure on the kinetics of carrier-mediated ion transport through lipid bilayers

Charge-pulse relaxation experiments of valinomycin-mediated Rb⁺ transport have been carried out in order to study the influence of membrane structure on carrier kinetics. From the experimental data the rate constants of association (k_R) and dissociation (k_D) of the ion-carrier complex as well as the rate constants of translocation of the complex (k_MS) and of the free carrier (k_S) could be obtained. The composition of the planar bilayer membrane was varied in a wide range. In a first series of experiments, membranes made from glycerolmonooleate dissolved in different n-alkanes (n-decane to n-hexadecane), as well as solvent-free membranes made from the same lipid by the Montal-Mueller technique were studied. The translocation rate constants k_S and k_MS were found to differ by less than a factor of two in the membranes of different solvent content. Much larger changes of the rate constants were observed if the structure of the fatty acid residue was varied. For instance, an increase in the number of double bonds in the C₂₀ fatty acid from one to four resulted in an increase of k_S by a factor of seven and in an increase of k_MS by a factor of twenty-four. The stability constant K = k_R/k_D of the ion-carrier complex as well as the translocation rate constants k_S and k_MS were found to depend strongly on the nature of the polar headgroup of the lipid. The incorporation of cholesterol into glycerolmonooleate membranes reduced k_R, k_MS and k_S up to seven-fold.     

Protonmotive force and motility of Bacillus subtilis.

Motility of Bacillus subtilis was inhibited within a few minutes by a combination of valinomycin and a high concentration of potassium ions in the medium at neutral pH. Motility was restored by lowering the concentration of valinomycin or potassium ions. The valinomycin concentration necessary for motility inhibition was determined at various concentrations of potassium ions and various pH's. At pH 7.5, valinomycin of any concentration did not inhibit the motility, when the potassium ion concentration was lower than 9 mM. In the presence of 230 mM potassium ion, the motility inhibition by valinomycin was not detected at pH lower than 6.1. These results are easily explained by the idea that the motility of B. subtilis is supported by the electrochemical potential difference of the proton across the membrane, or the protonmotive force. The electrochemical potential difference necessary for motility was estimated to be about -90 mV.     

Modification of valinomycin-mediated bilayer membrane conductance by 4,5,6,7-tetrachloro-2-methylbenzimidazole

Summary The compound, 4,5,6,7-tetrachloro-2-methylbenzimidazole (TMB), has been found to markedly modify the steady-state valinomycin-mediated conductance of potassium (K⁺) ions through lipid bilayer membranes. TMB alone does not contribute significantly to membrane conductance, being electrically neutral in solution. In one of two classes of experiments (I), valinomycin is first added to the aqueous phases then changes of membrane conductance accompanying stepwise addition of TMB to the water are measured. In a second class of experiments (II), valinomycin is added to the membrane-forming solution, follwed by TMB additions to the surrounding water. In both cases membrane conductance shows an initial increase with increasing TMB concentration which is more pronounced at lower K⁺ ion concentration. At TMB concentrations in excess of 10^–5 m, membrane conductance becomes independent of K⁺ ion concentration, in contrast to the linear dependence observed at TMB concentrations below 10^–7 m. This transition is accompanied by a change of high field current-voltage characteristics from superlinear (or weakly sublinear) to a strongly sublinear form. All of these observations may be correlated by the kinetic model for carriermedicated transport proposed by Läuger and Stark (Biochim. Biophys. Acta 211:458, 1970) from which it may be concluded that valinomycin-mediated ion transport is limited by back diffusion of the uncomplexed carrier at high TMB concentrations. Experiments of class I reveal a sharp drop of conductance at high (>10^–5 m) TMB concentration, not seen in class II experiments, which is attributed to blocked entry of uncomplexed carrier from the aqueous phases. Valinomycin initially in the membrane is removed by lateral diffusion to the surrounding torus. The time dependence of this removal has been studied in a separate series of experiments, leading to a measured coefficient of lateral diffusion for valinomycin of 5×10^–6 cm²/sec at 25°C. This value is about two orders of magnitude larger than the corresponding coefficient for transmembrane carrier diffusion, and provides further evidence for localization of valinomycin in the membrane/solution interfaces.     

3,5-Di-tert-butyl-4-hydroxybenzylidenemalononitrile. Effects of pH on its binding to liposomes and evidence for formation of a ternary complex with valinomycin and potassium ion

1. The properties of 3,5-di-tert-butyl-4-hydroxybenzylidenemalononitrile (SF 6847) were studied chemically and spectroscopically. Two molecular species of SF6847 were identified: the undissociated form (SFH; ?₃₆₃, 10 mM^?1) and the dissociated form (SF^?; ?₄₅₄, 35 mM^?1). The pK_a value of the molecule was determined to be 6.9.2. On the basis of these properties the interactions of SF6847 with liposomes and valinomycin · K⁺ were studied. The partition constants of SFH (Kⁿ_p and SF^? (K^?_p) to liposomes were determined separately; Kⁿ_p was 56 mM^?1 and was independent of the pH of the medium, whereas K^?_p dependend greatly on the pH, being 1.2 mM^?1 at pH 7.0 and 2.9 mM^?1 at pH 8.0. Using these values, the partition constant of total SF6847 (K_p) was calculated and found to be essentially the same as that calculated from the kinetics of proton uptake. It was concluded that the amount of SF^? bound to liposomes is rate limiting for proton uptake.3. The effects of membrane potential on partition constants were studied. The K^?_p decreased greatly upon generation of a membrane potential negative inside the liposomes but increased upon generation of a membrane potential positive inside the liposomes.4. The interaction of SF6847 with valinomycin in aqueous solution and in liposomes was demonstrated only in the presence of potassium ion. Potassium ion could not be replaced by sodium ion. Evidence was obtained for the formation of the ternary complex valinomycin · K⁺ · SF^? in liposomes and in hexane. It was concluded that SF^? became more soluble in the liposomal membranes on formation of this ternary complex. All these results support our proposed mechanism for the proton uptake cycle (Yamaguchi, A. and Anraku, Y. (1978) Biochim. Biophys. Acta 501, 136–149).