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

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.

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.

Increase of local hydrogen ion gradient near bilayer lipid membrane under the conditions of catalysis of proton transfer across the interface

The measurements of pH profiles in the unstirred layers (USLs) near planar bilayer lipid membranes (BLM) were applied for the evaluation of the hydrogen ion fluxes which were induced by nigericin in the presence of potassium ion gradients. It was shown that at high concentrations of KCl the increase in the concentration of citrate buffer caused an anomalous effect, namely, an increase in the local pH shifts in the USLs. The hydrogen ion flux rose 50 times upon the increase in the citrate concentration from 1 mM to 20 mM. Phosphate stimulated the flux 7 times under these conditions. In agreement with our previous results, at low KCl concentrations, when the process is limited by the K+-nigericin interaction, an increase in the buffer concentration led to a reduction of the local pH shifts, under these conditions the usual concentration dependence was observed. The data obtained favor the model implying the existence of the kinetic barrier for proton transfer at the membrane-water interface.

Proton conduction in gramicidin A and in its dioxolane-linked dimer in different lipid bilayers

Gramicidin A (gA) molecules were covalently linked with a dioxolane ring. Dioxolane-linked gA dimers formed ion channels, selective for monovalent cations, in planar lipid bilayers. The main goal of this study was to compare the functional single ion channel properties of natural gA and its covalently linked dimer in two different lipid bilayers and HCl concentrations (10-8000 mM). Two ion channels with different gating and conductance properties were identified in bilayers from the product of dimerization reaction. The most commonly observed and most stable gramicidin A dimer is the main object of this study. This gramicidin dimer remained in the open state most of the time, with brief closing flickers (tau(closed) approximately 30 micros). The frequency of closing flickers increased with transmembrane potential, making the mean open time moderately voltage dependent (tau(open) changed approximately 1.43-fold/100 mV). Such gating behavior is markedly different from what is seen in natural gA channels. In PEPC (phosphatidylethanolamine-phosphatidylcholine) bilayers, single-channel current-voltage relationships had an ohmic behavior at low voltages, and a marked sublinearity at relatively higher voltages. This behavior contrasts with what was previously described in GMO (glycerylmonooleate) bilayers. In PEPC bilayers, the linear conductance of single-channel proton currents at different proton concentrations was essentially the same for both natural and gA dimers. g(max) and K(D), obtained from fitting experimental points to a Langmuir adsorption isotherm, were approximately 1500 pS and 300 mM, respectively, for both the natural gA and its dimer. In GMO bilayers, however, proton affinities of gA and the dioxolane-dimer were significantly lower (K(D) of approximately 1 and 1.5 M, respectively), and the g(max) higher (approximately 1750 and 2150 pS, respectively) than in PEPC bilayers. Furthermore, the relationship between single-channel conductance and proton concentration was linear at low bulk concentrations of H+ (0.01-2 M) and saturated at concentrations of more than 3 M. It is concluded that 1) The mobility of protons in gramicidin A channels in different lipid bilayers is remarkably similar to proton mobilities in aqueous solutions. In particular, at high concentrations of HCl, proton mobilities in gramicidin A channel and in solution differ by only 25%. 2) Differences between proton conductances in gramicidin A channels in GMO and PEPC cannot be explained by surface charge effects on PEPC membranes. It is proposed that protonated phospholipids adjacent to the mouth of the pore act as an additional source of protons for conduction through gA channels in relation to GMO bilayers. 3) Some experimental results cannot be reconciled with simple alterations in access resistance to proton flow in gA channels. Said differences could be explained if the structure and/or dynamics of water molecules inside gramicidin A channels is modulated by the lipid environment and by modifications in the structure of gA channels. 4) The dioxolane ring is probably responsible for the closing flickers seen in the dimer channel. However, other factors can also influence closing flickers.