Interaction of polymyxin B nonapeptide with anionic phospholipids

Thrombin-Derived Peptides Potentiate the Activity of Gram-Positive-Specific Antibiotics against Gram-Negative Bacteria

The continued rise of antibiotic resistance threatens to undermine the utility of the world’s current antibiotic arsenal. This problem is particularly troubling when it comes to Gram-negative pathogens for which there are inherently fewer antibiotics available. To address this challenge, recent attention has been focused on finding compounds capable of disrupting the Gram-negative outer membrane as a means of potentiating otherwise Gram-positive-specific antibiotics. In this regard, agents capable of binding to the lipopolysaccharide (LPS) present in the Gram-negative outer membrane are of particular interest as synergists. Recently, thrombin-derived C-terminal peptides (TCPs) were reported to exhibit unique LPS-binding properties. We here describe investigations establishing the capacity of TCPs to act as synergists with the antibiotics erythromycin, rifampicin, novobiocin, and vancomycin against multiple Gram-negative strains including polymyxin-resistant clinical isolates. We further assessed the structural features most important for the observed synergy and characterized the outer membrane permeabilizing activity of the most potent synergists. Our investigations highlight the potential for such peptides in expanding the therapeutic range of antibiotics typically only used to treat Gram-positive infections.

Review: Towards antibacterial strategies: studies on the mechanisms of interaction between antibacterial peptides and model membranes

Lipopolysaccharides (LPSs) play a dual role as inflammation-inducing and as membrane-forming molecules. The former role attracts significantly more attention from scientists, possibly because it is more closely related to sepsis and septic shock. This review aims to focus the reader's attention to the other role, the function of LPS as the major constituent of the outer layer of the outer membrane of Gram-negative bacteria, in particular those of enterobacterial strains. In this function, LPS is a necessary component of the cell envelope and guarantees survival of the bacterial organism. At the same time, it represents the first target for attacking molecules which may either be synthesized by the host's innate or adaptive immune system or administered to the human body. The interaction of these molecules with the outer membrane may not only directly cause the death of the bacterial organism, but may also lead to the release of LPS into the circulation. Here, we review membrane model systems and their application for the study of molecular mechanisms of interaction of peptides such as those of the human complement system, the bactericidal/permeability-increasing protein (BPI), cationic antibacterial peptide 18 kDa (CAP18) as an example of cathelicidins, defensins, and polymyxin B (PMB). Emphasis is on electrical measurements with a reconstitution system of the lipid matrix of the outer membrane which was established in the authors' laboratory as a planar asymmetric bilayer with one leaflet being composed solely of LPS and the other of the natural phospholipid mixture. The main conclusion, which can be drawn from these investigations, is that LPS and in general its negative charges are the dominant determinants for specific peptide—membrane interactions. However, the detailed mechanisms of interaction, which finally lead to bacterial killing, may involve further steps and differ for different antibacterial peptides.

Characterization of the interactions of a polycationic, amphiphilic, terminally branched oligopeptide with lipid A and lipopolysaccharide from the deep rough mutant of Salmonella minnesota

The lipid A and lipopolysaccharide (LPS) binding and neutralizing activities of a synthetic, polycationic, amphiphilic peptide were studied. The branched peptide, designed as a functional analog of polymyxin B, has a six residue hydrophobic sequence, bearing at its N-terminus a penultimate lysine residue whose α- and E-amino groups are coupled to two terminal lysine residues. In fluorescence spectroscopic studies designed to examine relative affinities of binding to the toxin, neutralization of surface charge and fluidization of the acyl domains, the peptide was active, closely resembling the effects of polymyxin B and its nonapeptide derivative; however, the synthetic peptide does not induce phase transitions in LPS aggregates as do polymyxin B and polymyxin B nonapeptide. The peptide was also comparable with polymyxin B in its ability to inhibit LPS-mediated IL-1 and IL-6 release from human peripheral blood mononuclear cells. The synthetic compound is devoid of antibacterial activities and did not induce conductance fluxes in LPS-containing asymmetric planar membranes. These results strengthen the premise that basicity and amphiphilicity are necessary and sufficient physical properties that ascribe endotoxin binding and neutralizing activities, and further suggest that antibacterial/membrane perturbant and LPS neutralizing activities are dissociable, which may be of value in designing LPS-sequestering agents of low toxicity.

Interactions between Hepatitis C Virus and the Human Apolipoprotein H Acute Phase Protein: A Tool for a Sensitive Detection of the Virus

The Hepatitis C virus (HCV) infection exhibits a high global prevalence frequently associated with hepatocellular carcinoma, taking years to develop. Despite the standardization of highly sensitive HCV quantitative RT-PCR (qRT-PCR) detection methods, false-negative diagnoses may be generated with current methods, mainly due to the presence of PCR inhibitors and/or low viral loads in the patient’s sample. These false-negative diagnoses impact both public health systems, in developing countries, and an in lesser extent, in developed countries, including both the risk of virus transmission during organ transplantation and/or blood transfusion and the quality of the antiviral treatment monitoring. To adopt an appropriate therapeutic strategy to improve the patient’s prognosis, it is urgent to increase the HCV detection sensitivity. Based upon previous studies on HBV, we worked on the capacity of the scavenger acute phase protein, Apolipoprotein H (ApoH) to interact with HCV. Using different approaches, including immunoassays, antibody-inhibition, oxidation, ultracentrifugation, electron microscopy and RT-PCR analyses, we demonstrated specific interactions between HCV particles and ApoH. Moreover, when using a two-step HCV detection process, including capture of HCV by ApoH-coated nanomagnetic beads and a home-made real-time HCV-RT-PCR, we confirmed the presence of HCV for all samples from a clinical collection of HCV-seropositive patients exhibiting an RT-PCR COBAS® TaqMan® HCV Test, v2.0 (COBAS)-positive result. In contrast, for HCV-seropositive patients with either low HCV-load as determined with COBAS or exhibiting HCV-negative COBAS results, the addition of the two-step ApoH-HCV-capture and HCV-detection process was able to increase the sensitivity of HCV detection or more interestingly, detect in a genotype sequence-independent manner, a high-proportion (44%) of HCV/RNA-positive among the COBAS HCV-negative patients. Thus, the immune interaction between ApoH and HCV could be used as a sample preparation tool to enrich and/or cleanse HCV patient’s samples to enhance the detection sensitivity of HCV and therefore significantly reduce the numbers of false-negative HCV diagnosis results.

Different modes of membrane permeabilization by two RTX toxins: HlyA from Escherichia coli and CyaA from Bordetella pertussis

This study clarifies the membrane disruption mechanisms of two bacterial RTX toxins: alphahemolysin (HlyA) from Escherichia coli and a highly homologous adenylate cyclase toxin (CyaA) from Bordetella pertussis. For this purpose, we employed a fluorescence requenching method using liposomes (extruded through filters of different pore size - 1000 nm, 400 nm or 100 nm) with encapsulated fluorescent dye/quencher pair ANTS/DPX. We showed that both toxins induced a graded leakage of liposome content with different selectivities alpha for DPX and ANTS. In contrast to HlyA, CyaA exhibited a higher selectivity for cationic quencher DPX, which increased with vesicle diameter. Large unilamellar vesicles (LUV(1000)) were found to be more suitable for distinguishing between high alpha values whereas smaller ones (LUV(100)) were more appropriate for discriminating an all-or-none leakage (alpha=0) from the graded leakage with low values of alpha. While disrupting LUV(1000), CyaA caused a highly cation-selective leakage (alpha~15) whereas its mutated form with decreased channel K(+)/Cl(-) selectivity due to two substitutions in a predicted transmembrane segment (CyaA-E509K+E516K) exhibited much lower selectivity (alpha approximately 6). We concluded that the fluorescence requenching method in combination with different size of liposomes is a valuable tool for characterization of pore-forming toxins and their variants.

Direct UV Raman monitoring of 3(10)-helix and pi-bulge premelting during alpha-helix unfolding

We used UV resonance Raman (UVRR) spectroscopy exciting at approximately 200 nm within the peptide bond pi --> pi* transitions to selectively study the amide vibrations of peptide bonds during alpha-helix melting. The dependence of the amide frequencies on their Psi Ramachandran angles and hydrogen bonding enables us, for the first time, to experimentally determine the temperature dependence of the peptide bond Psi Ramachandran angle population distribution of a 21-residue mainly alanine peptide. These Psi distributions allow us to easily discriminate between alpha-helix, 3(10)-helix and pi-helix/bulge conformations, obtain their individual melting curves, and estimate the corresponding Zimm and Bragg parameters. A striking finding is that alpha-helix melting is more cooperative and shows a higher melting temperature than previously erroneously observed. These Psi distributions also enable the experimental determination of the Gibbs free energy landscape along the Psi reaction coordinate, which further allows us to estimate the free energy barriers along the AP melting pathway. These results will serve as a benchmark for the numerous untested theoretical studies of protein and peptide folding.

Histochemical evidence for lipid A (endotoxin) in eukaryote chloroplasts

Lipopolysaccharide (LPS) (a.k.a., endotoxin) is an essential component of the outer leaflet of the outer membrane of gram-negative bacteria and is a potent activator of the innate immune system of animals. Lipid A, the glycolipid core of LPS, is the agent responsible for disease and death from gram-negative sepsis, an important cause of human mortality and morbidity. Although it is generally accepted that lipid A is restricted to the prokaryotes, recent efforts to purify molecules from green algae with structural features unique to lipid A have met with success. Furthermore, the vascular plant Arabidopsis thaliana has been found to contain genes that encode all of the enzymes of the biosynthetic pathway for lipid A. It is not known whether vascular plants synthesize lipid A or where lipid A might be located in the tissues. For the present study, we used affinity reagents for lipid A to probe green alga and tissues of the garden pea for a light microscopic localization of lipid A in these eukaryote cells. We find staining for lipid A in free-living and endosymbiotic green algae and in the chloroplasts of vascular plants, indicating that this molecule is not restricted to prokaryotes, but is found also in select eukaryotes.

Bilayer fibril formation by genetically engineered polypeptides: preparation and characterization

A de novo, genetically engineered 687 residue polypeptide expressed in E. coli has been found to form highly rectilinear, beta-sheet containing fibrillar structures. Tapping-mode atomic force microscopy, deep-UV Raman spectroscopy, and transmission electron microscopy definitively established the tendency of the fibrils to predominantly display an apparently planar bilayer or ribbon assemblage. The ordered self-assembly of designed, extremely repetitive, high molecular weight peptides is a harbinger of the utility of similar materials in nanoscience and engineering applications.

Temperature dependence of the binding of endotoxins to the polycationic peptides polymyxin B and its nonapeptide

The interaction between endotoxins-free lipid A and various lipopolysaccharide (LPS) chemotypes with different sugar chain lengths-and the polycationic peptides polymyxin B and polymyxin nonapeptide has been investigated by isothermal titration calorimetry between 20 and 50 degrees C. The results show a strong dependence of the titration curves on the phase state of the endotoxins. In the gel phase (<30 degrees C for LPS and <45 degrees C for lipid A), an endothermic reaction is observed, for which the driving force is an entropically driven endotoxin-polymyxin interaction, due to disruption of the ordered water structure and cation assembly in the lipid A backbone and adjacent molecules. In the liquid crystalline phase (>35 degrees C for LPS and >47 degrees C for lipid A) an exothermic reaction takes place, which is mainly due to the strong electrostatic interaction of the polymyxins with the negative charges of the endotoxins, i.e., the entropic change DeltaS is much lower than in the gel phase. For endotoxins with short sugar chains (lipid A, LPS Re, LPS Rc) the stoichiometry of the polymyxin binding corresponds to pure charge neutralization; for the compounds with longer sugar chains (LPS Ra, LPS S-form) this is no longer valid. This can be related to the lower susceptibility of the corresponding bacterial strains to antibiotics.

Inner field compensation as a tool for the characterization of asymmetric membranes and Peptide-membrane interactions

Symmetric and asymmetric planar lipid bilayers prepared according to the Montal-Mueller method are a powerful tool to characterize peptide-membrane interactions. Several electrical properties of lipid bilayers such as membrane current, membrane capacitance, and the inner membrane potential differences and their changes can be deduced. The time-resolved determination of peptide-induced changes in membrane capacitance and inner membrane potential difference are of high importance for the characterization of peptide-membrane interactions. Intercalation and accumulation of peptides lead to changes in membrane capacitance, and membrane interaction of charged peptides induces changes in the charge distribution within the membrane and with that to changes in the membrane potential profile. In this study, we establish time-resolved measurements of the capacitance minimization potential DeltaPsi on various asymmetric planar lipid bilayers using the inner field compensation method. The results are compared to the respective ones of inner membrane potential differences DeltaPhi determined from ion carrier transport measurements. Finally, the time courses of membrane capacitances and of DeltaPsi have been used to characterize the interaction of cathelicidins with reconstituted lipid matrices of various Gram-negative bacteria.

Polymyxin B-lipid interactions in Langmuir-Blodgett monolayers ofEscherichia coli lipids: A thermodynamic and atomic force microscopy study

The dramatically increased frequency of antibiotic resistance has led to intensive efforts towards developing new families of antibiotics. Membrane-active antibiotic peptides such as polymyxin B (PxB) hold promise as the next generation of antibiotics, since they rarely spur the evolution of resistance. At low concentrations in the membrane, PxB forms vesicle-vesicle contacts and induces lipid exchange without leakage or fusion, a phenomenon that can explain its specificity towards gram-negative bacteria by contact formation between the two phospholipids interfaces in the periplasmatic space. In this work, the interaction of PxB and the nonantibiotic derivative polymyxin B nonapeptide (PxB-NP) with monolayers of Escherichia coli membrane lipids (ECL) has been studied by thermodynamic and structural methods. PxB inserts itself into ECL monolayers as a conformation that forms intermembrane contacts with vesicles injected underneath, and induces lipid exchange when the monolayer surface pressure is set at 32 mN/m (membrane equivalence pressure) or net transfer vesicle-to-monolayer at lower surface pressures. Thermodynamic analysis of the compression isotherms of mixed monolayers indicates that PxB inserts into the monolayer with an expansion of the mean molecular area, implying that peptide and lipids form nonideal mixtures. At low concentrations, corresponding to the membrane-membrane contact form of PxB, the mixed monolayers present positive excess energy values (deltaGm(Ex)), and atomic force microscopy (AFM) imaging reveals structures of approximately 120-nm diameter that protrude from the lipid surface approximately 0.7 nm. At concentrations of PxB above 4 mol %, thermodynamic analysis gives a very high deltaGm(Ex), corresponding to nonfavorable interactions, and AFM images show round structures of 20-30 nm diameter. PxB-NP behaves in a totally different way, in agreement with its inability to form vesicle-vesicle contacts and its lack of antibiotic effect. These results are discussed in the light of the mechanism of action of PxB on the membrane of gram-negative bacteria.

Stabilization of O-pyromellitylgramicidin channels in bilayer lipid membranes through electrostatic interaction with polylysines of different chain lengths

Functioning of membrane proteins, in particular ionic channels, can be modulated by alteration of their arrangement in membranes. We addressed this issue by studying the effect of different chain length polylysines on the kinetics of ionic channels formed in a bilayer lipid membrane (BLM) by O-pyromellitylgramicidin carrying three negative charges at the C-terminus. The method of sensitized photoinactivation was applied to the analysis of the channel association-dissociation kinetics (characterized by the exponential factor of the curve describing the time course of the flash-induced decrease in the transmembrane current, tau). Addition of polylysine to the bathing solutions of BLM led to the deceleration of the photoinactivation kinetics, i.e. to the increase in tau. It was shown here that for a series of polylysines differing in their chain lengths, the value of tau grew as their concentration increased above a threshold level until at a certain concentration of each polylysine tau reached maximum. At higher polylysine concentrations tau began to decrease and finally became close to the control level observed in the absence of polylysine. With lengthening of the polylysine chain the maximum value of tau increased, the concentration dependence became steeper, and the threshold concentration decreased. The increase in the ionic strength of the medium shifted the concentration dependence of tau to higher polylysine concentrations and decreased the maximum value of tau. It was concluded that the increase in tau was caused by the formation of domains of O-pyromellitylgramicidin molecules induced by binding of polylysines. This can be related to functional aspects of polycation-induced sequestering of negatively charged transmembrane peptides in neutral membranes.

Cationic peptide antimicrobials induce selective transcription of micF and osmY in Escherichia coli

Cationic antimicrobial peptides, such as polymyxin and cecropin, activated transcription of osmY and micF in growing Escherichia coli independently of each other. The micF response required the presence of a functional rob gene. It is intriguing that in this and other assays an identical response profile was also seen with hyperosmotic salt or sucrose gradient, two of the most commonly used traditional food preservatives. The osmY and micF transcription was not induced by hypoosmotic gradient, ionophoric peptides, uncouplers, or with other classes of membrane perturbing agents. The antibacterial peptides did not promote transcription of genes that respond to macromolecular or oxidative damage, fatty acid biosynthesis, heat shock, or depletion of proton or ion gradients. These and other results show that the antibacterial cationic peptides induce stasis in the early growth phase, and the transcriptional efficacy of antibacterial peptides correlates with their minimum inhibitory concentration, and also with their ability to mediate direct exchange of phospholipids between vesicles. The significance of these results is developed as the hypothesis that the cationic peptide antimicrobials stress growth of Gram-negative organisms by making contacts between the two phospholipid interfaces in the periplasmic space and prevent the hyperosmotic wrinkling of the cytoplasmic membrane. Broader significance of these results, and of the hypothesis that the peptide mediated contacts between the periplasmic phospholipid interfaces are the primary triggers, is discussed in relation to antibacterial resistance.

The dual role of lipopolysaccharide as effector and target molecule

Lipopolysaccharides (LPS) are major integral components of the outer membrane of Gram-negative bacteria being exclusively located in its outer leaflet facing the bacterial environment. Chemically they consist in different bacterial strains of a highly variable O-specific chain, a less variable core oligosaccharide, and a lipid component, termed lipid A, with low structural variability. LPS participate in the physiological membrane functions and are, therefore, essential for bacterial growth and viability. They contribute to the low membrane permeability and increase the resistance towards hydrophobic agents. They are also the primary target for the attack of antibacterial drugs and proteins such as components of the host's immune response. When set free LPS elicit, in higher organisms, a broad spectrum of biological activities. They play an important role in the manifestation of Gram-negative infection and are therefore termed endotoxins. Physico-chemical parameters such as the molecular conformation and the charges of the lipid A portion, which is responsible for endotoxin-typical biological activities and is therefore termed the 'endotoxic principle' of LPS, are correlated with the biological activity of chemically different LPS.

Cecropins induce the hyperosmotic stress response in Escherichia coli

Cecropin A and B, below or near their minimum inhibitory concentrations in viable Escherichia coli, interfered with the rapid NaCl-induced hyperosmotic shrinkage of the cytoplasmic volume (plasmolysis), and also activated the promoter of the hyperosmotic stress gene osmY. The same promoter was also expressed by hyperosmolar NaCl or sucrose, two of the most commonly used antimicrobial food preservatives. Stress responses were monitored during the logarithmic growth phase of E. coli strains that contain specific promoters fused to a luxCDABE operon on a plasmid. The luminescence assay, developed to monitor the transcriptional response to stresses, is based on the premise that organisms often respond and adapt to sublethal environmental adversities by increased expression of stress proteins to restore homeostasis. The luminescence response from these fusion strains to a specific stress occurs as the transcription at the promoter site is activated. Cecropins induced luminescence response only from the osmY-luxCDABE fusion, but not the corresponding stress promoter activation associated with macromolecular or oxidative damage, or leakage of the cytoplasmic content including the proton gradient. The inhibitory effect of cecropins on plasmolysis is interpreted to suggest that the primary locus of action of these antimicrobial peptides in the periplasmic space is on the coupling between the inner and outer membrane.

A new monofluorinated phosphatidylcholine forms interdigitated bilayers

16-Fluoropalmitic acid was synthesized from 16-hydroxypalmitic acid using diethylaminosulfur trifluoride. This monofluorinated fatty acid then was used to make 1-palmitoyl-2-[16-fluoropalmitoyl]-phosphatidylcholine (F-DPPC) as a fluorinated analog of dipalmitoylphosphatidylcholine (DPPC). Surprisingly, we found that the phase transition temperature (Tm) of F-DPPC occurs near 50 degrees C, approximately 10 degrees C higher than its nonfluorinated counterpart, DPPC, as judged by both differential scanning calorimetry and infrared spectroscopy. The pretransition observed for DPPC is absent in F-DPPC. A combination of REDOR, rotational-echo double-resonance, and conventional solid-state NMR experiments demonstrates that F-DPPC forms a fully interdigitated bilayer in the gel phase. Electron paramagnetic resonance experiments show that below Tm, the hydrocarbon chains of F-DPPC are more motionally restricted than those of DPPC. X-ray scattering experiments confirm that the thickness and packing of gel phase F-DPPC is similar to that of heptanetriol-induced interdigitated DPPC. F-DPPC is the first phosphoglyceride containing sn-1 and sn-2 ester-linked fatty acyl chains of equal length that spontaneously forms interdigitated bilayers in the gel state in the absence of inducing agents such as alcohols.

Osmotic stress in viable Escherichia coli as the basis for the antibiotic response by polymyxin B

Cationic antimicrobial peptides, such as polymyxin B (PxB), below growth inhibitory concentration induce expression of osmY gene in viable E. coli without leakage of solutes and protons. osmY expression is also a locus of hyperosmotic stress response induced by common food preservatives, such as hypertonic NaCl or sucrose. High selectivity of PxB against Gram-negative organisms and the basis for the hyperosmotic stress response at sublethal PxB concentrations is attributed to PxB-induced mixing of anionic phospholipid between the outer layer of the cytoplasmic membrane with phospholipids in the inner layer of the outer membrane. This explanation is supported by PxB-mediated rapid and direct exchange of anionic phospholipid between vesicles. This mechanism is consistent with the observation that genetically stable resistance against PxB could not be induced by mutagenesis.

Effect of natural amphipathic peptides on viability, membrane potential, cell shape and motility of mollicutes

The antibiotic activity of ten amphipathic peptides was investigated in six species of mollicutes belonging to the genera Acholeplasma, Mycoplasma and Spiroplasma. A. laidlawii was the most sensitive and M. mycoides subsp. mycoides SC the most resistant. Animal defence peptides (cecropins A and P1, and magainin 2) proved to be less potent than bee-venom mellitin and most of the peptides produced by bacteria (globomycin, gramicidin S, surfactin and valinomycin) or fungi (alamethicin). Gramicidin S was by far the most active peptide, with minimal inhibitory concentrations ranging from 2 to 50 nM. Alamethicin, gramicidin S, mellitin and surfactin had a cidal effect, whilst cecropins, globomycin, magainin 2, polymyxin B and valinomycin proved to be static. The peptides altered the membrane potential of spiroplasma cells with a potency independent of their linear or cyclic structure. However, globomycin depolarized the plasma membrane only weakly, whilst polymyxin B, in order to be active, required prior hyperpolarization of the membrane. The peptides also induced the loss of cell motility and helicity in spiroplasmas, suggesting that motility and cell shape in these bacteria are coupled to the transmembrane electrochemical gradient. Globomycin, an inhibitor of signal-peptidase II, prevented the growth of spiroplasmas, M. gallisepticum, and M. genitalium, but not that of A. laidlawii and M. mycoides subsp. mycoides SC, although the latter also synthesized membrane lipoproteins. Inhibition of spiralin processing by globomycin was demonstrated in S. citri and S. melliferum, with a more pronounced effect in the second species.

Monitoring exocytosis and release from individual mast cells by capillary electrophoresis with laser-induced native fluorescence detection

The complex temporal evolution of on-column exocytotic release of serotonin and proteins from individual rat peritoneal mast cells (RPMCs) was monitored by using capillary electrophoresis. Laser-induced native fluorescence detection with 275-nm excitation was used, and a detection limit of 1.7 amol (S/N = 3; rms) was obtained for serotonin. A physiological running buffer was used to ensure that the cell remained viable throughout. The secretagogue was polymyxin B sulfate (Pmx). Following the injection of a single mast cell into the capillary, electromigration of Pmx toward and past the cell induced degranulation and release of serotonin. The time course of release was registered in the electropherograms with subsecond resolution. Subsequent introduction of SDS caused the cell to lyse completely and allowed the residual serotonin to be quantified. The average amount of serotonin observed per RPMC was 1.6 +/- 0.6 fmol; the average percentage of serotonin released was 28 +/- 14%. Events that are consistent with released serotonin from single submicrometer granules (250 aL each) were evident, each of which contained an average amount of 5.9 +/- 3 amol.

Titration calorimetric studies to elucidate the specificity of the interactions of polymyxin B with lipopolysaccharides and lipid A

Lipopolysaccharide (LPS), the major cell wall constituent of Gram-negative bacteria, evokes a multitude of biological effects in mammals including pyrogenicity and toxic shock syndrome. Polymyxin B (PmB), a polycationic cyclic peptide, is known to neutralize most of its activities. The nature of the interaction of PmB with LPS and lipid A was investigated by isothermal titration calorimetry. PmB binds to LPS as well as lipid A stoichiometrically and non-co-operatively with micromolar affinity. These interactions are driven primarily by a favourable change in entropy (delta S) and are endothermic in nature. These positive changes in enthalpies decrease with increasing temperature, yielding a heat capacity change, delta Cp, of -2385 J.mol-1.degree-1 for PmB-LPS interactions while the binding of PmB to lipid A displays a delta Cp of -2259 J.mol-1.degree-1. The negative heat capacity changes provide strong evidence for the role of hydrophobic interactions as the driving force for the association of PmB with LPS and lipid A. A correlation of the energetics of these interactions with analyses of the molecular models of PmB suggests that a cluster of solvent-exposed non-polar amino acid side-chains that line one surface of the molecule, together with a ring of positively charged residues on its other surface, are responsible for its strong and stoichiometric binding to LPS.

Conformation of polymyxin B analogs in DMSO from NMR spectra and molecular modeling

The tertiary structures of two polymyxin analogues: [formula: see text] and [formula: see text] in DMSO, from solid-phase peptide synthesis and aerobic oxidation were determined from two-dimensional NMR spectra and distance geometry calculations followed by restrained molecular dynamics simulation. The backbone of peptide I had a rectangular shape stabilized by at least two hydrogen bonds and the hydrophilic side chains of five lysine residues, and the hydrophobic side chains of Phe and Leu resided at both sides to form an amphiphilic molecule. This amphiphilic structure of I is likely to interact with lipid A mainly via a hydrophobic interaction. Compared with I, peptide II, which lacks three N-terminal amino-acid residues, exhibits neither amphiphilic property nor binding ability with lipid A.

Lateral inhomogeneous lipid membranes: theoretical aspects

The physical concepts underlying the lateral distribution of the components forming a lamellar assembly of amphiphiles are discussed in this review. The role of amphiphiles' molecular structure and/or aqueous environment (ionic strength, water soluble substances) on formation and stability of lateral patterns is investigated. A considerable effort is devoted to the analysis of the properties of patterned structure which can be different from those of randomly mixed multi-component lamellae. Examples include adhesion and fusion among laterally inhomogeneous bilayers, enhanced interfacial adsorption of ions and polymers, enhanced transport across the bilayer, modified mechanical properties, local stabilization of non-planar geometries (pores, edges) and related phenomena (electroporation, budding transition and so on). Furthermore, an analysis of chemical reactivity within or at the water interface of a laterally inhomogeneous bilayer is briefly discussed. A link between these concepts and experimental findings taken from the biological literature is attempted throughout the review.

Interaction of cationic amphiphilic drugs with lipid A: implications for development of endotoxin antagonists

This report presents evidence for the interactions of several classes of cationic amphiphilic drugs including the phenothiazines, aminoquinolines, biguanides, and aromatic diamidines, with lipid A, the endotoxic principle of lipopolysaccharides. The interactions of the drugs were quantitatively assessed by fluorescence methods. The affinities of the drugs for lipid A parallel their endotoxin-antagonistic effects in the Limulus gelation assay. Dicationic compounds bind lipid A with greater affinity; the affinity of such molecules increases exponentially as a function of the distance between the basic moieties. The bis-amidine drug--pentamidine--examined in greater detail, binds lipid A with high affinity (apparent Kd: 0.12 microM), and LPS, probably due to simultaneous interactions of the terminal amidine groups with the anionic phosphates on lipid A. The sequestration of endotoxin by pentamidine reduces its propensity to bind to cells, and the complex exhibits attenuated toxicity in biological assays. These results have implications in the development of therapeutic strategies against endotoxin-related disease states.

Interdigitated gel phase bilayers formed by unsaturated synthetic and bacterial glycerolipids in the presence of polymyxin B and glycerol

The ability of synthetic phosphoglycerolipids with a cis mono-unsaturated acyl chain in the 2-position and a saturated chain in the 1-position of glycerol to form interdigitated gel phase bilayers in the presence of amphipathic substances was monitored using a fatty acid spin label, 16-doxylstearic acid, and a phosphatidylglycerol spin label containing 16-doxylstearic acid. These spin labels become significantly more motionally restricted in an interdigitated gel phase bilayer than in a non-interdigitated gel phase bilayer. The results indicated that polymyxin B and polymyxin B nonapeptide caused interdigitation of 1-palmitoyl,2-oleoyl-phosphatidylglycerol (POPG) and glycerol caused interdigitation of 1-stearoyl,2-oleoyl-phosphatidylcholine (SOPC), similar to their effects on disaturated lipids. The fluidity gradient present in non-interdigitated gel phase bilayers was abolished. However, glycerol did not cause POPG to become interdigitated, in contrast to SOPC. We reported earlier that there is a kinetic barrier to interdigitation of saturated PG in the presence of glycerol, in contrast to saturated PC. This barrier is even greater for the unsaturated species of PG. Furthermore, these compounds lowered the gel to liquid-crystalline phase transition temperatures of the unsaturated lipids more than of saturated lipids suggesting that the interdigitated bilayer of the former may be less ordered or less stable than that of the latter. Since polymyxin B is an antibiotic we also examined its effect on a lipid extract from the Gram-negative bacteria Pseudomonas aeruginosa in order to assess whether interdigitation might be involved in its mechanism of bactericidal or bacteriostatic effect. Polymyxin B and polymyxin B nonapeptide also caused motional restriction of a small percentage (about 13% at -2 degrees C and 25% at -14 degrees C for polymyxin B) of the spin label in the lipid extract at low temperatures, where the lipid is in the gel phase, consistent with formation of a small domain of interdigitated bilayer lipid. However, the degree of immobilization was less than that in the interdigitated bilayers of the synthetic unsaturated lipids. This may be a result of the heterogeneous nature of the lipids in the extract. However, it cannot be ruled out that the motional restriction of the spin label in this extract may be caused by something other than interdigitation. Thus the results with the lipid extract are less conclusive of interdigitation than for the synthetic lipids. A motionally restricted population was not detectable at higher temperatures.(ABSTRACT TRUNCATED AT 400 WORDS)

Agents that increase the permeability of the outer membrane

The outer membrane of gram-negative bacteria provides the cell with an effective permeability barrier against external noxious agents, including antibiotics, but is itself a target for antibacterial agents such as polycations and chelators. Both groups of agents weaken the molecular interactions of the lipopolysaccharide constituent of the outer membrane. Various polycations are able, at least under certain conditions, to bind to the anionic sites of lipopolysaccharide. Many of these disorganize and cross the outer membrane and render it permeable to drugs which permeate the intact membrane very poorly. These polycations include polymyxins and their derivatives, protamine, polymers of basic amino acids, compound 48/80, insect cecropins, reptilian magainins, various cationic leukocyte peptides (defensins, bactenecins, bactericidal/permeability-increasing protein, and others), aminoglycosides, and many more. However, the cationic character is not the sole determinant required for the permeabilizing activity, and therefore some of the agents are much more effective permeabilizers than others. They are useful tools in studies in which the poor permeability of the outer membrane poses problems. Some of them undoubtedly have a role as natural antibiotic substances, and they or their derivatives might have some potential as pharmaceutical agents in antibacterial therapy as well. Also, chelators (such as EDTA, nitrilotriacetic acid, and sodium hexametaphosphate), which disintegrate the outer membrane by removing Mg2+ and Ca2+, are effective and valuable permeabilizers.

Polymyxin B inhibits insulin-induced glucose transporter and IGF II receptor translocation in isolated adipocytes

In isolated adipocytes, polymyxin B inhibited insulin-induced glucose incorporation into lipids in a dose-dependent manner, while polymyxin E, a structurally related antibiotic, was ineffective. To approach the mechanism of this effect, the subcellular distribution of the glucose transporter Glut 4 was investigated. Adipocytes were pretreated without or with polymyxin B before insulin stimulation, subcellular fractionation was performed and Glut 4 was detected by immunodetection. Incubation of adipocytes with polymyxin B prevented the insulin-induced appearance of Glut 4 in the plasma membranes, but did not prevent their decrease from the low-density microsomal fraction. A lower purity of the plasma membrane fractions, a detergent effect of polymyxin B on the membranes or an interference of the substance with the immunodetection of the Glut 4 molecules were excluded. These results suggest that polymyxin B was interfering with the Glut 4 translocation process stimulated by insulin in adipocytes. In a similar fashion, polymyxin B inhibited the insulin-induced increase in IGF II binding to adipocytes. This resulted from a blockade of the appearance of IGF II receptors in the plasma membranes. Since low-molecular-mass GTP-binding proteins have been implicated in the regulation of vesicular trafficking, we have used [alpha-32P]GTP binding to analyze such proteins in adipocyte fractions, after SDS/PAGE and transfer to nitrocellulose. Specific and distinct subsets of GTP-binding proteins were revealed in plasma membrane and low-density microsomal fractions of control adipocytes, whether they were stimulated or not with insulin. Polymyxin B treatment of adipocytes markedly modified the profile of the low-molecular-mass GTP-binding proteins in plasma membranes, but not in low-density microsomal fractions. Our results suggest that polymyxin B was interfering with the exocytotic process of the Glut 4 and IGF II receptor-containing vesicles, perhaps at the fusion step between vesicles and plasma membranes.

A comparative study on interactions of alpha-aminoisobutyric acid containing antibiotic peptides, trichopolyn I and hypelcin A with phosphatidylcholine bilayers

Interactions of alpha-aminoisobutyric acid containing antibiotic peptides, trichopolyn I and hypelcin A with phosphatidylcholine bilayers were investigated to obtain some basic information on their bioactive mechanisms. Trichopolyn I as well as hypelcin A induced the leakage of a fluorescent dye, calcein, entrapped in sonicated egg yolk L-alpha-phosphatidylcholine vesicles. A quantitative analysis revealed that both the binding affinity and the 'membrane-perturbing activity' of trichopolyn I to the vesicles are about one-third of those of hypelcin A. The conformations and the orientations of the peptide and lipid molecules in the membranes were studied using polarized Fourier transform infrared-attenuated total reflection spectroscopy, circular dichroism, and differential scanning calorimetry. In phosphatidylcholine bilayers, both peptides mainly conformed to helical structures irrespective of the membrane physical state (gel or liquid-crystalline). The helix axes, penetrating the hydrophobic region of the bilayers, were oriented neither parallel nor perpendicular to the membrane normal. The disruption in the lipid packing induced by the peptide insertion seems to be responsible for the leakage by these peptides.

Chemiosmotic systems in medicine

The concept of chemiosmotic systems arises from the pioneering work of Peter Mitchell on two fronts. One is concerned with the mechanisms by which molecules are transported across membranes which are generally barriers to such transport. These mechanisms are inevitably molecular, and are now yielding their secrets to a combination of structural protein chemistry and molecular biology. The other front is more physiological, and explores the functional relationships between metabolism and transport. Nevertheless, the two fronts form a continuum of mutually related structure and function. Chemiosmotic systems provide a hierarchy of complexity, starting from say a uniporter reconstituted in a chemically defined bilayer, and proceeding to greater complexity in mitochondria, chloroplasts, eukaryotic and prokaryotic cell membranes, and multicellular systems. Their relationship to medicine is profound, because they provide many opportunities for therapeutic intervention. In this paper I present an overview of chemiosmotic systems at different levels of complexity, both molecular and biological, of their involvements in pathology, and of possible pharmacological treatment or prevention of disease.

Interactions of an antimicrobial peptide, tachyplesin I, with lipid membranes

Tachyplesin I, isolated from the acid extracts of hemocytes of Tachypleus tridentatus, is a cyclic broad-spectrum antimicrobial peptide forming a rigid, antiparallel beta-sheet because of two intramolecular S-S linkages. The strong binding of the peptide to lipopolysaccharides cannot explain the susceptibilities of gram positive bacteria and fungi to the peptide. We found that tachyplesin I caused a rapid K+ efflux from Escherichia coli cells, concomitant with a reduced cell viability. This result suggests that the peptide-induced permeability enhancement of the bacterial membranes may be a plausible action mechanism. Thus, we studied the interactions of tachyplesin I with various large unilamellar vesicles (LUVs) to reveal the molecular machinery of the antimicrobial activity. Tachyplesin I induced the leakage of calcein, a trapped fluorescent marker, from LUVs of acidic phospholipids, especially phosphatidylglycerol (PG), but not from phosphatidylcholine LUVs. A detailed analysis found that the affinity of the peptide to the PG membranes is very strong and that the binding of one peptide molecule to approx. 200 lipid molecules leads to a significant leakage. The location of tachyplesin I in membranes was estimated by use of the Trp-2 fluorescence of the peptide. The presence of PG LUVs caused a blue shift of the maximum wavelength, an increase in the quantum yield, and a complete protection from fluorescence quenching by an aqueous quencher, acrylamide. Moreover, the degree of fluorescence quenching of the Trp residue by n-doxylstearates was in the order n = 5 greater than 7 greater than 12 approximately equal to 16. These results show that the Trp residue of tachyplesin I seems to locate in a hydrophobic environment near the surface of the PG bilayers.

The susceptibility of conjugative resistance transfer in gram-negative bacteria to physicochemical and biochemical agents

Over thirty years of studies have established that conjugative transfer of plasmid-encoded resistance to drugs and heavy metals can take place at high frequency between various organisms under laboratory conditions. The detected transfer frequencies in soil, in aquatic environments, and in the urogenital and respiratory tracts of healthy animals and man have generally been low. However, the conversion of bacteria from susceptible to resistant to antibiotics has been observed often during antimicrobial therapy. This has formed a challenge for the antibacterial treatment of pathogenic bacteria and called for the evaluation of the extent of conjugative transfer in various environments. Several biochemical and physicochemical factors inhibit conjugation, show preferential toxicity against plasmid-bearing cells, or stimulate plasmid curing. These factors include various agents such as detergents, anesthetics, mutagens and antibiotics which affect membrane potential, membrane permeability, protein synthesis and the processing of DNA. The application of the data on these agents, summarized in this review, might be helpful in preventing drug multi-resistance from spreading. Also these data might be valuable in studies which use conjugation as a tool or which treat the molecular mechanisms involved in conjugation.

Physicochemical determinants for the interactions of magainins 1 and 2 with acidic lipid bilayers

Permeability enhancement of acidic lipid small unilamellar vesicles (dioleoylphosphatidylglycerol, DOPG; dipalmitoylphosphatidylglycerol, DPPG; bovine brain phosphatidylserine, PS) induced by magainins 1 and 2, basic antimicrobial peptides from Xenopus skin, was investigated at 30 degrees C based on leakage of calcein, an entrapped fluorescent marker. Both the peptide concentration and the lipid concentration dependencies of the leakage rate were analyzed to obtain the binding isotherms of the peptides to the membranes and the 'membrane-perturbing activities' of the membrane-bound peptides. For both peptides, the binding affinity was in the order DOPG greater than DPPG greater than PS, which coincided with the zeta potential order (-54, -39, and -9 mV, respectively). An increase in salt concentration of the medium reduced binding and leakage. Electrostatic interactions play a crucial role in the binding process. On the other hand, the membrane-perturbing activity is regulated by membrane fluidity: The fluid membranes (DOPG and PS) were leakier. A circular dichroism study suggested that at least 14 positively charged residues in the N-terminal regions can form amphiphilic helices which interact with the membranes. An even stronger binding of magainin 2 can be explained in terms of more positive charges in its N-terminal region. A tentative model for the magainin-lipid interactions is hypothesized.

Interaction of phospholipids with proteins, peptides and amino acids. New advances 1987-1989

1. The review deals with the recent achievements in the study of the various interactions of phospholipids with proteins, peptides and amino acids. The interactions are classified according to the hydrophobic, hydrophilic or mixed character of the interactive forces. The effect of the interaction on the structure and biological activity of the interacting biomolecules is discussed.

Phase separation in phospholipid bilayers induced by biologically active polycations

The interaction of various polyionenes with phospholipid bilayer membranes was explored by means of differential scanning calorimetry (DSC) with special reference to their antimicrobial activities. A strong interaction was observed between the polyionenes and acidic phospholipids, whereas zwitterionic phospholipid bilayers were not affected significantly by the polycations. Addition of the polyionenes was found to result in phase separation in mixed bilayer membranes composed of acidic and zwitterionic phospholipids. The ability to induce phase separation strongly depended on the structure of the polyionenes. Polyionenes with rigid spacers were found to be most effective to induce phase separation and to be most active in antimicrobial activity. Polyionenes with rigid and flexible spacers in the alternate fashion exhibited less activities which were similar to those of all flexible spacers. Furthermore, their mode of interaction with bilayers was again similar to those of all flexible spacers. Our results indicate that the rigid spacers are favorable for strong interaction with membranes which are assumed to be the target sites of the polycationic biocides, leading to the higher activity. Other factors affecting both the antimicrobial activity and the mode of interaction with membranes were molecular weight and hydrophobicity. With increasing molecular weight, both the activity and ability to induce phase separation increased. Introduction of hydrophilic groups into the spacers resulted in loss of activity and ability to induce phase separation. The antimicrobial activity and the mode of interaction with membranes were correlated and interpreted on the basis of conformational concept of the polyionenes in solution.

Use of spin labels to determine the percentage of interdigitated lipid in complexes with polymyxin B and polymyxin B nonapeptide

Long chain spin labels with the nitroxide group located near the terminal methyl of the chain were used to determine the percentage interdigitated lipid in complexes of polymyxin B (PMB) and polymyxin B nonapeptide (PMBN) with the acidic lipids dipalmitoylphosphatidylglycerol (DPPG) and dipalmitoylphosphatidic acid (DPPA) at varying mole ratios of drug to lipid and at different pH values. These spin labels are more motionally restricted in the interdigitated than in the non-interdigitated gel phase bilayer. This allows determination of the percentage interdigitated lipid by resolution of the spectrum into motionally restricted and more mobile components. At nonsaturating concentrations of PMB, significantly more DPPG than that which can be maximally PMB-bound, becomes interdigitated. As the temperature approaches the gel to liquid crystalline phase transition temperature, the bilayer becomes progressively non-interdigitated. The ESR spectrum indicates that PMB also causes interdigitation of DPPA. However, in contrast to DPPG, the amount of DPPA which is interdigitated at pH 6, is less than the amount which is expected to be PMB-bound. This is attributed to the ability of DPPA to participate in lateral interlipid hydrogen bonding interactions. Such lateral interactions would be abolished in the interdigitated bilayer and thus they are expected to inhibit its formation. At pH 9, where the interlipid interactions of DPPA are weakened, PMB induces even more lipid than that which is PMB-bound to become interdigitated. Indeed, the percentage interdigitated lipid is even greater than found for DPPG. This may be partly a result of the greater negative charge of DPPA at this pH. A greater repulsive negative charge is expected to favor interdigitation. PMBN is less effective than PMB at inducing interdigitation of DPPG and causes little or no interdigitation of DPPA at pH 6, even at saturating concentrations. PMBN also does not lower the phase transition temperature of DPPA at pH 6 as much as PMB. At pH 9, the effect of PMBN on DPPA is more similar to the effect of PMB. However, even for DPPG, and DPPA at pH 9, PMBN does not maintain interdigitation of the lipids at higher temperatures as effectively as PMB. PMBN's smaller perturbing effect and greatly decreased ability to cause interdigitation of DPPA at pH values below 9 may be related to a decreased ability to cause lateral separation of the lipid molecules, which is necessary in order to weaken the interlipid interactions.(ABSTRACT TRUNCATED AT 400 WORDS)

Interdigitation of phosphatidylcholine and phosphatidylethanolamine mixed with complexes of acidic lipids and polymyxin B or polymyxin B nonapeptide

A fatty acid spin label, 16-doxyl-stearic acid, was used to determine the percent interdigitated lipid in mixtures of a neutral phospholipid and an acidic phospholipid. Interdigitation of the acidic lipid was induced with polymyxin B (PMB) at a mole ratio of PMB to acidic lipid of 1:5. This compound does not bind significantly to neutral lipids or induce interdigitation of the neutral lipids by themselves. The neutral lipids used were dimyristoylphosphatidylcholine (DMPC), dipalmitoylphosphatidylcholine (DPPC), or dipalmitoylphosphatidylethanolamine (DPPE), and the acidic lipids were dipalmitoylphosphatidylglycerol (DPPG) or dipalmitoylphosphatidic acid (DPPA). The percent interdigitated lipid was determined from the percent of the spin label which is motionally restricted, assuming that the spin label is homogeneously distributed in the lipid. Assuming further that 100% of the acidic lipid is interdigitated at this saturating concentration of PMB, the percentage of the neutral lipid which can become interdigitated along with it was calculated. The results indicate that about 20 mole % DPPC can be incorporated into and become interdigitated in the interdigitated bilayer of PMB/DPPG at 4 degrees C. As the temperature approaches the phase transition temperature, the lipid becomes progressively less interdigitated; this occurs to a greater degree for the mixtures than for the single acidic lipid. Thus the presence of DPPC promotes transformation of the acidic lipid to a non-interdigitated bilayer at higher temperatures. At the temperature of the lipid phase transition little or none of the lipid in the mixture is interdigitated. Thus the lipid phase transition detected by calorimetry is not that of the interdigitated bilayer. The shorter chain length DMPC can be incorporated to a greater extent than DPPC, 30-50 mol%, in the interdigitated bilayer of PMB-DPPG. This may be a result of reduced exposure of the terminal methyl groups of the shorter myristoyl chains at the polar/apolar interface of the interdigitated bilayer. Less than 29% of the total lipid was interdigitated in a DPPC/DPPA/PMB 1:1:0.2 mixture indicating that none of the DPPC in this mixture becomes interdigitated. This is attributed to the lateral interlipid hydrogen bonding interactions of DPPA which inhibits formation of an interdigitated bilayer. DPPE was found to be incorporated into the interdigitated bilayer of PMB-DPPG to a similar extent as DPPC if the amount of PMB added is sufficient to bind to only the DPPG in the mixture. Differential scanning calorimetry showed that the remaining non-interdigitated DPPE-enriched mixture phase separates into its own domain.(ABSTRACT TRUNCATED AT 400 WORDS)

Behavior of spin labels in a variety of interdigitated lipid bilayers

The behavior of a number of spin labels in several lipid bilayers, shown by X-ray diffraction to be interdigitated, has been compared in order to evaluate the ability of the spin label technique to detect and diagnose the structure of lipid bilayers. The main difference between interdigitated and non-interdigitated gel phase bilayers which can be exploited for determination of their structure using spin labels, is that the former have a much less steep fluidity gradient. Thus long chain spin labels with the nitroxide group near the terminal methyl of the chain, such as 16-doxylstearic acid, its methyl ester, or a phosphatidylglycerol spin label containing 16-doxylstearic acid (PG-SL), are more motionally restricted and/or ordered in the interdigitated bilayer than in the non-interdigitated bilayer. This difference is large enough to be of diagnostic value for all three spin labels in the interdigitated bilayers of dihexadecylphosphatidylcholine, dipalmitoylphosphatidylcholine/ethanol, and 1,3-dipalmitoylphosphatidylcholine. However, it is not large enough to be of diagnostic value at low temperatures. Use of probes with the nitroxide group closer to the apolar/polar interface reveals that these latter interdigitated bilayers are more disordered or less closely packed. As the temperature is increased, however, the motion of the PG-SL does not increase as much in these interdigitated bilayers as in non-interdigitated bilayers. The difference in the motion and/or order of PG-SL between interdigitated and non-interdigitated bilayers is large enough at higher temperatures to be of value in diagnosing the structure of the bilayers. Thus by choice of a suitable spin label and a suitable temperature, this technique should prove useful for detection and diagnosis of lipid bilayer structure with a good degree of reliability. Caution must, of course be exercised, as with any spectroscopic technique. Spin labels will also be invaluable for more detailed studies of known interdigitated bilayers, which would be time- and material-consuming, if carried out using X-ray diffraction solely.

Basic amphipathic helical peptides induce destabilization and fusion of acidic and neutral liposomes

We have studied the fusion of small unilamellar vesicles composed of egg PC and of a mixture of egg PC plus egg PA using various basic amphipathic peptides. Fusion was monitored by carboxyfluorescein leakage assay, light scattering, membrane intermixing assay, contents mixing assay and electron microscopy. Ac-(L-Leu-L-Ala-L-Arg-L-Leu)3-NHCH3 (peptide 4(3] and Ac-(L-Leu-L-Ala-L-Lys-L-Leu)3-NHCH3 (peptide 4'3), which have high hydrophobic moments, caused transformation of small unilamellar vesicles into larger and relatively homogeneous ones. Ac-(L-Leu-L-Leu-L-Ala-L-Arg-L-Leu)2-NHCH3 (5(2], which has medium hydrophobic moment, induced weak but appreciable fusion, while Ac-(L-Ala-L-Arg-L-Leu)3-NHCH3 (3(3] which has no helical structure did not show any fusion. However, peptides 4(3), 4'3 and 5(2) caused massive leakage of the contents from small unilamellar vesicles. These results indicated that interaction of the peptides with artificial membranes caused extensive perturbation of the lipid bilayer, followed by fusion. The fusogenic capacity of model basic peptides was correlated with the hydrophobic moment of each peptide when the peptides adopted an alpha-helical structure in the presence of acidic liposomes. Peptides 4(3) and 4'3 also showed weak fusogenic ability for neutral liposomes, while 5(2) and 3(3) showed no ability, suggesting that highly amphipathic peptides, such as 4(3), interact weakly but distinctly with neutral liposomes to fuse them.

Intermixing of dipalmitoylphosphatidylcholine with phospho- and sphingolipids bearing highly asymmetric hydrocarbon chains

We have used high-sensitivity differential scanning calorimetry to investigate the mixing of dipalmitoylphosphatidylcholine (DPPC) with N-lignoceroylgalactocerebroside, N-lignoceroylsulfogalactocerebroside and 1-lauroyl-2-lignoceroylphosphatidylcholine. These three lignoceroyl species, whose two hydrocarbon chains are quite discrepant in length, are completely miscible with DPPC in the liquid-crystalline state. Mixtures of all three lignoceroyl lipids with DPPC show phase separation in the gel state, which is observed over a limited range of compositions (from less than 10 mol% to just over 40 mol% sulfatide) in the case of N-lignoceroylsulfatide and over a wide range of compositions in the cases of N-lignoceroylcerebroside (less than 10 mol% to greater than 90 mol% cerebroside) and 1-lauroyl-2-lignoceroyl-PC (roughly 10 mol% to 90 mol% lauroyl/lignoceroyl PC). The extensive solid-solid phase separation observed in mixtures of DPPC and 1-lauroyl-2-lignoceroyl-PC, which show eutectic behavior, is somewhat unexpected given the similar transition temperatures of the two components but appears to reflect the ability of the lignoceroyl species to form an interdigitated gel phase. However, we find no evidence that the N-lignoceroylsphingolipids are markedly more prone to segregate laterally in PC-rich bilayers than are previously studied sphingolipid species with shorter N-acyl chains. We suggest on the basis of these results that the primary biological importance of the very long N-acyl chains found in many sphingolipids may lie in some function other than the promotion of lateral segregation of sphingolipid-enriched domains in biological membranes.

Use of electron spin resonance spectroscopy of spin labels for studying drug-induced membrane perturbation

The use of electron spin resonance spectroscopy of spin labels is reviewed in the context of drug-induced membrane perturbation. The correlation between membrane perturbation and biological effects is also considered.

Influence of beta-adrenoceptor blocking drugs on lipid-protein interaction in synaptosomal membranes. An ESR study

The influence of the beta-adrenoceptor blocking drugs atenolol, doberol, propranolol and exaprolol on synaptosomal membranes was studied using ESR spectroscopy of stearic acid spin labeled at the 16th position. The drugs changed the ESR spectra of the label in the membranes, where in addition to changes of a fluid lipid component they increased the proportion of a motionally-restricted component. No motionally-restricted component was found in the samples prepared from brain total lipid liposomes treated with the drugs. The drug propensities at 20 mmol/l concentration to increase the proportion of the motionally-restricted component in the following order, control less than doberol approximately atenolol less than or equal to propranolol less than exaprolol did not correlate with their potency to influence the dynamics of the bulk lipid membrane phase. The motionally-restricted component induced by exaprolol increased with raising temperature and prolongation of time of the sample incubation. The results indicate that the beta-adrenoceptor blocking drugs influence lipid-protein interaction in the synaptosomal membranes, which could be important for elucidation of their mechanism of biological membrane activities.

The DNA synthesis of leukemic (L2C) guinea pig B lymphocytes involves a permanent activation of protein kinase C without corresponding phosphoinositide hydrolysis

L2C B lymphocytes have a constant high DNA synthesis due to their continuous proliferative state. The addition of polymyxin B (PmB), a rather selective inhibitor of protein kinase C, stopped (3H)thymidine incorporation with an IC50 of 10 microM when added 18 h before measuring DNA synthesis. Interestingly, PmB inhibition of DNA synthesis was suppressed when 4 nM 12-O-tetradecanoylphorbol-13-acetate was added along with PmB, indicating that PmB may act through inhibition of protein kinase C. In the node and spleen lymphocytes of normal guinea pigs, protein kinase C activity was entirely cytosolic and was eluted at 0.12 M NaCl when adsorbed on DEAE-cellulose. In L2C leukemic lymphocytes, total protein kinase C activity was of the same order of magnitude, but 20% of it was associated with the membrane fraction. The lipid-dependent activity, eluted at 0.12 M NaCl from cytosolic and membrane fractions, was suppressed by staurosporine with an IC50 of 10-40 nM and by polymyxin B with an IC50 of 2-6 microM. Phosphoinositide metabolism was studied in the transformed cells. Incorporation of 32Pi into polyphosphoinositides was considerable, whereas much more time was required for a tiny incorporation of inositol. We detected no release of radioactive inositol triphosphate. Taken together, these results suggest that protein kinase C function is indispensible for triggering L2C leukemic lymphocyte proliferation. The causes of this permanent activation merit further investigation.

Interaction of polymyxin B1 and polymyxin B1 nonapeptide with phosphatidic acid monolayer and bilayer membranes

The interactions of the antibiotic polymyxin B1 and its enzymatic cleavage product polymyxin B1 nonapeptide with phosphatidic acid monolayers and with bilayer membranes were investigated. Temperature-dependent pressure-area analysis of the monolayer reveals a linear increase of the lipid mean molecular area in the liquid condensed state for polymyxin concentrations between 10(-8) and 4 x 10(-7) M. Depending on the surface pressure, the area increase amounts to 30-70 A2. A linear dependence was also observed in the liquid expanded state but saturation is reached already at 10(-7) M polymyxin. The adsorption of polymyxin to phosphatidic acid bilayers is also linear and of a Langmuir type. Saturation is reached at a 1:4 polymyxin/lipid molar ratio. Polymyxin induces a phase separation in phosphatidic acid monolayers which was concluded from the thermotropic phase transition curves. In agreement with earlier bilayer experiments a second lowered phase transition appears in the presence of polymyxin. These fluidized domains again exhibit a linear polymyxin uptake comparable to the one of the liquid expanded monolayer at a temperature, where the undisturbed lipid is still in the condensed state. Polymyxin nonapeptide also causes an expansion of phosphatidic acid monolayers but only by maximally 10 A2. The thermotropic phase transition of the monolayer is reduced and considerably broadened by the nonapeptide. In phosphatidic acid bilayers we observed a decrease of the lipid phase transition temperature by 24 degrees C. The lateral chain packing is considerably disturbed by the peptide part of polymyxin.(ABSTRACT TRUNCATED AT 250 WORDS)

Interdigitated lipid bilayers of long acyl chain species of cerebroside sulfate. A fatty acid spin label study

The metastable phase behavior of semi-synthetic species of cerebroside sulfate (CBS), with hydroxy and non-hydroxy fatty acids from 16 to 26 carbons in length, was compared in Li+ and K+ using differential scanning calorimetry. The structure of the metastable and various stable phases formed in the presence of these two cations was investigated using a fatty acid spin label, 16-doxylstearate. A number of stable phases with successively higher phase transition temperatures and enthalpies occur in the presence of K+ (see the preceding paper). Li+ prevents formation of the most stable phases with the highest transition temperatures and enthalpies for all species of CBS. However, it does not prevent a transition from the metastable phase to the first stable phase of the longer chain C24 and C26 species. Furthermore, it allows C24:0h-CBS to undergo a similar transition, in contrast to a high K+ concentration, which prevents it. The spin label has anisotropic motion in the metastable gel phase formed by all species of CBS on cooling from the liquid crystalline phase. The spectra resemble those in gel phase phospholipids. The spin label is partially insoluble in the most stable phases formed by all the lipids, including the unsaturated C24:1 species, preventing further elucidation of their structure using this technique. However, the spin label is soluble in the first stable phase formed on cooling by the longer chain C24:0 and C26:0-CBS in Li+ and K+ and by C24:0h-CBS in Li+, and is motionally restricted in this phase. The motional restriction is similar to that observed in the mixed interdigitated bilayers of asymmetric species of phosphatidylcholine and fully interdigitated bilayers formed by symmetric phospholipids. It strongly suggests that the highly asymmetric long chain species of CBS form a mixed interdigitated bilayer in their first stable gel phases while the metastable phase of these and the shorter chain lipids may be partially interdigitated. The metastable phase of C24:1-CBS is more disordered suggesting that it may not be interdigitated at all. Thus the results suggest that (i) the hydroxy fatty acid inhibits but does not prevent formation of a mixed interdigitated bilayer by long chain species of CBS, (ii) an increase in non-hydroxy fatty acid chain length from 24 to 26 carbons promotes it, and (iii) a cis double bond probably prevents any form of interdigitation. These results may be relevant to the physiological and pathological roles of these structural modifications of CBS.