Membrane fusion and the lamellar-to-inverted-hexagonal phase transition in cardiolipin vesicle systems induced by divalent cations

The polymorphic phase behavior of bovine heart cardiolipin (CL) in the presence of different divalent cations and the kinetics of CL vesicle fusion induced by these cations have been investigated. (31)P-NMR measurements of equilibrium cation-CL complexes showed the lamellar-to-hexagonal (L(alpha)-H(II)) transition temperature (T(H)) to be 20-25 degrees C for the Sr(2+) and Ba(2+) complexes, whereas in the presence of Ca(2+) or Mg(2+) the T(H) was below 0 degrees C. In the presence of Sr(2+) or Ba(2+), CL large unilamellar vesicles (LUVs) (0.1 microm diameter) showed kinetics of destabilization, as assessed by determination of the release of an aqueous fluorescent dye, which strongly correlated with the L(alpha)-H(II) transition of the final complex: at temperatures above the T(H), fast and extensive leakage, mediated by vesicle-vesicle contact, was observed. On the other hand, mixing of vesicle contents was limited and of a highly transient nature. A different behavior was observed with Ca(2+) or Mg(2+): in the temperature range of 0-50 degrees C, where the H(II) configuration is the thermodynamically favored phase, relatively nonleaky fusion of the vesicles occurred. Furthermore, with increasing temperature the rate and extent of leakage decreased, with a concomitant increase in fusion. Fluorescence measurements, involving incorporation of N-NBD-phosphatidylethanolamine in the vesicle bilayer, demonstrated a relative delay in the L(alpha)-H(II) phase transition of the CL vesicle system in the presence of Ca(2+). Freeze-fracture electron microscopy of CL LUV interaction products revealed the exclusive formation of H(II) tubes in the case of Sr(2+), whereas with Ca(2+) large fused vesicles next to H(II) tubes were seen. The extent of binding of Ca(2+) to CL in the lamellar phase, saturating at a binding ratio of 0.35 Ca(2+) per CL, was close to that observed for Sr(2+) and Ba(2+). It is concluded that CL LUVs in the presence of Ca(2+) undergo a transition that favors nonleaky fusion of the vesicles over rapid collapse into H(II) structures, despite the fact that the equilibrium Ca(2+)-CL complex is in the H(II) phase. On the other hand, in the presence of Sr(2+) or Ba(2+) at temperatures above the T(H) of the respective cation-CL complexes, CL LUVs rapidly convert to H(II) structures with a concomitant loss of vesicular integrity. This suggests that the nature of the final cation-lipid complex does not primarily determine whether CL vesicles exposed to the cation will initially undergo a nonleaky fusion event or collapse into nonvesicular structures.

Low pH induces an interdigitated gel to bilayer gel phase transition in dihexadecylphosphatidylcholine membrane

We have investigated the influence of pH on the structures and phase behaviors of multilamellar vesicles of the ether-linked dihexadecylphosphatidylcholine (DHPC-MLV). This phospholipid is known to be in the interdigitated gel (L(beta)I) phase in excess water at 20 degrees C at neutral pH. The results of X-ray diffraction experiments indicate that a phase transition from L(beta)I phase to the bilayer gel phase occurred in DHPC-MLV in 0.5 M KCl around pH 3.9 with a decrease in pH, and that at low pH values, less than pH 2.2, DHPC-MLVs were in L(beta') phase. The results of fluorescence and light scattering method indicate that the gel to liquid-crystalline phase transition temperature (T(m)) of DHPC-MLV increased with a decrease in pH. On the basis of a thermodynamic analysis, we conclude that the main mechanism of the low-pH induced L(beta)I to bilayer gel phase transition in DHPC-MLV and the increase in its T(m) is connected with the decrease in the repulsive interaction between the headgroups of these phospholipids. As pH decreases, the phosphate groups of the headgroups begin to be protonated, and as a result, the apparent positive surface charges appear. However, surface dipoles decrease and the interaction free energy of the hydrophilic segments with water increases. The latter effect dominates the pure electrostatic repulsion between the charged headgroups, and thereby, the total repulsive interaction in the interface decreases.

Lipid composition determines the effects of arbutin on the stability of membranes

Arbutin (hydroquinone-beta-D-glucopyranoside) is an abundant solute in the leaves of many freezing- or desiccation-tolerant plants. Its physiological role in plants, however, is not known. Here we show that arbutin protects isolated spinach (Spinacia oleracea L.) thylakoid membranes from freeze-thaw damage. During freezing of liposomes, the presence of only 20 mM arbutin led to complete leakage of a soluble marker from egg PC (EPC) liposomes. When the nonbilayer-forming chloroplast lipid monogalactosyldiacylglycerol (MGDG) was included in the membranes, this leakage was prevented. Inclusion of more than 15% MGDG into the membranes led to a strong destabilization of liposomes during freezing. Under these conditions arbutin became a cryoprotectant, as only 5 mM arbutin reduced leakage from 75% to 20%. The nonbilayer lipid egg phosphatidylethanolamine (EPE) had an effect similar to that of MGDG, but was much less effective, even at concentrations up to 80% in EPC membranes. Arbutin-induced leakage during freezing was accompanied by massive bilayer fusion in EPC and EPC/EPE membranes. Twenty percent MGDG in EPC bilayers completely inhibited the fusogenic effect of arbutin. The membrane surface probes merocyanine 540 and 2-(6-(7-nitrobenz-2-oxa-1, 3-diazol-4-yl)amino)hexanoyl-1-hexadecanoyl-sn-glycero-3-phosph ocholi ne (NBD-C(6)-HPC) revealed that arbutin reduced the ability of both probes to partition into the membranes. Steady-state anisotropy measurements with probes that localize at different positions in the membranes showed that headgroup mobility was increased in the presence of arbutin, whereas the mobility of the fatty acyl chains close to the glycerol backbone was reduced. This reduction, however, was not seen in membranes containing 20% MGDG. The effect of arbutin on lipid order was limited to the interfacial region of the membranes and was not evident in the hydrophobic core region. From these data we were able to derive a physical model of the perturbing or nonperturbing interactions of arbutin with lipid bilayers.

Short-chain alcohols promote an early stage of membrane hemifusion

Hemifusion, the linkage of contacting lipid monolayers of two membranes before the opening of a fusion pore, is hypothesized to proceed through the formation of a stalk intermediate, a local and strongly bent connection between membranes. When the monolayers' propensity to bend does not support the stalk (e.g., as it is when lysophosphatidylcholine is added), hemifusion is inhibited. In contrast, short-chain alcohols, reported to affect monolayer bending in a manner similar to that of lysophosphatidylcholine, were here found to promote hemifusion between fluorescently labeled liposomes and planar lipid bilayers. Single hemifusion events were detected by fluorescence microscopy. Methanol or ethanol (1.2-1.6 w/w %) added to the same compartment of the planar bilayer chamber as liposomes caused a 5-50 times increase in the number of hemifusion events. Alcohol-induced hemifusion was inhibited by lysophosphatidylcholine. Promotion of membrane hemifusion by short-chain alcohol was also observed for cell-cell fusion mediated by influenza virus hemagglutinin (HA). Alcohol promoted a fusion stage subsequent to the low pH-dependent activation of HA. We propose that binding of short-chain alcohol to the surface of membranes promotes hemifusion by facilitating the transient breakage of the continuity of each of the contacting monolayers, which is required for their subsequent merger in the stalk intermediate.

Direct evidence that the N-terminal heptad repeat of Sendai virus fusion protein participates in membrane fusion

Recent studies have demonstrated the importance of heptad repeat regions within envelope proteins of viruses in mediating conformational changes at various stages of viral infection. However, it is not clear if heptad repeats have a direct role in the actual fusion event. Here we have synthesized, fluorescently labeled and functionally and structurally characterized a wild-type 70 residue peptide (SV-117) composed of both the fusion peptide and the N-terminal heptad repeat of Sendai virus fusion protein, two of its mutants, as well as the fusion peptide and heptad repeat separately. One mutation was introduced in the fusion peptide (G119K) and another in the heptad repeat region (I154K). Similar mutations have been shown to drastically reduce the fusogenic ability of the homologous fusion protein of Newcastle disease virus. We found that only SV-117 was active in inducing lipid mixing of egg phosphatidylcholine/phosphatidyiglycerol (PC/PG) large unilamellar vesicles (LUV), and not the mutants nor the mixture of the fusion peptide and the heptad repeat. Functional characterization revealed that SV-117, and to a lesser extent its two mutants, were potent inhibitors of Sendai virus-mediated hemolysis of red blood cells, while the fusion peptide and SV-150 were negligibly active alone or in a mixture. Hemagglutinin assays revealed that none of the peptides disturb the binding of virions to red blood cells. Further studies revealed that SV-117 and its mutants oligomerize similarly in solution and in membrane, and have similar potency in inducing vesicle aggregation. Circular dichroism and FTIR spectroscopy revealed a higher helical content for SV-117 compared to its mutants in 40 % tifluorethanol and in PC/PG multibilayer membranes, respectively, ATR-FTIR studies indicated that SV-117 lies more parallel with the surface of the membrane than its mutants. These observations suggest a direct role for the N-terminal heptad repeat in assisting the fusion peptide in mediating membrane fusion.

Interaction of chlorpromazine and imipramine with model membranes

The aim of the present study is to examine the effect of phospholipids on the lipid destabilization induced by chlorpromazine (CPZ) and imipramine (IP) at different levels of pH. The large unilamellar vesicles (LUV) are formed in the presence of calcein (60 mM). The vesicles containing calcein have been incubated in the presence of CPZ and IP at pH 4.5 and pH 8. At pH 4.5 CPZ and IP induce a rapid release of the calcein encapsulated in the liposomes. Calcein release, at equal concentrations of pharmacological agent, is more important by CPZ than by IP. At pH 8, the calcein release was more important than at pH 4.5; this effect appears to be more significant for the CPZ than for the IP. In conclusion, the insertion of chlorpromazine and imipramine into large unilamellar vesicles is accompanied by a strong destabilization of the vesicles. These effects appear more significant for chlorpromazine than for imipramine.

Pore-forming action of mastoparan peptides on liposomes: a quantitative analysis

We have investigated the wasp venom peptides mastoparan X and polistes mastoparan regarding their apparent potential to induce pore-like defects in phosphatidylcholine unilamellar vesicles. Based on a fundamental theoretical model, the pore activation and deactivation kinetics have been evaluated from the observed efflux of liposome entrapped carboxyfluorescein in relation to the bound peptide to lipid ratio. We can quantitatively describe our experimental data very well in terms of a specific reaction scheme resulting in only a few short-lived pores. They evidently emerge rapidly from a prepore nucleus being produced by two rate-limiting monomeric states of bound peptide. These peculiar states would be favorably populated in an early stage of bilayer perturbation, but tend to die out in the course of a peptide/lipid restabilization process.

Spectroscopic detection of endovesiculation by large unilamellar phosphatidylcholine vesicles: effects of chlorpromazine, dibucaine, and safingol

Endovesiculation by large unilamellar vesicles (LUVs) induced by cationic amphiphiles is described in this work. A recent procedure to monitor phagocytosis of vesicles by macrophages by determining the amount of the simultaneously internalized water_soluble fluorescent dye HPTS with external quencher was adapted to LUVs (Daleke, D. L.; Hong, K.; Papahadjopoulos, D. Biochim. Biophys. Acta 1990, 1024, 352). Compared to dibucaine and safingol, the local anesthetic chlorpromazine (CPZ) was found to be the most efficient inducer of HPTS-internalization by LUVs. Control experiments using LUVs with entrapped HPTS indicated that the observed dye-internalization does not originate from transient lysis. A strong increase in activity above the critical micelle concentration of CPZ implies the importance of CPZ-micelles for endovesiculation. The significantly less efficient CPZ-induced HPTS-internalization by LUVs with 68 nm compared to 176 nm diameter further diminishes the likelihood of a micelle/bilayer fusion mechanism and supports the presence of 'zipper-type' endovesiculation by LUVs with diameters as small as 68 nm.

Effect of gramicidin A on the dipole potential of phospholipid membranes

The effect of channel-forming peptide gramicidin A on the dipole potential of phospholipid monolayers and bilayers has been studied. Surface pressure and surface potential isotherms of monolayers have been measured with a Langmuir trough equipped with a Wilhelmy balance and a surface potential meter (Kelvin probe). Gramicidin has been shown to shift pressure-area isotherms of phospholipids and to reduce their monolayer surface potentials. Both effects increase with the increase in gramicidin concentration and depend on the kind of phosphatidylcholine used. Application of the dual-wavelength ratiometric fluorescence method using the potential-sensitive dye RH421 has revealed that the addition of gramicidin A to dipalmitoylphosphatidylcholine liposomes leads to a decrease in the fluorescence ratio of RH421. This is similar to the effect of phloretin, which is known to decrease the dipole potential. The comparison of the concentration dependences of the fluorescence ratio for gramicidin and phloretin shows that gramicidin is as potent as phloretin in modifying the membrane dipole potential.

A mechanism for tamoxifen-mediated inhibition of acidification

Tamoxifen has been reported to inhibit acidification of cytoplasmic organelles in mammalian cells. Here, the mechanism of this inhibition is investigated using in vitro assays on isolated organelles and liposomes. Tamoxifen inhibited ATP-dependent acidification in organelles from a variety of sources, including isolated microsomes from mammalian cells, vacuoles from Saccharomyces cerevisiae, and inverted membrane vesicles from Escherichia coli. Tamoxifen increased the ATPase activity of the vacuolar proton ATPase but decreased the membrane potential (Vm) generated by this proton pump, suggesting that tamoxifen may act by increasing proton permeability. In liposomes, tamoxifen increased the rate of pH dissipation. Studies comparing the effect of tamoxifen on pH gradients using different salt conditions and with other known ionophores suggest that tamoxifen affects transmembrane pH through two independent mechanisms. First, as a lipophilic weak base, it partitions into acidic vesicles, resulting in rapid neutralization. Second, it mediates coupled, electroneutral transport of proton or hydroxide with chloride. An understanding of the biochemical mechanism(s) for the effects of tamoxifen that are independent of the estrogen receptor could contribute to predicting side effects of tamoxifen and in designing screens to select for estrogen-receptor antagonists without these side effects.

Dopamine and DOPA cause release of iron from ferritin and lipid peroxidation of liposomes

We investigated dopamine (DA)- and DOPA-related release of iron from ferritin, and lipid peroxidation of liposomes induced by the released iron. Iron release increased with increasing DA or DOPA concentrations. Effects of SOD and an oxygen-reduced environment indicated that superoxide was partly responsible for iron release. The released iron induced lipid peroxidation at relatively low concentrations of DA or DOPA, while at high concentrations, peroxidation was inhibited. These findings indicate that the risk of lipid peroxidation depends on the DA/iron or DOPA/iron ratio even if the iron concentration is low. Our findings suggest that DA-containing neurons are always at risk of oxidative damage. Furthermore, DOPA therapy may modify the nigral degeneration by reducing or accelerating ferritin iron-dependent lipid peroxidation.

Linoleic acid hydroperoxide favours hypochlorite- and myeloperoxidase-induced lipid peroxidation

Liposomes composed of soybean phosphatidylcholine were peroxidized using the reagent sodium hypochlorite or the myeloperoxidase-hydrogen peroxide-Cl- system. Linoleic acid hydroperoxide previously prepared from linoleic acid by means of lipoxidase was incorporated into liposomes. The yield of thiobarbituric acid reactive substances (TBARS) continuously increased with higher amounts of hydroperoxide groups after the initiation of lipid peroxidation by hypochlorous acid producing systems. The accumulation of TBARS was inhibited by scavengers of free radicals such as butylated hydroxytoluene and by the scavengers of hypochlorous acid, taurine and methionine. Lipid peroxidation was also prevented by sodium azide or chloride free medium in the myeloperoxidase-hydrogen peroxide-Cl- system. Here we show for the first time that the reaction of hypochlorous acid with a biologically relevant hydroperoxide yields free radicals able to cause further oxidation of lipid molecules.

Antioxidant activity of retinol, glutathione, and taurine in bovine photoreceptor cell membranes

The antioxidant activities of compounds endogenous to mammalian rod outer segments (ROS) were investigated in vitro by measuring the oxidative loss of polyunsaturated fatty acids (PUFA's) from the membranes of intact ROS and from liposomes made from ROS phospholipids (PL) to which lipid soluble compounds had been added. The membranes were exposed to the water-soluble oxidant 2, 2'-azobis(2-amidinopropane) dihydrochloride (AAPH). Retinol protected PUFA's in ROS liposome PL's, whereas retinaldehyde promoted lipid peroxidation. When isolated ROS were stimulated to produce endogenous retinol, PUFA loss was inhibited by up to 17%. These findings suggest an antioxidant function for the enzymatic reduction of retinaldehyde to retinol during the visual cycle. Water-soluble antioxidants, taurine and reduced glutathione (GSH), were investigated individually and in combination with retinol in ROS PL liposomes. GSH protected PUFA's in ROS PL liposomes. Taurine alone showed little antioxidant activity, but in combination with retinol it protected lipids twice as much as retinol alone. These results support previous findings that taurine protects ROS lipids during exposure to cyclic light.

Immobilized artificial membrane (IAM)-HPLC for partition studies of neutral and ionized acids and bases in comparison with the liposomal partition system

PURPOSE

To study the partitioning of model acids ((RS)-warfarin and salicylic acid), and bases (lidocaine, (RS)-propranolol and diazepam), with immobilized artificial membrane (IAM)-HPLC, as compared to partitioning in the standardized phosphatidylcholine liposome/buffer system.

METHODS

The pH-dependent apparent partition coefficients D were calculated from capacity factors (k'IAM) obtained by IAM-HPLC, using a 11-carboxylundecylphosphocholine column. For lipophilic compounds k'IAM, values were determined with organic modifiers and extrapolation to 100% water phase (k'IAMw) was optimized. Temperature dependence was explored (23 to 45 degrees C), and Gibbs free energy (deltaG), partial molar enthalpy (deltaH) and change in entropy (deltaS) were calculated. Equilibrium dialysis was used for the partitioning studies with the liposome/buffer system.

RESULTS

For extrapolation of k'IAMw, linear plots were obtained both with the respective dielectric constants and the mole fractions of the organic modifier. All tested compounds showed a similar pH-D diagram in both systems; however, significant differences were reproducibly found in the pH range of 5 to 8. In all cases, deltaG and deltaH were negative, whereas deltaS values were negative for acids and positive for bases.

CONCLUSIONS

In both partitioning systems, D values decreased significantly with the change from the neutral to the charged ionization state of the solute. The differences found under physiological conditions, i.e. around pH 7.4, were attributed to nonspecific interactions of the drug with the silica surface of the IAM column.

The membranotropic activity of cyclic acyldepsipeptides from bacterium Bacillus pumilus, associated with the marine sponge Ircinia sp

The isolate of Bacillus pumilus associated with the marine sponge Ircinia sp. produced the surfactin-like lipopeptides, cyclic acyldepsipeptides. The hemolytic activity of individual cyclic acyldepsipeptides, bacircines (BI) 2, 3, 4, 5 and 5A having different acyl side chain structures (anteiso-C13, iso-C14, normal-C14, anteiso-C15, and iso-C15, respectively) was studied. The hemolytic power of bacircines depended on both the structure of the side chain (n->iso->anteiso-) and pH values (5.6 and 6.5 > 7.4). Hemolytic potency as a function of BI 5 concentration was given for pH 6.5; 7.4; 8.0; 9.0. pH dependent hemolysis induced by BI 5 was shown to be reversible. The membrane damaging potential of bacircine 5 (5 microM) at pH 6.5 was characterized by a higher rate of hemolysis and by a shorter time between the introduction of BI 5 solution into the RBC samples and the onset of hemolysis. Under this condition, BI 5 decreased abnormally the microviscosity of erythrocyte ghosts bilayer. The damaging potency of BI 5 decreased with an increase pH from 6.5 to 7.4 or its decrease from 6.5 to 4.9. It was shown that fatty acid bacircine fragment penetrated into the lipid bilayer to a depth of minimum 7 carbon atoms. Constants of dissociation of the Asp (pK 4.75) and Glu (pK 6.65) residues of bacircine in the lipid bilayer were obtained. These results showed that at pH 6.5 BI 5 possessed membranotropic activity in the monoionic form.

Permeability and stability properties of membranes formed by lipids extracted from Lactobacillus acidophilus grown at different temperatures

Lactobacillus acidophilus CRL 640 grown at 25 and 37 degrees C showed a high content of cardiolipin, phosphatidylglycerol, and glycolipids. Cultures grown at 25 degrees C showed a twofold increase in glycolipids in relation to phospholipids, a twofold increase in the C16:0 and a fourfold increase in the C18:2 fatty acids. In contrast, the C19-cyc and the 10-hydroxy acid (C18:0-10 OH) species showed a noticeable decrease. Extracts of total lipids of bacteria grown at 25 and 37 degrees C dispersed in water yielded particles having a high negative surface potential as measured by electrophoretic mobility. Vesicles prepared by extrusion of these dispersions through polycarbonate membranes of 100-nm pore diameter showed high trapping of carboxyfluorescein (CF), which remained unchanged for at least 20 h. The fluorescence anisotropy measured with diphenylhexatriene (DPH) and the generalized polarization of Laurdan were significantly lower in vesicles prepared with lipids containing the highest glycolipid ratio, in comparison to those of bacteria grown at 37 degrees C. No phase transition was detected between 5 and 50 degrees C as measured with both probes. In accordance with these results, no significant release of the trapped CF in this range of temperature was detected. Bile salts and NaCl promoted an increase in the fluorescence, which is interpreted as a change in the permeability properties of the membrane. This effect was lower with KCl, while CaCl2 did not cause any change. The greater permeability change was observed in vesicles with a low glycolipid/phospholipid ratio. NaCl did not affect the packing of the interface as measured with Laurdan, in contrast to CaCl2. The action of Ca+2 may be ascribed to the binding to the negatively charged lipids, such as phosphatidyl glycerol and cardiolipin. It is concluded that the higher glycolipid/phospholipid ratio and the fatty acids C18:2 and C16:0 enhance the lipid membrane stability and decrease the organization in the interfacial and hydrocarbon zones. These results are congruent with the behavior of entire bacteria subject to osmotic and freeze/thaw stresses.

Inhibition of excessive neuronal apoptosis by the calcium antagonist amlodipine and antioxidants in cerebellar granule cells

Neuronal cell death as a result of apoptosis is associated with cerebrovascular stroke and various neurodegenerative disorders. Pharmacological agents that maintain normal intracellular Ca2+ levels and inhibit cellular oxidative stress may be effective in blocking abnormal neuronal apoptosis. In this study, a spontaneous (also referred to as age-induced) model of apoptosis consisting of rat cerebellar granule cells was used to evaluate the antiapoptotic activities of voltage-sensitive Ca2+ channel blockers and various antioxidants. The results of these experiments demonstrated that the charged, dihydropyridine Ca2+ channel blocker amlodipine had very potent neuroprotective activity in this system, compared with antioxidants and neutral Ca2+ channel blockers (nifedipine and nimodipine). Within its effective pharmacological range (10-100 nM), amlodipine attenuated intracellular neuronal Ca2+ increases elicited by KCl depolarization but did not affect Ca2+ changes triggered by N-methyl-D-aspartate receptor activation. Amlodipine also inhibited free radical-induced damage to lipid constituents of the membrane in a dose-dependent manner, independent of Ca2+ channel modulation. In parallel experiments, spontaneous neuronal apoptosis was inhibited in dose- and time-dependent manners by antioxidants (U-78439G, alpha-tocopherol, and melatonin), nitric oxide synthase inhibitors (N-nitro-L-arginine and N-nitro-D-arginine), and a nitric oxide chelator (hemoglobin) in the micromolar range. These results suggest that spontaneous neuronal apoptosis is associated with excessive Ca2+ influx, leading to further intracellular Ca2+ increases and the generation of reactive oxygen species. Agents such as amlodipine that block voltage-sensitive Ca2+ channels and inhibit cellular oxidative stress may be effective in the treatment of cerebrovascular stroke and neurodegenerative diseases associated with excessive apoptosis.

The ectodomain of HA2 of influenza virus promotes rapid pH dependent membrane fusion

To better understand the roles of different regions of influenza hemagglutinin in membrane fusion, we have studied the fusion properties of large unilamellar vesicles in the presence of constructs comprising the 127 amino acid ectodomain of the HA2 fragment (FHA2) as well as mutated forms of FHA2 containing single amino acid substitutions, the 95 amino acid truncated form of FHA2 lacking the N-terminal fusion peptide (SHA2), the 20 amino acid N-terminal fusion peptide and the ten amino acid peptide corresponding to the kinked loop region of FHA2. The 100 nm liposomes were made from dioleoylphosphatidylethanolamine, dioleoylphosphatidylcholine and cholesterol in equimolar ratio. At pH 5 a high rate of lipid mixing was observed with FHA2 present, even at very low molar concentrations, whereas much lower rates were observed using the shorter constructs: SHA2, the fusion peptide, and the loop peptide. Concentrations of FHA2 which promoted extensive lipid mixing also induced leakage of aqueous contents. Marked effects of FHA2 were also observed with liposomes of egg phosphatidylcholine. All of the changes observed with the liposomes were highly pH-dependent, with only negligible changes occurring at pH 7. The results demonstrate the potent action of FHA2 in promoting lipid mixing and demonstrate the contribution of other regions of the ectodomain of FHA2, in addition to the fusion peptide, to the mechanism of acceleration of membrane fusion. The results also indicate that the pH dependence of fusion is not due solely to changes in the interactions between the HA1 and HA2 subunits. Thus, the "spring loaded energy" is not required to bring about the apposition of the two membranes, considering that FHA2 is already in its thermostable conformation. The acidic amino acid residues in the kinked loop region appear to play a particularly important role in the pH-dependent fusion process as demonstrated by the marked loss of lipid mixing activity of mutant forms of FHA2.

Medicinal plants from Nepal; II. Evaluation as inhibitors of lipid peroxidation in biological membranes

In an ethnopharmacological screening of selected Nepalese medicinal plants, the inhibitory effect on lipid peroxidation of 36 methanolic extracts of 28 different plant species were evaluated using bovine brain phospholipid liposomes as model membranes. The most potent inhibitor with an IC50 value of 0.93 microg/ml was the extract obtained from the bark of Clerodendrum indicum. The extracts obtained from Aglaia roxburghiana fruits, Euonymus pendulus barks and Emblica officinalis fruits were also active and inhibited lipid peroxidation with IC50 values of 10, 12 and 13 microg/ml, respectively. The most active extracts were from indigenous plants traditionally used to treat inflammatory diseases.

Effects of urea and trimethylamine N-oxide on fluidity of liposomes and membranes of an elasmobranch

The effects on membrane fluidity of two solutes of biological importance in elasmobranch fishes, urea and trimethylamine oxide (TMAO), were determined using elasmobranch red blood cell plasma membranes and artificial liposomes. Fluorescence polarizations of three probes with differing sites of insertion (1, 6-diphenylhexatriene, cis-parinaric acid, and trans-parinaric acid) were used to study the effects of physiological levels of urea (400 mM) and TMAO (200 mM) separately and together in a 2:1 urea:TMAO ratio (400 mM:200 mM). In the elasmobranch erythrocyte membrane, there was a trend toward an increase in the order of the gel-phase domains when treated with urea, although this was not statistically significant. This effect was counteracted by the presence of TMAO. To determine if the organic solutes were acting directly on the membrane lipids or on the integral proteins, phase-transition profiles of protein-free dipalmitoyl phosphatidylcholine liposomes were determined. These profiles showed that urea again increased the order of the gel-phase domains of the bilayer; however, this effect was not counteracted by the presence of TMAO. We suggest that the increased order in the gel-phase domains may be an indirect effect of a decrease in the order of the fluid-phase domains. This increase in fluidity may be due either to a disruptive effect of urea on the hydrophobic core of the membrane or to indirect effects mediated by changes in the integral membrane proteins. This study is the first to demonstrate that urea and TMAO may act as counteracting solutes in the elasmobranch erythrocyte membrane and that the counteraction appears to be at the level of the integral proteins rather than the membrane lipids.

Total synthesis and membrane modifying properties of the lipopeptaibol trikoningin KB II and its analogues with acyl chains of different length at the N- and C-termini

Trikoningin KB II, a ten-amino acid residue lipopeptaibol blocked at the N-terminus by the n-octanoyl group and at the C-terminus by the 1,2-amino alcohol L-leucinol, and extracted from the fungus Trichoderma koningii, exhibits membrane-modifying properties. We have synthesized by solution-phase methods trikoningin KB II and several analogues with acyl chains of different length at the N- and C-termini. Permeability measurements showed that an appropriate length of the linear acyl chain is a more important characteristic for the onset of significant membrane-modifying activity than its position in the peptide chain.

Conformation and antimicrobial activity of linear derivatives of tachyplesin lacking disulfide bonds

Tachyplesin is a potent antimicrobial peptide isolated from the hemocytes of the horseshoe crab, Tachypleus tridentatus. Previous studies have shown that the 17-residue peptide has an intrinsic amphipathic structure conferred by two antiparallel beta-sheets held rigidly by two disulfide bonds. Taking its short length into account and the potential of such a small polypeptide to take on multiple conformational states, one may assume that the disulfide bonds are relevant determinants of function. However, in order to gain a global perspective on the tolerance of cysteine residues in tachyplesin to amino acid substitutions, a series of linear peptides have been synthesized and their physicochemical properties analyzed. In these linear peptides, the cysteines have been replaced with amino acids possessing different side-chain properties, i.e., aliphatic hydrophobic (Ala, Leu, Ile, Val, and Met), aromatic hydrophobic (Phe and Tyr), and acidic (Asp). Activity assays using natural and synthetic membranes, and conformational measurements, highlight the subtle influence and variability of the amino acid side-chain properties on peptide structure. While an unequivocal interpretation of the results will have to await more refined structural measurements, our results indicate that a rigidly held disulfide-bonded beta-pleated sheet structure may not be absolutely essential for antimicrobial activity. Furthermore, the results challenge the accepted dogma of structure-activity relationships among antimicrobial peptides and suggest that the maintenance of peptide hydrophobic-hydrophilic balance may be a critical parameter, in addition to structure, in the design of peptides with pharmaceutical relevance.

Electron paramagnetic resonance studies of membrane fluidity in ozone-treated erythrocytes and liposomes

Doxyl stearate spin probes which differed in the attachment of the nitroxide free radical to the fatty acid have been used to study membrane fluidity in ozone-treated bovine erythrocytes and liposomes. Analysis of EPR spectra of spin labels incorporated into lipid bilayer of the erythrocyte membranes indicates an increase in the mobility and decrease in the order of membrane lipids. In isolated erythrocyte membranes (ghosts) the most significant changes were observed for 16-doxylstearic acid. In intact erythrocytes statistically significant were differences for 5-doxylstearic acid. The effect of ozone on liposomes prepared from a lipid extract of erythrocyte lipids was marked in the membrane microenvironment sampled by all spin probes. Ozone apparently leads to alterations of membrane dynamics and structure but does not cause increased rigidity of the membrane.

A comparative study on the structure and function of a cytolytic alpha-helical peptide and its antimicrobial beta-sheet diastereomer

Antimicrobial peptides which adopt mainly or only beta-sheet structures have two or more disulfide bonds stabilizing their structure. The disruption of the disulfide bonds results in most cases in a large decrease in their antimicrobial activity. In the present study we examined the effect of d-amino acids incorporation on the structure and function of a cytolytic alpha-helical peptide which acts on erythrocytes and bacteria. The influence of a single or double d-amino acid replacement in alpha-helical peptides on their structure was reported previously in 50% 2,2,2, trifluoroethanol/water [Krause et al. (1995) Anal. Chem. 67, 252-258]. Here we used Attenuated Total Reflectance Fourier-Transform Infrared (ATR-FTIR) spectroscopy and found that the predominant structure of the wild-type peptide is alpha-helix in phospholipid membranes, whereas the structure of the diastereomer is beta-sheet. However, the linear, beta-sheet diastereomer preserved its cytolytic activity on bacteria but not on erythrocytes. Previous studies have shown that the ability of antimicrobial peptides to lyse bacteria but not normal mammalian cells correlated with their ability to disintegrate preferentially negatively charged, but not zwitterionic phospholipid membranes. In contrast, the diastereomer described here disrupts zwitterionic and negatively charged vesicles with similar potencies to those of the hemolytic wild-type peptide. Interestingly, whereas addition of a positive charge to the N-terminus of the wild-type peptide (which caused a minor effect on its structure) increased activity only towards some of the bacteria tested, similar modification in the diastereomer increased activity towards all of them. Furthermore, the modified wild-type peptide preserved its potency to destabilize zwitterionic and negatively charged vesicles, whereas the modified diastereomer had a reduced potency on zwitterionic vesicles but increased potency on negatively charged vesicles. Overall our results suggest that this new class of antimicrobial diastereomeric peptides bind to the membrane in 'carpet-like' manner followed by membrane disruption and breakdown, rather than forming a transmembrane pore which interfere with the bacteria potential. These studies also open a way to design new broad-spectrum antibacterial peptides.

Pb2+ promotes lipid oxidation and alterations in membrane physical properties

Experimental evidence suggests that cellular damage mediated by oxidants could be involved in the pathology associated with lead (Pb) toxicity. We investigated the effect of Pb2+ on lipid oxidation in liposomes using different initiators. In the presence of Fe2+, Pb2+ (12.5-200 microM) stimulated lipid oxidation in phosphatidylcholine:phosphatidylserine-containing liposomes, measured as 2-thiobarbituric acid-reactive substances (TBARS) and conjugated dienes. This stimulatory effect depended on the presence of membrane negative charges and on bilayer integrity. Pb2+ did not stimulate TBARS formation in the presence of 25 mM 2,2'-azo-bis (2,4 dimethylvaleronitrile (AMVN) and 2,2' azobis (2-amidinopropane) (AAPH). Pb2+ significantly stimulated TBARS production and NADH oxidation in the presence of photoactivated rose Bengal. The use of specific inhibitors indicated that several reactive oxygen species were involved in the pro-oxidant action of Pb2+. Pb2+ (12.5-200 microM) caused membrane lateral phase separation and this effect was positively correlated with its capacity to stimulate Fe2+ and rose Bengal-initiated TBARS production. Pb2+ could bind to the membrane and act to stimulate lipid oxidation by causing changes in membrane physical properties. Through this mechanism Pb2+ would favor the propagation of lipid oxidation. By causing lateral phase separation and/or by increasing lipid oxidation rates, Pb2+ could be cytotoxic by altering membrane-related processes.