Transport of protons and hydrochloric acid through lipid bilayer membranes

Transport of protons and hydrochloric acid through lipid bilayer membranes was studied by a combination of electrical conductance and pH electrode techniques. In the presence of large pH gradients, proton transport occurs primarily by diffusion of molecular HCl. The permeability of egg phosphatidylcholine/decane bilayers to HCl is about 3 cm . s-1, seven to nine order of magnitude higher than the permeability to H+, OH- or Cl-. The HCl permeability of phosphatidylserine or egg phosphatidylcholine/cholesterol (1 : 1) bilayers is about 50% lower than the permeability of egg phosphatidylcholine bilayers. Diffusion of molecular HCl may be an important process in tissues exposed to high HCl concentrations, e.g., gastric mucosa. However, at neutral pH the diffusion of molecular HCl is too slow to contribute significantly to net movements of H+ or Cl-.     

Permeability and electrical properties of planar lipid membranes from thylakoid lipids.

Electrical measurements were carried out on planar lipid membranes from thylakoid lipids. The specific capacitance of membranes formed from decane-containing monogalactosyldiacylglycerol (MGDG), which accounts for 57% of the total lipid content of thylakoids, showed that it adopted a bilayer structure. Solvent-free bilayers of MGDG were not formed, with very rare exceptions, indicating that decane is required to stabilize the planar conformation. However, this cone-shaped lipid produces bilayer structures in combination with other cylindrical thylakoid lipids even in the absence of organic solvent. We compared the properties of solvent-free and decane-containing bilayers from MGDG, soybean lecithin, and the quaternary mixture of lipids similar to that found in vivo. The conductance of decane-MGDG was 26 times higher than that of decane-lecithin. The flux through the decane-lecithin bilayer was found to be slightly dependent on pH, whereas the decane-MGDG membrane was not. The specific conductance of bilayers formed from the quaternary mixture of lipids was 5 to 10 times larger than lecithin (with alkane or not). Further experiments with bilayers made in the presence of a KCl gradient showed that decane-MGDG, decane-MGDG/DGDG/SQDG/PG, and solvent-free MGDG/DGDG/SQDG/PG were cation-selective. The permeability coefficient for potassium ranged from 4.9 to 8.3 x 10(-11) cm s-1. The permeability coefficient for protons in galactolipids, however, was determined to be about six orders of magnitude higher than the value for potassium ions. The HCl permeation mechanism through the lipid membranes was determined from diffusion potentials measured in HCl gradients. Our results suggest that HCl was not transported as neutral molecules. The data is discussed with regard to the function of galactolipids in the ion transport through thylakoid membranes.     

Translocation of phospholipids and dithionite permeability in liquid-ordered and liquid-disordered membranes

We present a detailed study of the translocation rate of two headgroup-labeled phospholipid derivatives, one with two acyl chains, NBD-DMPE, and the other with a single acyl chain, NBD-lysoMPE, in lipid bilayer membranes in the liquid-disordered state (POPC) and in the liquid-ordered states (POPC/cholesterol (Chol), molar ratio 1:1, and sphingomyelin (SpM)/Chol, molar ratio 6:4). The study was performed as a function of temperature and the thermodynamic parameters of the translocation process have been obtained. The most important findings are 1), the translocation of NBD-DMPE is significantly faster than the translocation of NBD-lysoMPE for all bilayer compositions and temperatures tested; and 2), for both phospholipid derivatives, the translocation in POPC bilayers is approximately 1 order of magnitude faster than in POPC/Chol (1:1) bilayers and approximately 2-3 orders of magnitude faster than in SpM/Chol (6:4) bilayers. The permeability of the lipid bilayers to dithionite has also been measured. In liquid disordered membranes, the permeability rate constant obtained is comparable to the translocation rate constant of NBD-DMPE. However, in liquid-ordered bilayers, the permeability of dithionite is significantly faster then the translocation of NBD-DMPE. The change in enthalpy and entropy associated with the formation of the activated state in the translocation and permeation processes has also been obtained.     

Conductance routes for protons across membrane barriers

Simple phospholipid bilayers show a high level of permeability to protons; in spite of this fact, large proton gradients existing across such bilayers may decay very slowly. In sealed systems, the free movement of protons across a membrane barrier is severely restricted by the coincident development of a proton diffusion potential. Using the fluorescent weak acid N-[5-(dimethylamino)naphth-1-ylsulfonyl]glycine strongly buffered systems movement of the small number of protons giving rise to this electrical potential is insufficient to perturb the proton concentration gradient; significant flux of protons (and hence significant collapse of the concentration gradient) can only occur if protons traverse the membrane as part of an electroneutral complex or if there is a balancing flow of appropriate counterions. In both instances, proton flux is obligatorily coupled to the translocation of species other than protons. In weakly buffered systems, the small initial uncoupled electrogenic flux of protons may significantly alter the concentration gradient. This initial rapid gradient collapse caused by uncoupled electrogenic proton movements is then superimposed upon the residual collapse attributable to tightly coupled proton flux. The initial uncoupled electrogenic proton flux shows a temperature dependence very similar to that demonstrated for water permeation across simple lipid bilayers; upon cooling, there is a sharp decrease in flux at the temperature coinciding with the main gel-liquid-crystalline phase transition of the lipid. The coupled proton flux shows a markedly different temperature dependence with no dramatic change in rate at the phase transition temperature and strong similarity to the behavior previously seen with solutes known to be permeating as electrically neutral compounds.(ABSTRACT TRUNCATED AT 250 WORDS)     

Mechanism of proton permeation through chloroplast lipid membranes.

Electrical measurements were carried out to investigate the contribution of chloroplast lipids to the passive proton permeability of both the thylakoid and inner-envelope membranes. Permeability coefficient and conductance to protons were measured for solvent-free bilayers made from monogalactosyldiglyceride:digalactosyldiglycerid: sulfoquinovosyldiglyceride:phosphatidylglycerol (2:1:0.5:0.5, w/w) in the presence of a pH gradient of 7.4/8.1. The permeability coefficient for protons in glycolipids was 5.5 +/- 1.1 x 10(-4) cm s-1 (n = 14). To determine whether this high H+ permeability could be explained by the presence of lipid contaminants such as weak acids, we investigated the effects of (a) bovine serum albumin, which can remove some amphiphilic molecules such as free fatty acids, (b) 6-ketocholestanol, which increases the membrane dipole potential, (c) oleic acid, and (d) chlorodecane, which increases the dielectric constant of the lipid bilayer. Our results show that free fatty acids are inefficient protonophores, as compared with carbonylcyanide-m-chlorphenythydrazone, and that the hypothesis of a weak acid mechanism is not valid with glycolipid bilayers. In the presence of deuterium oxide the H+ conductane was reduced significantly, indicating that proton transport through the glycolipid matrix could occur directly by a hydrogen bond process. The passive transport of H+ through the glycolipid matrix is discussed with regard to the activity of the thylakoid ATP synthase and the inner-envelope H(+)-ATPase.     

Proton permeation of lipid bilayers

Proton permeation of the lipid bilayer barrier has two unique features. First, permeability coefficients measured at neutral pH ranges are six to seven orders of magnitude greater than expected from knowledge of other monovalent cations. Second, proton conductance across planar lipid bilayers varies at most by a factor of 10 when pH is varied from near 1 to near 11. Two mechanisms have been proposed to account for this anomalous behavior: proton conductance related to contaminants of lipid bilayers, and proton translocation along transient hydrogen-bonded chains (tHBC) of associated water molecules in the membrane. The weight of evidence suggests that trace contaminants may contribute to proton conductance across planar lipid membranes at certain pH ranges, but cannot account for the anomalous proton flux in liposome systems.Two new results will be reported here which were designed to test the tHBC model. These include measurements of relative proton/potassium permeability in the gramicidin channel, and plots of proton flux against the magnitude of pH gradients. (1) The relative permeabilities of protons and potassium through the gramicidin channel, which contains a single strand of hydrogenbonded water molecules, were found to differ by at least four orders of magnitude when measured at neutral pH ranges. This result demonstrates that a hydrogen-bonded chain of water molecules can provide substantial discrimination between protons and other cations. It was also possible to calculate that if approximately 7% of bilayer water was present in a transient configuration similar to that of the gramicidin channel, it could account for the measured proton flux. (2) The plot of proton conductance against pH gradient across liposome membranes was superlinear, a result that is consistent with one of three alternative tHBC models for proton conductance described by Nagle elsewhere in this volume.     

The effect of general anesthetics on the proton and potassium permeabilities of liposomes

The pump-leak hypothesis of general anesthesia proposes that anesthetics act by increasing the functional proton permeability of membranes, particularly those of synaptic vesicles. Since transmembrane proton gradients are required for neurotransmitter accumulation, decay of such gradients by an uncompensated anesthetic-induced leak would result in loss of neurotransmitter from the vesicles, followed by synaptic block and anesthesia. We have tested this hypothesis by determining the effect of four different general anesthetics on the relative permeabilities of liposome membranes to protons and potassium ions. In all cases, physiologically relevant levels of anesthetics caused a 200 to 500 percent increment in ionic permeability. There was no marked preference for protons, suggesting that the anesthetics did not induce a leak specific for this ionic species. Instead the anesthetics appeared to produce a more general defect available to both protons and potassium ions which resulted in a functional increment in proton permeability. These observations were compared with available data on proton transport rates by synaptic vesicle ATPase enzymes. The magnitude of the anesthetic-induced leak could not be compensated by the ATPase, which is only capable of a 40 percent increase in rate when uncoupled. We consider these results to be consistent with the pump-leak hypothesis.     

Plant sterol inhibition of abscisic acid-induced perturbations in phospholipid bilayers

Abscisic acid (ABA)-induced phospholipid bilayer perturbations (permeability and lipid vesicle aggregation) are shown to be reversed by incorporation of a commercially available mixture of plant sterols (60% beta-sitosterol, 27% campesterol and 13% dihydrobrassicasterol) into the membranes. As little and 5 membrane mol% plant sterol inhibits ABA-stimulated permeability of both saturated and unsaturated mixed phosphatidylcholine/phosphatidylethanolamine bilayers to the fluorescent anion carboxyfluorescein by more than 50%. The same conclusion was reached by an osmotic swelling technique for the uncharged permeant solute erythritol. Hormone-induced carboxyfluorescein permeability to mixed acyl chain phosphatidylcholine bilayers was similarly inhibited by the sterols, but only if the membranes were tested at a temperature where liquid crystal and gel states coexist. The plant sterols were also shown to prevent the ABA-induced fusion of mixed phosphatidylcholine/phosphatidylethanolamine bilayers. The ABA effect on membranes is inhibited equally by plant sterols as well as cholesterol. From these experiments a possible role is suggested for plant sterols in controlling the mode of action of ABA.     

The effect of the polar moiety of lipids on bilayer conductance induced by uncouplers of oxidative phosphorylation

Summary Bilayer membranes were formed from decane, cholesterol, and three lipids isolated fromStaphylococcus aureus: positively charged lysyl phosphatidylglycerol (LysPG), negatively charged phosphatidylglycerol (PG), and neutral diglucosyldiglyceride (DiGluDiGly). The uncouplers of oxidative phosphorylation, 2,4-dinitrophenol (DNP) and 3-t-butyl,5-chloro,2-chloro,4-nitrosalicylanilide (S 13), increased the electrical conductance of all three differently charged bilayers. S 13 was found to be the most effective reagent of the known uncouplers in increasing conductance of the bilayers. The conductance induced by uncouplers was investigated as a function of pH and uncoupler concentration. The pH of maximum conductance for each uncoupling agent was dependent on both the uncoupler and the lipid; it was lower for each uncoupler in LysPG and higher in PG compared to DiGluDiGly bilayers. At a pH below the optimum for LysPG, the conductance of the positively charged membrane was 500 times and of the neutral one 10 times higher than that of the negatively charged bilayer at equal uncoupler concentration and pH. Above the pH optimum for DiGluDiGly, the conductance was approximately equal for the positive and neutral membranes, but was lower in PG bilayers. Conductance depended linearly on uncoupler concentration. The bilayer conductance induced by S 13 was entirely due to increased proton permeability in all three lipids. The findings are consistent with the role of uncouplers as carriers for protons across the hydrocarbon interior of lipid membranes. The differences in conductance of differently charged lipid bilayers at equal uncoupler concentration, as well as the change of pH optimum of conductance with lipid charge, can be explained in terms of an electrostatic energy contribution of the fixed lipid charges to the distribution of the uncoupler anion between the aqueous and the membrane phases.     

Attenuation of proton currents by methanol in a dioxolane-linked gramicidin A channel in different lipid bilayers.

The mobility of protons in a dioxolane-linked gramicidin A channel (D1) is comparable to the mobility of protons in aqueous solutions (Cukierman, S., E. P. Quigley, and D. S. Crumrine. 1997. Biophys. J. 73:2489-2502). Aliphatic alcohols decrease the mobility of H+ in aqueous solutions. In this study, the effects of methanol on proton conduction through D1 channels were investigated in different lipid bilayers and at different HCl concentrations. Methanol attenuated H+ currents in a voltage-independent manner. Attenuation of proton currents was also independent of H+ concentrations in solution. In phospholipid bilayers, methanol decreased the single channel conductance to protons without affecting the binding affinity of protons to bilayers. In glycerylmonooleate membranes, the attenuation of single channel proton conductances qualitatively resembled the decrease of conductivities of HCl solutions by methanol. However, in both types of lipid bilayers, single channel proton conductances through D1 channels were considerably more attenuated than the conductivities of different HCl solutions. This suggests that methanol modulates single proton currents through D1 channels. It is proposed that, on average, one methanol molecule binds to a D1 channel, and attenuates H+ conductance. The Gibbs free energy of this process (DeltaG0) is approximately 1.2 kcal/mol, which is comparable to the free energy of decrease of HCl conductivity in methanol solutions (1.6 kcal/mol). Apolar substances like urea and glucose that do not transport protons in HCl solutions and do not permeate D1 channels decreased solution conductivity and single channel conductance by a considerably larger proportion than methanol. Cs+ currents through D1 channels were considerably less (fivefold) attenuated by methanol than proton currents. It is proposed that methanol partitions inside the pore of gramicidin channels and delays the transfer of protons between water and methanol molecules, causing a significant attenuation of the single channel proton conductance. Gramicidin channels offer an interesting experimental model to study proton hopping along a single chain of water molecules interrupted by a single methanol molecule.     

Proton permeability of liposomes from natural phospholipid mixtures

A method to determine the proton permeability coefficient of phospholipid membrane with the fluorescent probe pyranine is described. Very high proton permeability coefficients of liposomes from natural extracts are measured with great accuracy. The proton permeability appears to be linearly related to the fluidity of the bilayers. This relation as well as the comparison of the activation energies of proton permeability and fluidity support the hypothesis (Nichols J.W. and Deamer D.W. (1980) Proc. Natl. Acad. Sci. USA 77, 2038–2042) of a transfer process along a network of hydrogen bonded water molecules. It is suggested that the common lipid characteristics of biological membranes (net surface charge and unsaturation) favor a high proton permeability.     

Interactions of the Australian tree frog antimicrobial peptides aurein 1.2, citropin 1.1 and maculatin 1.1 with lipid model membranes: differential scanning calorimetric and Fourier transform infrared spectroscopic studies

The interactions of the antimicrobial peptides aurein 1.2, citropin 1.1 and maculatin 1.1 with dimyristoylphosphatidylcholine (DMPC), dimyristoylphosphatidylglycerol (DMPG) and dimyristoylphosphatidylethanolamine (DMPE) were studied by differential scanning calorimetry (DSC) and Fourier-transform infrared (FTIR) spectroscopy. The effects of these peptides on the thermotropic phase behavior of DMPC and DMPG are qualitatively similar and manifested by the suppression of the pretransition, and by peptide concentration-dependent decreases in the temperature, cooperativity and enthalpy of the gel/liquid-crystalline phase transition. However, at all peptide concentrations, anionic DMPG bilayers are more strongly perturbed than zwitterionic DMPC bilayers, consistent with membrane surface charge being an important aspect of the interactions of these peptides with phospholipids. However, at all peptide concentrations, the perturbation of the thermotropic phase behavior of zwitterionic DMPE bilayers is weak and discernable only when samples are exposed to high temperatures. FTIR spectroscopy indicates that these peptides are unstructured in aqueous solution and that they fold into alpha-helices when incorporated into lipid membranes. All three peptides undergo rapid and extensive H-D exchange when incorporated into D(2)O-hydrated phospholipid bilayers, suggesting that they are located in solvent-accessible environments, most probably in the polar/apolar interfacial regions of phospholipid bilayers. The perturbation of model lipid membranes by these peptides decreases in magnitude in the order maculatin 1.1>aurein 1.2>citropin 1.1, whereas the capacity to inhibit Acholeplasma laidlawii B growth decreases in the order maculatin 1.1>aurein 1.2 congruent with citropin 1.1. The higher efficacy of maculatin 1.1 in disrupting model and biological membranes can be rationalized by its larger size and higher net charge. However, despite its smaller size and lower net charge, aurein 1.2 is more disruptive of model lipid membranes than citropin 1.1 and exhibits comparable antimicrobial activity, probably because aurein 1.2 has a higher propensity for partitioning into phospholipid membranes.     

Effect of ethylene oxide and propylene oxide block copolymers on the permeability of bilayer lipid membranes to small solutes including doxorubicin

The effects of ethylene oxide and propylene oxide block copolymers (pluronics) on the permeability of several weak acids and bases through bilayer lipid membranes have been studied by the methods of monitoring (1) pH shifts near planar bilayers, (2) doxorubicin fluorescence quenching inside liposomes, and (3) current transients in the presence of hydrophobic anions. It has been shown that pluronics facilitate the permeation of comparatively large molecules (such as 2-n-undecylmalonic acid and doxorubicin) across lipid bilayers, while the permeation of small solutes (such as ammonium and acetic acid) remains unaffected. Pluronics also accelerate the translocation of large hydrophobic anions (tetraphenylborate). The effect of pluronics correlates with the content of propylene oxide units: it is enhanced when the portion of polypropylene oxide block in the copolymer is increased. The action of the pluronic on lipid membrane permeability differs from the effect of the conventional detergent Triton X-100, which does not affect doxorubicin transport if added at concentrations similar to those used for pluronics. It has been proposed that pluronics accelerate the processes of solute diffusion within lipid bilayers (in a structure-dependent manner) rather than influencing the rate of solute adsorption/desorption on the membrane surface. We suppose that the effect of pluronics on doxorubicin permeation across lipid bilayers along with the known effect on the multidrug resistance protein determines its influence on the therapeutic activity of anthracycline drugs.     

Lipids, curvature stress, and the action of lipid prodrugs: free fatty acids and lysolipid enhancement of drug transport across liposomal membranes

Molecular shape and its impact on bilayer curvature stress are powerful concepts for describing the effects of lipids and fatty acids on fundamental membrane properties, such as passive permeability and derived properties like drug transport across liposomal membranes. We illustrate these relationships by studying the effects of fatty acids and lysolipids on the permeation of a potent anti-cancer drug, doxorubicin, across the bilayer of a liposome in which the drug is encapsulated. Using a simple fluorescence assay, we have systematically studied the passive permeation of doxorubicin across liposomal membranes in different lipid phases: the solid-ordered phase (DPPC bilayers), the liquid-disordered phase (POPC lipid bilayers), and the liquid-ordered phase induced by high levels of cholesterol (DOPC + cholesterol lipid bilayers). The effect of different free fatty acids (FA) and lysolipids (LL), separately and in combination, on permeability was assessed to elucidate the possible mechanism of phospholipase A₂-triggered release in cancer tissue of liposomal doxorubicin formulations. In all cases, FAs applied separately lead to significant enhancement of permeability, most pronounced in liquid-disordered bilayers and less pronounced in solid and solid-ordered bilayers. LLs applied separately had only a marginal effect on permeability. FA and LL applied in combination lead to a synergistic enhancement of permeability in solid bilayers, whereas in liquid-disordered bilayers, the combined effect suppressed the otherwise strong permeability enhancement due to the FAs.     

Comparative study of molecular dynamics,diffusion, and permeability for ligands in biomembranes of different lipid composition

A comparative study of several model lipid bilayers of different composition, which included analysis of kinetic parameters of model lipid bilayers and permeability of bilayer membranes for small molecules, has been carried out. The conformity of results of numeric experiments to experimental data (structure of membrane lipid bilayers, lateral diffusion coefficients, and relative permeability of biomembranes for ligands) is discussed in the framework of a standard molecular dynamics protocol.     

Weak acid permeability through lipid bilayer membranes. Role of chemical reactions in the unstirred layer

The premeabilities of planar lipid bilayer (egg phosphatidylcholine- decane) membranes to butyric and formic acids were measured by tracer and pH electrode techniques. The purposes of the study were (a) to establish criteria for the applicability of each method and (b) to resolve a discrepancy between previously published permeabilities determined using the different techniques. Tracer fluxes of butyric acid were measured at several concentrations and pH's. Under symmetrical conditions the one-way flux of butyric acid(J) is described by 1/J = 1/Pul ([HA] + [A-]) + 1/Pm([HA]), where Pul and Pm are the unstirred layer and membrane permeability coefficients. Pm determined in this manner is 950 x 10(4) cm s-1. Published values for the butyric acid permeability for egg phosphatidylcholine-decane bilayers are 11.5 x 10(-4) (Wolosin and Ginsburg, 1975) and 640 x 10(-4) cm s-1 (Orbach and Finkelstein, 1980). Wolosin and Ginsburg measured net fluxes from a solution of pH = Pka into an unbuffered solution containing a pH electrode. Orbach and Finkelstein measured tracers fluxes under symmetrical conditions at pH 7.4. We reproduced the results of Wolosin and Ginsburg and showed that their apparently low Pm was caused by unstirred layer effects in their poorly buffered solutions. The permeability to formic acid (pKa = 3.75) measured by both tracer and pH electrode techniques was approximately 10(-2) cm s-1. However, if pm greater than Pul, the pH electrode technique cannot be used for measuring the permeabilities of weak acids with pKa's greater than approximately 4.     

Water permeability of asymmetric planar lipid bilayers: leaflets of different composition offer independent and additive resistances to permeation

To understand how plasma membranes may limit water flux, we have modeled the apical membrane of MDCK type 1 cells. Previous experiments demonstrated that liposomes designed to mimic the inner and outer leaflet of this membrane exhibited 18-fold lower water permeation for outer leaflet lipids than inner leaflet lipids (Hill, W.G., and M.L. Zeidel. 2000. J. Biol. Chem. 275:30176-30185), confirming that the outer leaflet is the primary barrier to permeation. If leaflets in a bilayer resist permeation independently, the following equation estimates single leaflet permeabilities: 1/P(AB) = 1/P(A) + 1/P(B) (Eq. l), where P(AB) is the permeability of a bilayer composed of leaflets A and B, P(A) is the permeability of leaflet A, and P(B) is the permeability of leaflet B. Using for the MDCK leaflet-specific liposomes gives an estimated value for the osmotic water permeability (P(f)) of 4.6 x 10(-4) cm/s (at 25 degrees C) that correlated well with experimentally measured values in intact cells. We have now constructed both symmetric and asymmetric planar lipid bilayers that model the MDCK apical membrane. Water permeability across these bilayers was monitored in the immediate membrane vicinity using a Na+-sensitive scanning microelectrode and an osmotic gradient induced by addition of urea. The near-membrane concentration distribution of solute was used to calculate the velocity of water flow (Pohl, P., S.M. Saparov, and Y.N. Antonenko. 1997. Biophys. J. 72:1711-1718). At 36 degrees C, P(f) was 3.44 +/- 0.35 x 10(-3) cm/s for symmetrical inner leaflet membranes and 3.40 +/- 0.34 x 10(-4) cm/s for symmetrical exofacial membranes. From, the estimated permeability of an asymmetric membrane is 6.2 x 10(-4) cm/s. Water permeability measured for the asymmetric planar bilayer was 6.7 +/- 0.7 x 10(-4) cm/s, which is within 10% of the calculated value. Direct experimental measurement of P(f) for an asymmetric planar membrane confirms that leaflets in a bilayer offer independent and additive resistances to water permeation and validates the use of.     

Lysolipid incorporation in dipalmitoylphosphatidylcholine bilayer membranes enhances the ion permeability and drug release rates at the membrane phase transition

The enhanced permeability of lipid bilayer membranes at their gel-to-liquid phase transition has been explained using a "bilayer lipid heterogeneity" model, postulating leaky interfacial regions between still solid and melting liquid phases. The addition of lysolipid to dipalmitoylphosphatidylcholine bilayers dramatically enhances the amount of, and speed at which, encapsulated markers or drugs are released at this, already leaky, phase transition through these interfacial regions. To characterize and attempt to determine the mechanism behind lysolipid-generated permeability enhancement, dithionite permeability and doxorubicin release were measured for lysolipid and non-lysolipid, containing membranes. Rapid release of contents from lysolipid-containing membranes appears to occur through lysolipid-stabilized pores rather than a simple enhancement due to increased drug solubility in the bilayer. A dramatic enhancement in the permeability rate constant begins about two degrees below the calorimetric peak of the thermal transition, and extends several degrees past it. The maximum permeability rate constant coincides exactly with this calorimetric peak. Although some lysolipid desorption from liquid state membranes cannot be dismissed, dialyzation above T(m) and mass spectrometry analysis indicate lysolipid must, and can, remain in the membrane for the permeability enhancement, presumably as lysolipid stabilized pores in the grain boundary regions of the partially melted solid phase.     

Chemiluminescence of Acanthamoeba castellanii.

We have investigated ferrocytochrome c-induced proton ejection from reconstituted cytochrome c oxidase-containing vesicles using careful control of the number of enzyme turnovers. Ferrocytochrome c caused the appearance of protons at the vesicle exterior, and this could be abolished by using a protonophore. In addition, its decay was dependent on the permeability of the vesicle membranes to protons and the number of turnovers of the oxidase. These observations indicate that the ejection of protons was the result of genuine translocation. The possibility of this translocation occurring via a Mitchellian loop as a result of the presence of a reduced hydrogen carrier contaminating the enzyme was considered and excluded. Proton-translocating activity in this reconstituted system depended critically on the ratio of enzyme to lipid used in the reconstitution process and we propose a rationale to account for this. We conclude that our data provide strong support for the proposal that cytochrome c oxidase acts as a proton pump and that approx. 0.9 H+ is excluded per ferrocytochrome c molecule oxidized.     

Interactions of the Australian tree frog antimicrobial peptides aurein 1.2, citropin 1.1 and maculatin 1.1 with lipid model membranes: Differential scanning calorimetric and Fourier transform infrared spectroscopic studies

The interactions of the antimicrobial peptides aurein 1.2, citropin 1.1 and maculatin 1.1 with dimyristoylphosphatidylcholine (DMPC), dimyristoylphosphatidylglycerol (DMPG) and dimyristoylphosphatidylethanolamine (DMPE) were studied by differential scanning calorimetry (DSC) and Fourier-transform infrared (FTIR) spectroscopy. The effects of these peptides on the thermotropic phase behavior of DMPC and DMPG are qualitatively similar and manifested by the suppression of the pretransition, and by peptide concentration-dependent decreases in the temperature, cooperativity and enthalpy of the gel/liquid-crystalline phase transition. However, at all peptide concentrations, anionic DMPG bilayers are more strongly perturbed than zwitterionic DMPC bilayers, consistent with membrane surface charge being an important aspect of the interactions of these peptides with phospholipids. However, at all peptide concentrations, the perturbation of the thermotropic phase behavior of zwitterionic DMPE bilayers is weak and discernable only when samples are exposed to high temperatures. FTIR spectroscopy indicates that these peptides are unstructured in aqueous solution and that they fold into α-helices when incorporated into lipid membranes. All three peptides undergo rapid and extensive H-D exchange when incorporated into D₂O-hydrated phospholipid bilayers, suggesting that they are located in solvent-accessible environments, most probably in the polar/apolar interfacial regions of phospholipid bilayers. The perturbation of model lipid membranes by these peptides decreases in magnitude in the order maculatin 1.1 > aurein 1.2 > citropin 1.1, whereas the capacity to inhibit Acholeplasma laidlawii B growth decreases in the order maculatin 1.1 > aurein 1.2 ≅ citropin 1.1. The higher efficacy of maculatin 1.1 in disrupting model and biological membranes can be rationalized by its larger size and higher net charge. However, despite its smaller size and lower net charge, aurein 1.2 is more disruptive of model lipid membranes than citropin 1.1 and exhibits comparable antimicrobial activity, probably because aurein 1.2 has a higher propensity for partitioning into phospholipid membranes.