Temperature jump as a new technique to study the kinetics of fast transport of protons across membranes

Efficiency of mitochondrial uncoupling by modified butyltriphenylphosphonium cations and fatty acids correlates with lipophilicity of cations: Protonophoric vs leakage mechanisms

In order to determine the share of protonophoric activity in the uncoupling action of lipophilic cations a number of analogues of butyltriphenylphosphonium with substitutions in phenyl rings (C4TPP-X) were studied on isolated rat liver mitochondria and model lipid membranes. An increase in the rate of respiration and a decrease in the membrane potential of isolated mitochondria were observed for all the studied cations, the efficiency of these processes was significantly enhanced in the presence of fatty acids and correlated with the octanol-water partition coefficient of the cations. The ability of C4TPP-X cations to induce proton transport across the lipid membrane of liposomes loaded with a pH-sensitive fluorescent dye increased also with their lipophilicity and depended on the presence of palmitic acid in the liposome membrane. Of all the cations, only butyl[tri(3,5-dimethylphenyl)]phosphonium (C4TPP-diMe) was able to induce proton transport by the mechanism of formation of a cation-fatty acid ion pair on planar bilayer lipid membranes and liposomes. The rate of oxygen consumption by mitochondria in the presence of C4TPP-diMe increased to the maximum values corresponding to conventional uncouplers; for all other cations the maximum uncoupling rates were significantly lower. We assume that the studied cations of the C4TPP-X series, with the exception of C4TPP-diMe at low concentrations, cause nonspecific leak of ions through lipid model and biological membranes which is significantly enhanced in the presence of fatty acids.

Single-chain heteropolymers transport protons selectively and rapidly

Precise protein sequencing and folding are believed to generate the structure and chemical diversity of natural channels^1,2, both of which are essential to synthetically achieve proton transport performance comparable to that seen in natural systems. Geometrically defined channels have been fabricated using peptides, DNAs, carbon nanotubes, sequence-defined polymers and organic frameworks^3-13. However, none of these channels rivals the performance observed in their natural counterparts. Here we show that without forming an atomically structured channel, four-monomer-based random heteropolymers (RHPs)¹⁴ can mimic membrane proteins and exhibit selective proton transport across lipid bilayers at a rate similar to those of natural proton channels. Statistical control over the monomer distribution in an RHP leads to segmental heterogeneity in hydrophobicity, which facilitates the insertion of single RHPs into the lipid bilayers. It also results in bilayer-spanning segments containing polar monomers that promote the formation of hydrogen-bonded chains^15,16 for proton transport. Our study demonstrates the importance of the adaptability that is enabled by statistical similarity among RHP chains and of the modularity provided by the chemical diversity of monomers, to achieve uniform behaviour in heterogeneous systems. Our results also validate statistical randomness as an unexplored approach to realize protein-like behaviour at the single-polymer-chain level in a predictable manner.

Ultrafast proton transport in sub-1-nm diameter carbon nanotube porins

Proton transport plays an important role in many biological processes due to the ability of protons to rapidly translocate along chains of hydrogen-bonded water molecules. Molecular dynamics simulations have predicted that confinement in hydrophobic nanochannels should enhance the rate of proton transport. Here, we show that 0.8-nm-diameter carbon nanotube porins, which promote the formation of one-dimensional water wires, can support proton transport rates exceeding those of bulk water by an order of magnitude. The transport rates in these narrow nanotube pores also exceed those of biological channels and Nafion. With larger 1.5-nm-diameter nanotube porins, proton transport rates comparable to bulk water are observed. We also show that the proton conductance of these channels can be modulated by the presence of Ca(2+) ions. Our results illustrate the potential of small-diameter carbon nanotube porins as a proton conductor material and suggest that strong spatial confinement is a key factor in enabling efficient proton transport.

N-terminally glutamate-substituted analogue of gramicidin A as protonophore and selective mitochondrial uncoupler

Limited uncoupling of oxidative phosphorylation could be beneficial for cells by preventing excessive generation of reactive oxygen species. Typical uncouplers are weak organic acids capable of permeating across membranes with a narrow gap between efficacy and toxicity. Aimed at designing a nontoxic uncoupler, the protonatable amino acid residue Glu was substituted for Val at the N-terminus of the pentadecapeptide gramicidin A (gA). The modified peptide [Glu1]gA exhibited high uncoupling activity in isolated mitochondria, in particular, abolishing membrane potential at the inner mitochondrial membrane with the same or even larger efficacy as gA. With mitochondria in cell culture, the depolarizing activity of [Glu1]gA was observed at concentrations by an order of magnitude lower than those of gA. On the contrary, [Glu1]gA was much less potent in forming proton channels in planar lipid bilayers than gA. Remarkably, at uncoupling concentrations, [Glu1]gA did not alter cell morphology and was nontoxic in MTT test, in contrast to gA showing high toxicity. The difference in the behavior of [Glu1]gA and gA in natural and artificial membranes could be ascribed to increased capability of [Glu1]gA to permeate through membranes and/or redistribute between different membranes. Based on the protective role of mild uncoupling, [Glu1]gA and some other proton-conducting gA analogues may be considered as prototypes of prospective therapeutic agents.

Mechanism of response of potential-sensitive dyes studied by time-resolved fluorescence

The mechanism of response of two potential-sensitive dyes, diOC(2)(5) (3,3'-diethyloxadicarbocyanine iodide) and oxonol V (bis-[3-phenyl-5-oxoisoxazol-4-yl]pentamethine oxonol), were studied by using steady-state and time-resolved fluorescence techniques. The lipid concentration dependence of the Deltapsi (membrane potential)-induced change in total fluorescence intensity was quite different for these two dyes. Time-resolved fluorescence measurements showed that the fluorescence decay of these dyes in membranes could be resolved into at least three exponentials. Deltapsi-induced changes in the levels of these three populations were also measured under a variety of conditions. In the case of diOC(2)(5) an inside negative Deltapsi increased the levels of the bound forms. This shows that diOC(2)(5) responds to Deltapsi mainly by an "on-off" mechanism whereby Deltapsi perturbs the membrane-water partition coefficient of the dye. The Deltapsi-induced changes approached zero when the dye was totally membrane bound. In contrast, the Deltapsi-induced response of oxonol V increased with increased membrane binding. An inside negative Deltapsi decreased the level of the bound form with a longer lifetime. This shows that the mechanism of response of oxonol V is a Deltapsi-induced shift in the equilibrium between bound forms of the dye.

Lipid raft components cholesterol and sphingomyelin increase H+/OH- permeability of phosphatidylcholine membranes

H+/OH- permeation through lipid bilayers occurs at anomalously high rates and the determinants of proton flux through membranes are poorly understood. Since all life depends on proton gradients, it is important to develop a greater understanding of proton leak phenomena. We have used stopped-flow fluorimetry to probe the influence of two lipid raft components, chol (cholesterol) and SM (sphingomyelin), on H+/OH- and water permeability. Increasing the concentrations of both lipids in POPC (palmitoyl-2-oleoyl phosphatidylcholine) liposomes decreased water permeability in a concentration-dependent manner, an effect that correlated with increased lipid order. Surprisingly, proton flux was increased by increasing the concentration of chol and SM. The chol effect was complex with molar concentrations of 17.9, 33 and 45.7% giving 2.8-fold (P<0.01), 2.2-fold (P<0.001) and 5.1-fold (P<0.001) increases in H+/OH- permeability from a baseline of 2.4x10(-2) cm/s. SM at 10 mole% effected a 2.8-fold increase (P<0.01), whereas 20 and 30 mole% enhanced permeability by 3.6-fold (P<0.05) and 4.1-fold respectively (P<0.05). Supplementing membranes containing chol with SM did not enhance H+/OH- permeability. Of interest was the finding that chol addition to soya-bean lipids decreased H+/OH- permeability, consistent with an earlier report [Ira and Krishnamoorthy (2001) J. Phys. Chem. B 105, 1484-1488]. We speculate that the presence of proton carriers in crude lipid extracts might contribute to this result. We conclude that (i) chol and SM specifically and independently increase rates of proton permeation in POPC bilayers, (ii) domains enriched in these lipids or domain interfaces may represent regions with high H+/OH- conductivity, (iii) H+/OH- fluxes are not governed by lipid order and (iv) chol can inhibit or promote H+/OH- permeability depending on the total lipid environment. Theories of proton permeation are discussed in the light of these results.

Voltage-gated proton channels and other proton transfer pathways

Proton channels exist in a wide variety of membrane proteins where they transport protons rapidly and efficiently. Usually the proton pathway is formed mainly by water molecules present in the protein, but its function is regulated by titratable groups on critical amino acid residues in the pathway. All proton channels conduct protons by a hydrogen-bonded chain mechanism in which the proton hops from one water or titratable group to the next. Voltage-gated proton channels represent a specific subset of proton channels that have voltage- and time-dependent gating like other ion channels. However, they differ from most ion channels in their extraordinarily high selectivity, tiny conductance, strong temperature and deuterium isotope effects on conductance and gating kinetics, and insensitivity to block by steric occlusion. Gating of H(+) channels is regulated tightly by pH and voltage, ensuring that they open only when the electrochemical gradient is outward. Thus they function to extrude acid from cells. H(+) channels are expressed in many cells. During the respiratory burst in phagocytes, H(+) current compensates for electron extrusion by NADPH oxidase. Most evidence indicates that the H(+) channel is not part of the NADPH oxidase complex, but rather is a distinct and as yet unidentified molecule.

Rotational dynamics of surface probes in lipid vesicles

Translational and rotational diffusion of fluorescent molecules on the surface of small biological systems such as vesicles, proteins and micelles depolarize the fluorescence. A recent study has treated the case of the translational dynamics of surface probes (M.M.G. Krishna, R. Das, N. Periasamy and R. Nityananda, J. Chem. Phys., 112 (2000) 8502-8514) using Monte Carlo and theoretical methods. Here we extend the application of the methodologies to apply the case of rotational dynamics of surface probes. The corresponding fluorescence anisotropy decays were obtained using the Monte Carlo simulation methods for the two cases: surface probes undergoing rotational dynamics on a plane and on a sphere. The results were consistent with the theoretical equations which show that Monte Carlo methods can be used to simulate the surface diffusion problems. The anisotropy decay for the rotational diffusion of a molecule on a planar surface is single exponential and the residual anisotropy is zero. However, residual anisotropy is finite for the case of rotational diffusion on a sphere because of the spatial averaging of the anisotropy function. The rotational correlation time in both the cases is (4Drot)(-1) with Drot being the rotational diffusion coefficient. Rotational dynamics of a surface bound dye in a single giant liposome and in sonicated vesicles were studied and the results were explained according to the theoretical equations. A fast component of fluorescence depolarization was also observed for sonicated vesicles which was interpreted as wobbling-in-cylinder dynamics of the surface-bound dye.

Effect of phloretin on ionophore mediated electroneutral transmembrane translocations of H(+), K(+) and Na(+) in phospholipid vesicles

Rates of M(+)/H(+) exchange (M(+)=K(+), Na(+)) across phospholipid membranes by ionophore mediated electroneutral translocations and transports through channels could either increase or decrease or change negligibly on adding the polar molecule phloretin to the membrane. The changes depend on pH, the concentration and choice of M(+) and choice of ionophore/channel. Such diverse behaviours have been inferred from studies on the decay of the pH difference across soybean phospholipid vesicular membrane (=Delta pH). The transporters used in this study are (a) the exchange ionophores: nigericin, monensin; (b) combinations of alkali metal ion carriers, valinomycin or nonactin with weak acids carbonyl cyanide m-chlorophenylhydrazone or 2,4-dinitrophenol and (c) channels formed by gramicidin A. All the diverse results can be rationally explained if we take note of the following. (i) The rate limiting steps are associated with the transmembrane translocations involving the rate limiting species identified in the literature. (ii) Phloretin in the membrane decreases the apparent M(+) dissociation constant, K(M), of the M(+) bound ionophores/channels which has the effect of increasing the concentration of these species. (iii) The concentrations of H(+) bound ionophores/channels decrease on adding phloretin. (iv) Phloretin inhibits ternary complex formation (involving valinomycin or nonactin, M(+) and an anion) by forming 1:2 complexes with valinomycin-M(+) or nonactin-M(+). (v) On adding 6-ketocholestanol to the membrane (instead of phloretin) K(M) increases. The decreases/increases in K(M) mentioned above are consistent with the consequences of a hypothesis in which phloretin decreases and 6-ketocholestanol increases the positive internal membrane dipole potential.

Location and orientation of DODCI in lipid bilayer membranes: effects of lipid chain length and unsaturation

The location and orientation of a linear dye molecule, DODCI, in lipid bilayer membrane were determined by the effect of viscosity and refractive index of the aqueous medium on the fluorescence properties of the dye bound to the membrane. The membrane-bound dye is solubilized in two sites, one near the surface (short fluorescence lifetime) and another in the interior of the membrane (long lifetime). The ratio of the dye in the two locations and the orientation of the dye (parallel or perpendicular to the membrane) are sensitive to the lipid chain length and unsaturation in the alkyl chain. The fraction of the dye in the interior region is higher for short alkyl chains (C12>C14>C16>>C18C20) and in unsaturated lipids (C14:1>C14:0, C16:1>C16:0). These experimental results are consistent with the general principle that the penetration of an amphiphilic organic molecule in the interior region of the membrane is more when the structure of th bilayer is more fluid-like.

Probing the dynamics of planar supported membranes by Nile red fluorescence lifetime distribution

The structure and dynamics of planar supported membranes were studied by using the fluorescence probe Nile red. The width of fluorescence lifetime distribution of Nile red was used to infer the heterogeneity of membranes. The width of fluorescence lifetime was larger and the lifetime was shorter in supported membranes when compared to vesicle membranes. This was interpreted as due to the presence of water-filled membrane discontinuity leading to a heterogeneous surface in supported membranes. Microdomain causing agents such as cholesterol, sphingomyelin, etc. caused a larger level of heterogeneity in supported membranes when compared to vesicle membranes.

Evidence for dimer participation and evidence against channel mechanism in A23187-mediated monovalent metal ion transport across phospholipid vesicular membrane

The decay of the pH difference (DeltapH) across soybean phospholipid vesicular membrane by ionophore A23187 (CAL)-mediated H+/M+ exchange (M+ = Li+, Na+, K+, and Cs+) has been studied in the pH range 6-7.6. The DeltapH in these experiments were created by temperature jump. The observed dependence of DeltapH relaxation rate 1/tau on the concentration of CAL, pH, and the choice of M+ in vesicle solutions lead to the following conclusions. 1) The concentrations of dimers and other oligomers of A23187 in the membrane are small compared to the total concentration of A23187 in the membrane, similar to that in chloroform solutions reported in the literature. 2) In the H+ transport cycle leading to DeltapH decay, the A23187-mediated H+ translocation across the membrane is a fast step, and the rate-limiting step is the A23187-mediated M+ translocation. 3) Even though the monomeric Cal-H is the dominant species translocating H+, Cal-M is not the dominant species translocating M+ (even at concentrations higher than [Cal-H]), presumably because its dissociation rate is much higher than its translocation rate. 4) The pH dependence of 1/tau shows that the dimeric species Cal2LiLi, Cal2NaNa, Cal2KH, and Cal2CsH are the dominant species translocating M+. The rate constant associated with their translocation has been estimated to be approximately 5 x 10(3) s-1. With this magnitude for the rate constants, the dimer dissociation constants of these species in the membrane have been estimated to be approximately 4, 1, 0.05, and 0.04 M, respectively. 5) Contrary to the claims made in the literature, the data obtained in the DeltapH decay studies do not favor the channel mechanism for the ion transport in this system. 6) However, they support the hypothesis that the dissociation of the divalent metal ion-A23187 complex is the rate limiting step of A23187-mediated divalent metal ion transport.

Spectrally constrained global analysis of fluorescence decays in biomembrane systems

Dynamic and steady-state fluorescence spectroscopic properties of a dye probe measured in a multicomponent biological system are often required to be separated into the spectra and lifetimes of individual spectroscopically distinct species. The conventional method of obtaining decay-associated spectra fails when the lifetimes of the fluorophore in the membrane phase and in the aqueous phase are very close to each other. This paper describes a global analysis method which takes advantage of the known spectrum and lifetime of the dye in the aqueous phase. This method is used to identify the spectra for two fluorescent species (lifetimes, 0.84 and 1.97 ns) of the dye DODCI in EggPC vesicle membranes by keeping the spectrum and lifetime (0.68 ns) of the dye in the aqueous phase as fixed parameters. The structural identity of the two membrane-bound dye species was established by the effect of refractive index and/or viscosity of the aqueous medium on the lifetimes.

Cell type and spatial location dependence of cytoplasmic viscosity measured by time-resolved fluorescence microscopy

Information on the cell type and spatial location dependence of cytoplasmic viscosity would be very useful in understanding some of the processes occurring in the cell. For this purpose, fluorescent dye kiton red (sulforhodamine B) was loaded into a variety of cells such as Swiss 3T3 fibroblasts, human mononuclear cells, Sarcoma-180 tumor cells, Chinese hamster ovary cells, plant cells from Digitalis lanata, stamen hair cells of Tradescantia, and guard mother cells of Allium cepa. Space-resolved measurements of cytoplasmic viscosity were carried out by using an experimental set-up wherein a picosecond laser system was coupled with an epifluorescence microscope. The spatial resolution of this set-up was approximately 1.0 micron, and reliable dynamic fluorescence measurements could be obtained from 10(2) to 10(3) fluorescent molecules. Fluorescence lifetime measurements showed that a large fraction (approximately 70%) of kiton red was in the free form. Fluorescence anisotropy decay of kiton red in cells was analyzed by a two population (free and bound) model. The microviscosity of cytoplasm was estimated from the anisotropy decay kinetics of the free probe. It was found that the cytoplasmic viscosity is dependent on both the cell type and spatial location within a cell. Furthermore, both the average value of viscosity and spatial variation within a cell were larger in the plant cells when compared to the animal cells. Model studies in various simpler systems have shown that the higher viscosity observed in some part of the cell could be due to either physical restriction and/or the presence of high concentrations of small solutes and macromolecules.

Role of metal ion free valinomycin-carbonyl cyanide m-chlorophenylhydrazone complex in the enhancement of the rates of gramicidin facilitated net H+, Li+ and Na+ transport across phospholipid vesicular membrane

The studies on the decay of the pH difference, delta pH, across soyabean phospholipid vesicular membrane have shown that the rates of net proton transport and the associated Li+ and Na+ ion transport across the membrane can be enhanced by the combined action of gramicidin, valinomycin and carbonyl cyanide m-chlorophenylhydrazone (CCCP) in K(+)-free vesicle solutions. The data obtained under different experimental conditions suggest that this enhancement is a consequence of facilitation of CCCP- transport (1) by complexing CCCP- with the highly membrane permeant valinomycin without the metal ion bound to it and (2) by the associated Li+ or Na+ transport through the gramicidin channel such that no net charge is transported across the membrane. The dissociation constant of the weak valinomycin-CCCP- complex has been estimated to be > 200 mM in the membrane. The delta pH in these experiments were created by temperature jump.

Two mechanisms of H+/OH- transport across phospholipid vesicular membrane facilitated by gramicidin A

Two rate-limiting mechanisms have been proposed to explain the gramicidin channel facilitated decay of the pH difference across vesicular membrane (delta pH) in the pH region 6-8 and salt (MCI, M+ = K+, Na+) concentration range 50-300 mM. 1) At low pH conditions (approximately 6), H+ transport through the gramicidin channel predominantly limits the delta pH decay rate. 2) At higher pH conditions (approximately 7.5), transport of a deprotonated species (but not through the channel) predominantly limits the rate. The second mechanism has been suggested to be the hydroxyl ion propogation through water chains across the bilayer by hydrogen bond exchange. In both mechanisms alkali metal ion transport providing the compensating flux takes place through the gramicidin channels. Such an identification has been made from a detailed study of the delta pH decay rate as a function of 1) gramicidin concentration, 2) alkali metal ion concentration, 3) pH, 4) temperature, and 5) changes in the membrane order (by adding small amounts of chloroform to vesicle solutions). The apparent activation energy associated with the second mechanism (approximately 3.2 kcal/mol) is smaller than that associated with the first mechanism (approximately 12 kcal/mol). In these experiments, delta pH was created by temperature jump, and vesicles were prepared using soybean phospholipid or a mixture of 94% egg phosphatidylcholine and 6% phosphatidic acid.

H+, K+, and Na+ transport across phospholipid vesicular membrane by the combined action of proton uncoupler 2,4-dinitrophenol and valinomycin

The decay of the pH difference (delta pH) across soyabean phospholipid vesicular membrane (created by temperature jump), by the combined action of valinomycin and 2,4-dinitrophenol (DNP) has been monitored with the help of fluorescence from pyranine entrapped inside the vesicles under a variety of concentration conditions. The results suggest the following for the pH region of our interest (pH approximately 6 to pH approximately 8): (i) The rate limiting step in the proton transport cycle is not the transport of proton as DNPH, but the back transport of DNP- and the alkali metal ion M+ as Val-M(+)-DNP- across the membrane. The rate constant associated with the transport of the ternary complex has been estimated to be approximately 1.5 x 10(3) s-1. (ii) The dissociation constant of the ternary complex Val-M(+)-DNP- in the membrane are approximately 1 mM for M+ = K+ and approximately 0.001 mM for M+ = Na+. (iii) The reduction in the cation selectivity of valinomycin on complexing with DNP- is much more than that observed with the anionic form of carbonyl cyanide m-chlorophenylhydrazone (CCCP). The results also provide a verification of a corollary of Mitchell's hypothesis: an experimental strategy which enhances the delta pH decay rate should also be a strategy for the efficient uncoupling of oxidative and photophosphorylation.

The voltage-activated hydrogen ion conductance in rat alveolar epithelial cells is determined by the pH gradient

Voltage-activated H+ currents were studied in rat alveolar epithelial cells using tight-seal whole-cell voltage clamp recording and highly buffered, EGTA-containing solutions. Under these conditions, the tail current reversal potential, Vrev, was close to the Nernst potential, EH, varying 52 mV/U pH over four delta pH units (delta pH = pHo - pHi). This result indicates that H+ channels are extremely selective, PH/PTMA > 10(7), and that both internal and external pH, pHi, and pHo, were well controlled. The H+ current amplitude was practically constant at any fixed delta pH, in spite of up to 100-fold symmetrical changes in H+ concentration. Thus, the rate-limiting step in H+ permeation is pH independent, must be localized to the channel (entry, permeation, or exit), and is not bulk diffusion limitation. The instantaneous current-voltage relationship exhibited distinct outward rectification at symmetrical pH, suggesting asymmetry in the permeation pathway. Sigmoid activation kinetics and biexponential decay of tail currents near threshold potentials indicate that H+ channels pass through at least two closed states before opening. The steady state H+ conductance, gH, as well as activation and deactivation kinetic parameters were all shifted along the voltage axis by approximately 40 mV/U pH by changes in pHi or pHo, with the exception of the fast component of tail currents which was shifted less if at all. The threshold potential at which H+ currents were detectably activated can be described empirically as approximately 20-40(pHo-pHi) mV. If internal and external protons regulate the voltage dependence of gH gating at separate sites, then they must be equally effective. A simpler interpretation is that gating is controlled by the pH gradient, delta pH. We propose a simple general model to account for the observed delta pH dependence. Protonation at an externally accessible site stabilizes the closed channel conformation. Deprotonation of this site permits a conformational change resulting in the appearance of a protonation site, possibly the same one, which is accessible via the internal solution. Protonation of the internal site stabilizes the open conformation of the channel. In summary, within the physiological range of pH, the voltage dependence of H+ channel gating depends on delta pH and not on the absolute pH.

Enhancement of rates of H+, Na+ and K+ transport across phospholipid vesicular membrane by the combined action of carbonyl cyanide m-chlorophenylhydrazone and valinomycin: temperature-jump studies

Enhancement of delta pH relaxation rate by the combined action of valinomycin (VAL) and carbonyl cyanide m-chlorophenylhydrazone (CCCP) has been studied under a variety of concentration conditions in soyabean phospholipid (SBPL) vesicles after creating a pH gradient across the vesicular membrane delta pH by temperature jump. After taking note of the changes by VAL and CCCP induced membrane disorder (using nigericin and monensin mediated delta pH decay as probes) the following could be inferred about the mechanism of enhancement of delta pH decay rate: (i) in solutions containing KCl, the rate limiting species have been identified to be (a) Val-K(+)-CCCP-, at low [Val]0 and [CCCP]0 (with translocation rate constant k2 approximatley 3.2 x 10(3) s-1); (b) CCCPH, at high [Val]0 (with translocation rate constant k1 approximately 2 x 10(5) s-1); (c) the neutral valinomycin species Val, at high [CCCP]0. (ii) In solutions containing NaCl, in our concentration range, the rate limiting species are Val-Na(+)-CCCP-. (iii) The apparent dissociation constant K*M of Val-M+ decreases with pH in SBPL vesicles but is independent of pH in vesicles prepared from PC + 6% PA. (iv) The differences in the ionic strength dependencies of kinetic data shows that the environments of Na+ and K+ binding sites on VAL are different. (v) In vesicle solutions containing 100 mM MCl, the cation selectivity of VAL (towards K+ in preference to Na+) is reduced when CCCP- is already bound to it in the membrane. The CCCP- dissociation constant of Val-M(+)-CCCP- is smaller with M+ = Na+ (approximatley 0.22 mM at 100 mM NaCl) when compared to that with M+ = K+ (approximately 2 mM at 100 mM KCl). Attributing these differences to the differences in electrostatic interaction between CCCP- and M+ in Val-M(+)-CCCP-, we can say that CCCP- binds closer to the Na+ binding site than to the K+ binding site on VAL.

Anomalous response of oxonol-V to membrane potential in mitochondrial proton pumps

The response of the fluorescent membrane potential probe oxonol-V (bis[3-phenyl 5 oxoisoxazol-4-yl]pentamethine oxonol) in submitochondrial particles (SMP) was dependent upon whether the potential (inside positive) was generated by active proton pumps or by valinomycin-aided passive K+ influx. The fluorescence intensity showed a decrease in the former case and an increase in the latter situation. This anomalous behavior was not observed with other similar anionic probes. Gradual inhibition of proton pumping activity showed that the difference in the response of oxonol-V is not due to possible difference in the magnitude of membrane potential generated in these two situations. In the presence of membrane permeant anions such as TPB- (tetraphenyl boron) or chlorate, the direction of response of oxonol-V fluorescence was the same in both situations. Time-resolved fluorescence of the oxonol-V-SMP system showed three populations: one free form (fluorescence lifetime approximately 60 ps) and two SMP-bound forms (lifetimes of 0.45 ns and 1.4 ns). A fourth population was created during the action of proton pumps. The shorter lifetime (approximately 250 ps) of this new bound form suggest this population to be an aggregated form. This population was absent during the action of proton pumps in the presence of TPB- or chlorate. These results suggest the creation of a charge separated state during the action of proton pumps. The decrease in fluorescence intensity could be the result of aggregation of oxonol-V around the positive end of a proton pump existing in a dipole or charge separated state.

Sodium transport by an ionizable and a neutral mobile carrier: effects of membrane structure on the apparent activation energy

Temperature-jump relaxation experiments on Na+ transport by (221)C10-cryptand (ionizable mobile carrier) and nonactin (neutral mobile carrier) were carried out in order to study the effects of cholesterol and the degree of acyl chain unsaturation, and their temperature-dependence on ion transport through thin lipid membranes. The experiments were performed on large, negatively charged unilamellar vesicles (LUV) prepared from mixtures of phosphatidylcholine (egg phosphatidylcholine, dioleoylphosphatidylcholine and dilinoleolylphosphatidylcholine), phosphatidic acid and cholesterol (mole fractions 0-0.43), at various temperatures and carrier concentrations. The apparent rate constants of Na+ translocation by (221)C10 and nonactin increased with the carrier concentration, the degree of acyl chain unsaturation and the temperature. The incorporation of cholesterol into the membranes significantly reduced the carrier concentration-, acyl chain unsaturation- and temperature-dependence of this parameter. The apparent energy required to activate the transport decreased significantly with increasing (221)C10 concentrations and remained constant with increasing those of nonactin at any given cholesterol molar fraction and degree of acyl chain unsaturation. It increased significantly with increasing the cholesterol molar fraction at any given carrier concentration to an extent depending on the degree of acyl chain unsaturation. Our interpretation of the action of cholesterol on these transport systems is based on the assumption that the adsorption plane of Na(+)-(221)C10 and Na(+)-nonactin complexes is likely to be located towards the aqueous and the hydrocarbon side of the dipole layer, respectively. The results are discussed in terms of the structural, physico-chemical and electrical characteristics of carriers and complexes, and of the interactions occurring between an ionizable or a neutral mobile carrier and the membrane.

Temperature-dependent effects of cholesterol on sodium transport through lipid membranes by an ionizable mobile carrier

Temperature-jump relaxation experiments on Na+ transport by (221)C10-cryptand were carried out in order to study the influence of cholesterol and its temperature-dependence on ion transport through thin lipid membranes. The experiments were performed on large, negatively charged unilamellar vesicles (LUV) prepared from mixtures of dioleoylphosphatidylcholine, phosphatidic acid and cholesterol (mole fractions 0-0.43), at various temperatures and carrier concentrations. The initial rates of Na+ transport and the apparent rate constants of its translocation by (221)C10 increased with the carrier concentration and the temperature. The incorporation of cholesterol into the membranes significantly reduced the carrier concentration- and temperature-dependence of these two parameters. The apparent energy required to activate the transport decreased significantly with increasing carrier concentrations at any given cholesterol molar fraction, and increased significantly with the cholesterol molar fraction at any given carrier concentration. Our interpretation of the action of cholesterol on this transport system is based on the assumption that the binding cavity of cryptands is likely to be located towards the aqueous side of the dipole layer. The results are discussed in terms of the structural, physico-chemical and electrical characteristics of carriers and complexes, and of the interactions occurring between an ionizable mobile carrier and the membrane.

Monensin-mediated transports of H+, Na+, K+ and Li+ ions across vesicular membranes: T-jump studies

Theoretical expression for the rate of decay of delta pH across vesicular membrane due to carrier-mediated ion transports, 1/tau, has been modified taking note of carrier states (such as mon- and mon-H-M+) for which the translocation rate constants in the membrane are small. The rates of delta pH decay due to monensin-mediated H+ and M+ transports (M+ = Na+, K+, Li+) observed in our experiments in the pH range 6-8, and [M+] range 50-250 mM at 25 degrees C have been analysed with the help of this expression. delta pH across soybean phospholipid vesicular membranes were created by temperature jump in our experiments. The following could be inferred from our studies. (a) At low pH (approximately 6) 1/tau in a medium of Na+ is greater than that in a medium of K+. In contrast with this, at higher pH (approximately 7.5) 1/tau is greater in a medium of K+. Such contradictory observations could be understood with the help of our equation and the parameters determined in this work. The relative concentrations of the rate-limiting species (mon-H, mon-K, and mon-Li at Ph approximately 7 in vesicle solutions having Na+, K+ and Li+, respectively) can explain such behaviours. (b) The proton dissociation constant KH for mon-H in the lipid medium (pKH approximately 6.55) is larger than the reported KH in methanol. (c) The concentrations of mon- and mon-H-Na+ are not negligible under the conditions of our experiments. The latter species cause a [Na+]-dependent inhibition of ion transports. (d) The relative magnitudes of metal ion dissociation constants KHM (approximately 0.05 M) for mon-H-Na+ and KM (approximately 0.03 M) for mon-Na suggest that the carboxyl group involved in the protonation may not be dominantly involved in the metal ion complexation. (e) The estimates of KM (approximately 0.03 M for Na+, 0.5 M for K+ and 2.2 M for Li+) follow the ionophore selectivity order. (f) The rate constants k1 and k2 for the translocations of mon-H and mon-M (M+ = Na+, K+ and Li+) are similar in magnitude (approximately 9 x 10(3) s-1) and are higher than that for nig-H and nig-M (approximately 6 x 10(3) s-1) which can be expected from the relative molecular sizes of the ion carriers.

Nigericin-mediated H+, K+ and Na+ transports across vesicular membrane: T-jump studies

The decay of delta pH across vesicular membranes by nigericin-mediated H+ and metal ion (M+) transports has been studied at 25 degrees C after creating delta pH by temperature jump (T-jump). In these experiments K+ or Na+ were chosen as M+ for the compensating flux. Theoretical expressions derived to analyse these data suggest a method for estimating the intrinsic rate constants for the translocation of nig-H (k1) and for the translocation of nig-M (k2) across membrane, from the pH dependence of the delta pH decay. The following could be inferred from the analysis of data. (a) At pH approximately 7.5 and 250 mM ion concentrations, nigericin-mediated H+ and M+ transport rates are lower in a medium of K+ than in a medium of Na+, although ionophore selectivity of nigericin towards K+ is 25-45-times higher than that towards Na+. However, at lower [M+] (approximately 50 mM) the transport rates are higher in a medium of K+ than in a medium of Na+. Such behaviours can be understood with the help of parameters determined in this work. (b) The intrinsic rate constants k1 and k2 associated with the translocations of nig-H and nig-K or nig-Na across membrane are similar in magnitude. (c) At pH approximately 7.5 translocation of nig-H is the dominant rate-limiting step in a medium containing K+. In contrast with this, at this pH, translocation of nig-M is the dominant rate-limiting step when metal ion is Na+. (d)k1 approximately k2 approximately 6.10(3) s-1 could be estimated at 25 degrees C in vesicles prepared from soyabean phospholipid, and lipid mixtures of 80% phosphatidylcholine (PC) + 20% phosphatidylethanolamine and 92% PC + 8% phosphatidic acid. (e) The apparent dissociation constants of nig-M in vesicles were estimated to be approximately 1.5.10(-3) M for K+ and 6.4.10(-2) M for Na+ (at 50 mM ion concentrations) using approximately 10(-8.45) M for the apparent dissociation constant of nig-H.

Temperature-jump method for studying the fast transport of Na+ by (221) C10-cryptand across lipid membranes

The kinetics of Na+ transport by (221)C10-cryptand through thin lipid membranes were determined by performing temperature-jump relaxation experiments on large unilamellar vesicles (L.U.V.) loaded with a fluorescent pH indicator. Applying temperature jumps of 4 to 7 degrees C to liposomes having phosphate as internal buffer and Tris as external buffer resulted in transmembrane delta pH's of about 0.104 to 0.182. After a temperature-jump, a decay in the delta pH was observed which corresponded to a Na+/H+ exchange occurring through membranes in the simultaneous presence of the cryptand and a proton carrier. The transport of Na+ ions by (221)C10 was found to be a fast kinetic process. Its initial rate increased with both the temperature and the cryptand concentrations. In addition, the temperature-induced changes in the apparent rate constants of the translocation of Na+ by (221)C10 were carrier concentration-dependent, and the apparent activation energy required to activate the transport decreased significantly with increasing cryptand concentrations. The results are discussed in terms of the structural, physico-chemical and electrical characteristics of carriers and complexes.

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).

Enhancement of transmembrane proton conductivity of protonophores by membrane-permeant cations

The rate of protonophore-mediated decay of pH gradient across lipid vesicular membranes was found to be enhanced by orders of magnitude by valinomycin-K+. Experiments in the presence of gramicidin have shown that the observed rate enhancement by valinomycin-K+ is not due to collapse of the diffusion potential alone. The enhancement of the rate showed hyperbolic dependence on the concentration of valinomycin. Rate enhancement was observed in the presence of the membrane permeant cation tetraphenylphosphonium (TTP+) also. Several factors which might enhance the intrinsic H+ conductivity of protonophores were analyzed. The level of partitioning of the protonophore into the membrane and the pK of membrane-bound protonophores were measured. Valinomycin-K+ did not alter both these parameters significantly. TPP+ increased the partitioning of protonophores and decreased the pK values of membrane-bound protonophores. However, these changes were too small to explain the observed rate enhancements. We suggest that valinomycin-K+ and TPP+ enhance the H+ conductivity of protonophores by increasing the permeability of the ionized form of protonophores by forming an ion pair.

Mutual inactivation of valinomycin and protonophores by complex formation in liposomal membranes

The stimulation presence of a protonophore [3,5-di(ter-butyl)-4-hydroxybenzylidenemalononitrile or carbonyl cyanide m-chlorophenylhydrazone] and valinomycin in a liposome suspension results in time-dependent inactivation of ion transport by both the protonophore and valinomycin. Correlation of the inactivation with spectrophotometric observations on the formation of a complex between the protonophore and valinomycin strongly suggests that the complex observed has no (or very low) activity for the transport of either H+ or K+. The stoichiometry of valinomycin and the protonophore in the inactive complex is shown to be 1:1.

Proton conductance through phospholipid bilayers: water wires or weak acids?

The proton/hydroxide (H+/OH-) permeability of phospholipid bilayer membranes at neutral pH is at least five orders of magnitude higher than the alkali or halide ion permeability, but the mechanism(s) of H+/OH- transport are unknown. This review describes the characteristics of H+/OH- permeability and conductance through several types of planar phospholipid bilayer membranes. At pH 7, the H+/OH- conductances (GH/OH) range from 2-6 nS cm-2, corresponding to net H+/OH- permeabilities of (0.4-1.7) X 10(-5) cm sec-1. Inhibitors of GH/OH include serum albumin, phloretin, glycerol, and low pH. Enhancers of GH/OH include chlorodecane, fatty acids, gramicidin, and voltages greater than 80 mV. Water permeability and GH/OH are not correlated. The characteristics of GH/OH in fatty acid (weak acid) containing membranes are qualitatively similar to the controls in at least eight different respects. The characteristics of GH/OH in gramicidin (water wire) containing membranes are qualitatively different from the controls in at least four different respects. Thus, the simplest explanation for the data is that GH/OH in unmodified bilayers is due primarily to weakly acidic contaminants which act as proton carriers at physiological pH. However, at low pH or in the presence of inhibitors, a residual GH/OH remains which may be due to water wires, "hydrated defects," or other mechanisms.