Measurements of non-electrogenic fluxes through lipid bilayers induced by Men+/nH+-exchangers

In the presence of concentration gradient of metal ions on bilayer lipid membrane (BLM) the addition of non-electrogenic carriers results in a formation of concentration gradient of hydrogen ions in the unstirred layers near membrane. Addition of protonophore under these conditions brings about the formation of diffusion potential of hydrogen ions. This effect underlies the method of measuring non-electrogenic fluxes on BLM initiated in the presence of Men+/nH+ - exchangers. The proposed method was tested on the following Men+/nH+ - exchangers: nigericin, monensin and A23187. The order of cationic selectivity of the given carriers obtained by measuring the potentials on BLM in the presence of protonophores agrees with literature data, which were obtained by direct measurements of ionic fluxes.     

The use of phospholipid-impregnated millipore filters for recording nonelectrogenic cation flows in the presence of Men+/nH+ exchangers

It has been shown that with a cation (K+, Na+, Ca2+) concentration gradient on a Millipore filter impregnated with a decane solution of phospholipid, in the presence of a Men+/nH+ exchanger (nigericin, monensin, A23187), addition of a protonophore induces the formation of an electric potential positively charged on the side where the concentration of the cation is lower. The formation of the potential is induced by the hydrogen ion concentration gradient in the filter and in the unstirred layers as a result of the Men+/nH+ exchange. In such a system, with a pH gradient on the filter in the presence of monensin and valinomycin, a potential is generated with the plus on the side of the lower concentration of hydrogen. The effect is the result of the formation of a potassium ion concentration gradient in the unstirred layers in the course of the K+/H+ exchange. It is concluded that phospholipid-impregnated filters can be used for search and identification of electroneutral membrane ionophores of the Men+/nH+ exchanger type.     

Effect of phloretin on the carrier-mediated electrically silent ion fluxes through the bilayer lipid membrane: measurements of pH shifts near the membrane by pH microelectrode.

The effect of phloretin on the carrier-mediated electrically silent ion fluxes through the bilayer lipid membrane (BLM) was studied. The measurements were carried out according to our conventional technique, i.e. electrical potential recording in the presence of a protonophore, and by a new method--direct measurements of pH shifts in the unstirred layers of the BLM by pH microelectrode. Both techniques gave similar results. It was shown that the addition of phloretin increased the rate of cation/H+ exchange induced by nigericin and decreased the rate of anion/OH(-)-exchange induced by tributyltin. The effect of phloretin was higher in the presence of cholesterol in the BLM. Cholesterol decreased the nigericin- and tributyltin-induced fluxes under our experimental conditions. The application of an external voltage to the membrane had no effect on the ion fluxes thereby showing that these fluxes were electroneutral. The most probable explanation of these results bases on the effect of the membrane dipole potential on the electroneutral fluxes of ions. The possible mechanism of the dipole potential effect on the carrier-mediated electrically silent ion fluxes was discussed in terms of two competing hypotheses--the translocation through the membrane or the reactions at the membrane surface being the rate-limiting steps of the whole transport process.     

Effect of changes in cation concentration near bilayer lipid membrane on the rate of carrier-mediated cation fluxes and on the carrier apparent selectivity

A new approach was applied for the measurements of ion transport through bilayer lipid membranes (BLM) induced by electrically neutral cation/H+ exchangers. This is an improved version of the method of the measurements of the cation/H+ exchange rate based on recording pH shifts in the unstirred layers near the BLM. Using this approach, the pH gradient in the unstirred layers induced by the cation/H+ exchanger was reduced by successive addition of the acetate on one side of the BLM until the pH shift reached zero. The difference in acetate concentration across the membrane is a measure of the cation/H+ exchange rate. In the second part of the work we found that the changes in cation concentration in the unstirred layers under the conditions imposed when measuring cation selectivity (according to Antonenko, Yu.N. and Yaguzhinsky, L.S., Biochim. Biophys. Acta 1988; 938, 125-130) can significantly decrease the apparent value of cation selectivity. It was shown that more accurate results can be obtained if low concentrations of the carrier are used. The values of nigericin cation selectivity for the alkali metals were measured (K+/Rb+ 19 +/- 1, Rb+/Na+ 1.9 +/- 0.2, Na+/Cs+ 8 +/- 0.5, Cs+/Li+ 1.8 +/- 0.3).     

Nigericin-induced charge transfer across membranes.

The electric properties of the bilayer lecithin membranes have been studied in the presence of the antibiotic nigericin. When the antibiotic concentration is about 10(-7) ohm-1 cm-2. The potassium ion concentration gradient gives rise to a transmembrane potential of the order of 40 mV per 10-fold concentration gradient with the side of the higher potassium concentration negative. The transmembrane potential produced by the hydrogen ion concentration gradient is a function of the potassium ion concentration which is equal on both sides of the membrane. For low potassium ion concentrations the hydrogen potential has the expected polarity with the solution having higher concentration of protons negative. For potassium ion concentrations exceeding 0.03 M the hydrogen potential has the reverse polarity. This unexpected result cannot be accounted for in terms of the available simple hypotheses about the charge transport mechanism for nigericin in BLM. In order to account for the experimental results obtained, a theoretical approach has been developed based on the assumption that charge is transported across the membrane by nigericin dimers. The theoretical predicitons are in satisfactory agreement with the experimental results. The model also yields some predictions which may be verified in future experiments.     

Non-ohmic proton conductance of mitochondria and liposomes

Direct measurements of the proton/hydroxyl ion flux across rat liver mitochondria and liposome membranes are reported. H+/OH- fluxes driven by membrane potential (delta psi) showed nonlinear dependence on delta psi both in mitochondria and in liposomes whereas delta pH-driven H+/OH- flux shows linear dependence on delta pH in liposomes. In the presence of low concentrations of a protonophore the H+/OH- flux was linearly dependent on delta psi and showed complex dependence on delta pH. The nonlinearity of H+/OH- permeability without protonophore is described by an integrated Nernst- Plank equation with trapezoidal energy barrier. Permeability coefficients depended on the driving force but were in the range 10(-3) cm/s for mitochondria and 10(-4)-10(-6) cm/s for liposomes. The nonlinear dependence of H+/OH- flux on delta psi explains the nonlinear dependence of electrochemical proton gradient on the rate of electron transport in energy coupling systems.     

Nigericin-induced charge transfer across membranes

Summary The electric properties of the bilayer lecithin membranes have been studied in the presence of the antibiotic nigericin. When the antibiotic concentration is about 10^–6 m the conductivity of the BLM is increased up to 10^–7 ohm^–1 cm^–2. The potassium ion concentration gradient gives rise to a transmembrane potential of the order of 40 mV per 10-fold concentration gradient with the side of the higher potassium concentration negative. The transmembrane potential produced by the hydrogen ion concentration gradient is a function of the potassium ion concentration which is equal on both sides of the membrane. For low potassium ion concentrations the hydrogen potential has the expected polarity with the solution having higher concentration of protons negative. For potassium ion concentrations exceeding 0.03m the hydrogen potential has the reverse polarity. This unexpected result cannot be accounted for in terms of the available simple hypotheses about the charge transport mechanism for nigericin in BLM. In order to account for the experimental results obtained, a theoretical approach has been developed based on the assumption that charge is transported across the membrane by nigericin dimers. The theoretical predictions are in satisfactory agreement with the experimental results. The model also yields some predictions which may be verified in future experiments.     

The use of phospholipid-impregnated millipore filters for recording nonelectrogenic cation flows in the presence of Me/nH exchangers

It has been shown that with a cation (K⁺, Na⁺, Ca²⁺) concentration gradient on a Millipore filter impregnated with a decane solution of phospholipid, in the presence of a Meⁿ⁺/nH⁺ exchanger (nigericin, monensin, A23187), addition of a protonophore induces the formation of an electric potential positively charged on the side where the concentration of the cation is lower. The formation of the potential is induced by the hydrogen ion concentration gradient in the filter and in the unstirred layers as a result of the Meⁿ⁺/nH⁺ exchange. In such a system, with a pH gradient on the filter in the presence of monensin and valinomycin, a potential is generated with the plus on the side of the lower concentration of hydrogen. The effect is the result of the formation of a potassium ion concentration gradient in the unstirred layers in the course of the K⁺/H⁺ exchange. It is concluded that phospholipid-impregnated filters can be used for search and identification of electroneutral membrane ionophores of the Meⁿ⁺/nH⁺ exchanger type.     

Uptake of Magnesium by Chromaffin Granules In Vitro: Role of the Proton Electrochemical Gradient

Abstract: The chromaffin granule membrane in vitro is impermeable to protons as well as to Mg²⁺; however, when granules are incubated in the presence of the proton ionophore carbonyl cyanide p -trifluoromethoxy-phenylhydrazone or an inhibitor of the granule membrane Mg²⁺-dependent ATPase, the metal ion is accumulated inside the granules. This accumulation is dependent upon the granule transmembrane potential. The simultaneous presence of the ATPase inhibitor and the proton ionophore markedly increases metal ion incorporation. Mg²⁺ incorporation is also promoted by nigericin in the presence of potassium or sodium ions, indicating that Mg²⁺ accumulation is also dependent upon the transmembrane pH gradient. Concomitant with the Mg²⁺ accumulation, there is a significant loss of endogenous catecholamines. It is concluded that Mg²⁺ accumulation is determined by the electrochemical gradient maintained across the membrane. Once the metal ion has accumulated into the granules it displaces catecholamines from their storage sites.     

Proton dissociation from nigericin at the membrane-water interface, the rate-limiting step of K+/H+ exchange on the bilayer lipid membrane.

The rate of K+/H+ exchange through bilayer lipid membranes (BLM) induced by nigericin was measured by the method of pH gradient offset according to Antonenko, Yu.N. and Yaguzhinsky L.S. [(1990) Biochim. Biophys. Acta 1026, 236-240]. It was shown that under the conditions of high potassium ion concentration the rate of nigericin-mediated K+/H+ exchange increased with an increase in the concentrations of such buffer compounds as citric acid and MES. The concentration dependence was different for citrate and MES. The buffer concentration effect was absent at low potassium ion concentrations. Citrate increased the rate of K+/H+ exchange being added to the side of BLM where the K+ concentration was higher and had no effect at the opposite side. At high KCl and citrate concentrations, the rate of K+/H+ exchange was about 6 times lower in D2O when compared to H2O solutions. It is concluded that under certain experimental conditions the overall rate of the K+/H+ exchange induced by nigericin is determined by the rate of proton dissociation from nigericin at the membrane-water interface.     

Calcium transport driven by a proton motive force in vacuolar membrane vesicles of Saccharomyces cerevisiae

Vacuolar membrane vesicles of Saccharomyces cerevisiae accumulate Ca2+ ion in the presence of ATP, not in the presence of ADP or adenyl-5'-yl imidodiphosphate. Calcium transport showed saturation kinetics with a Km value of 0.1 mM and optimal pH of 6.4. Ca2+ ion incorporated in the vesicles was exchangeable and released completely by a protonophore uncoupler, 3,5-di-tert-butyl-4-hydroxybenzilidenemalononitrile (SF6847), or calcium-specific ionophore, A23187. The transport required Mg2+ ion but was inhibited by Cu2+ or Zn2+ ions, inhibitors of H+-ATPase of the vacuolar membrane. The transport activity was sensitive to the H+-ATPase inhibitor N,N'-dicyclohexylcarbodiimide, but not to oligomycin or sodium vanadate. SF6847 or nigericin blocked Ca2+ uptake completely, but valinomycin stimulated it 1.35-fold. These results indicate that an electrochemical potential difference of protons is a driving force for this Ca2+ transport. The ATP-dependent formation of the deltapH in the vesicles and its partial dissipation by CaCl2 were demonstrated by fluorescence quenching of quinacrine. This Ca2+ uptake by vacuolar membrane vesicles is suggested to be catalyzed by a Ca2+/H+ antiport system.     

Transport of mercury compounds across bimolecular lipid membranes: effect of lipid composition, pH and chloride concentration

The use of bimolecular lipid membranes (BLM) as model membrane allows the analysis of the transport of mercury compounds across the lipidic barriers of biological membranes. The results of flux measurements show that two mercury compounds--HgCl2 and CH3HgCl--cross the BLM but the overall permeabilities are dependent on the pH of the aqueous media, and are not apparently influenced by the different phospholipid constituents of the bilayers. On the other hand, electrical measurements show that, function of the chemical speciation, the transport of this metal is done essentially in the neutral form.     

The movement of ion pairs across bilayer lipid membranes.

If a polyhalide concentration gradient exists across a bilayer lipid membrane (BLM), ion pair movement occurs. The term ion pair indicates a lipid soluble complex of cation and anion with stoichiometry dictated by the respective charges. In a mixture of metal halide (MX_n, X = I, Cl, Br) and iodine, the ion pair is of the form M(I₂X)_n. The flux of ion pairs was monitored by measuring the flow of metal ions or polyhalide ions across the BLM. The flux of ion pairs across the BLM depended on cation crystal radius, fluidity of the membrane, strength of the ion pair complex and on the osmotic gradient (i.e., there exists a coupling between water and ion pair fluxes). The relationship between ion pairing and the electrical conductivity of BLM is briefly discussed.     

Kinetic study of the antiport mechanism of an Escherichia coli zinc transporter, ZitB

ZitB is a member of the cation diffusion facilitator (CDF) family that mediates efflux of zinc across the plasma membrane of Escherichia coli. We describe the first kinetic study of the purified and reconstituted ZitB by stopped-flow measurements of transmembrane fluxes of metal ions using a metal-sensitive fluorescent indicator encapsulated in proteoliposomes. Metal ion filling experiments showed that the initial rate of Zn2+ influx was a linear function of the molar ratio of ZitB to lipid and was related to the concentration of Zn2+ or Cd2+ by a hyperbola with a Michaelis-Menten constant (K(m)) of 104.9 +/- 5.4 microm and 90.1 +/- 3.7 microm, respectively. Depletion of proton stalled Cd2+ transport down its diffusion gradient, whereas tetraethylammonium ion substitution for K+ did not affect Cd2+ transport, indicating that Cd2+ transport is coupled to H+ rather than to K+. H+ transport was inferred by the H+ dependence of Cd2+ transport, showing a hyperbolic relationship with a Km of 19.9 nm for H+. Applying H+ diffusion gradients across the membrane caused Cd2+ fluxes both into and out of proteoliposomes against the imposed H(+) gradients. Likewise, applying outwardly oriented membrane electrical potential resulted in Cd2+ efflux, demonstrating the electrogenic effect of ZitB transport. Taken together, these results indicate that ZitB is an antiporter catalyzing the obligatory exchange of Zn2+ or Cd2+ for H+. The exchange stoichiometry of metal ion for proton is likely to be 1:1.     

Controlled search for the producers of ionophore antibiotics among Streptomycetes

A procedure was developed for directed screening of cultures producing ionophore antibiotics among streptomycetes. The procedure is based on measuring the membrane potential generated in the presence of the Men+/nH+ = -expchanger-protonophore couple. It provided isolation of cultures producing ionophore antibiotics at the fermentation broth stage. It was possible to use the procedure in screening both electrogenic and nonelectrogenic ionophores and to rapidly differentiate them. 5 cultures producing ionophore antibiotics were detected with this procedure; 3 of them carry out nonelectrogenic transport of the cations. The cation transport in the other two cultures was electrogenic. Cation selectivity of the antibiotics produced by the cultures was determined with the procedure. An antibiotic identical to indanomycin was isolated from the culture fluid of Streptomyces chromogenes.     

Enhancement of the light-triggered electrical response in plant cells following their de-energization with uncouplers

Light-triggered membrane potential changes in cells of a liverwort Anthoceros are greatly enhanced by the ionophorous uncouplers nigericin and monesin. Stimulation of the light-triggered electrical response (LTER) by nigericin occurred concomitantly with inhibition of a slow decline in the chlorophyll fluorescence, which suggests that the transmembrane pH gradient in thylakoids is not essential for generation of LTER at the plasma membrane. The extent of monensin-stimulated LTER remained high under a diminished driving force for the ionophore-induced proton-cation exchange across the plasma membrane (elevation of the external Na⁺ concentration from 1 to 50 m M ), which indicates that energy uncoupling in chloroplasts is more related to the electric response enhancement than the induction of the H⁺/K⁺(Na⁺) exchange at the plasma membrane. Enhancement of LTER by ionophores occurs in parallel with stimulation of light-triggered pH changes (alkalinization) in the vicinity of the cell surface, which suggests an association of trans-membrane H⁺ fluxes with LTER. The results are consistent with the hypothesis that illumination produces a temporary inhibition of the plasma membrane H⁺ pump with a subsequent activation of gated channels and transient rapid depolarization of the cell.     

Na+/H+ exchange is present in basolateral membranes from rabbit small intestine

Fluorescence quenching of the pH gradient sensitive dye acridine orange and that of the membrane potential sensitive dye Di-S-C3(5) have been studied in purified basolateral membrane vesicles obtained from rabbit small intestine. Basolateral membranes contain an electroneutral, carrier mediated, Na+/H+ exchange activity. They also appear to contain an electrogenic pathway for H+ movement. Based on the comparison of acridine orange fluorescence quenching in the presence of an outwardly directed Na+ gradient and in the presence of known K+ diffusion gradients it can be estimated that at least 50% of the observed proton fluxes are due to the activity of the exchanger. Acridine orange fluorescence recovery measurements have been used to assess the kinetic properties of the exchanger.     

Proton gradient-dependent transport of glycine in rabbit renal brush-border membrane vesicles

This study describes evidence for the existence of a H+/glycine symport system in rabbit renal brush-border membrane vesicles. An inward proton gradient stimulates glycine transport across the brush-border membrane, and this H+-driven glycine uptake is attenuated by the protonophore carbonyl cyanide p-trifluoromethoxyphenylhydrazone. It is a positive rheogenic process, i.e. the H+-dependent glycine uptake is further enhanced by an intravesicular negative potential. Glycine uptake is stimulated to a lesser degree by an inward Na+ gradient. H+-dependent glycine uptake is inhibited by sarcosine (69%), an analog amino acid, imino acids (proline 81%, hydroxy proline 67%), and beta-alanine (31%), but not by neutral (L-leucine) or basic (L-lysine) amino acids. The results demonstrate that H+ glycine co-transport system in rabbit renal brush-border membrane vesicles is a carrier-mediated electrogenic process and that transport is shared by imino acids and partially by beta-alanine.     

Measurements of local pH changes near bilayer lipid membrane by means of a pH microelectrode and a protonophore-dependent membrane potential. Comparison of the methods.

Shifts of pH near the bilayer lipid membrane (BLM) were measured in the absence of pH difference between bulk solutions by two methods, i.e. pH microelectrode and membrane potential recordings in the presence of a protonophore. A quantitative agreement of the results of both methods was obtained. The kinetics of the generation of potential induced by the addition of ammonium chloride was accounted for by the time of the diffusion through the unstirred layers. The thickness of the unstirred BLM layers was determined in the experiment.     

Active amino acid transport in plasma membrane vesicles from Simian virus 40-transformed mouse fibroblasts. Characteristics of electrochemical Na+ gradient-stimulated uptake.

Selectively permeable membrane vesicles isolated from Simian virus 40-transformed mouse fibroblasts catalyzed Na+ gradient-coupled active transport of several neutral amino acids dissociated from intracellular metabolism. Na+-stimulated alanine transport activity accompanied plasma membrane material during centrifugation in discontinuous dextran 110 gradients. Carrier-mediated transport into the vesicle was demonstrated. When Na+ was equilibrated across the membrane, countertransport stimulation of L-[3H]alanine uptake occurred in the presence of accumulated unlabeled L-alanine, 2-aminoisobutyric acid, or L-methionine. Competitive interactions among neutral amino acids, pH profiles, and apparent Km values for Na+ gradient-stimulated transport into vesicles were similar to those previously described for amino acid uptake in Ehrlich ascites cells, which suggests that the transport activity assayed in vesicles is a component of the corresponding cellular uptake process. Both the initial rate and quasi-steady state of uptake were stimulated as a function of a Na+ gradient (external Na+ greater than internal Na+) applied artificially across the membrane and were independent of endogenous (Na+ + K+)-ATPase activity. Stimulation by Na+ was decreased when the Na+ gradient was dissipated by monensin, gramicidin D or Na+ preincubation. Na+ decreased the apparent Km for alanine, 2-aminoisobutyric acid, and glutamine transport. Na+ gradient-stimulated amino acid transport was electrogenic, stimulated by conditions expected to generate an interior-negative membrane potential, such as the presence of the permeant anions NO3- and SCN-. Na+-stimulated L-alanine transport was also stimulated by an electrogenic potassium diffusion potential (K+ internal greater than K+ external) catalyzed by valinomycin; this stimulation was blocked by nigericin. These observations provide support for a mechanism of active neutral amino acid transport via the "A system" of the plasma membrane in which both a Na+ gradient and membrane potential contribute to the total driving force.