Selective binding of polymyxin B to negatively charged lipid monolayers

Candida glabrata complex from patients with healthcare-associated infections in Mansoura University Hospitals, Egypt: distribution, antifungal susceptibility and effect of fluconazole and polymyxin B combination

Introduction

Candida glabrata complex is composed of three cryptic species, Candida glabrata sensu stricto, C. bracarensis and C. nivariensis. Its reduced susceptibility to fluconazole is responsible for treatment failure of its infections. Combination therapy is recommended to treat these resistant strains. This study assessed the distribution of C. glabrata complex collected from patients in Mansoura University Hospitals and their antifungal susceptibility, with evaluation of fluconazole and polymyxin B combination against them.

Methods

C. glabrata complex was collected from patients with healthcare-associated infections. The isolates were identified biochemically then the species were detected using multiplex PCR. The susceptibility of isolates to antifungals and polymyxin B was tested by the microdilution assay. The effect of fluconazole and polymyxin B combination was assessed by checkerboard microdilution and time kill assays.

Results

This study included 45 isolates of Candida glabrata complex. The common isolate was Candida glabrata sensu stricto (38 isolates). There were 4 isolates of C. bracarensis and three isolates of C. nivariensis. All isolates were susceptible to amphotericin B, and 17.8% were susceptible to itraconazole. Regarding fluconazole, 48.9% of isolates were susceptible dose-dependent and 51.1% were resistant. Synergistic effect of the combination was observed in 68.9% of isolates by the checkerboard method and in 66.7% of isolates using the time kill assay.

Conclusions

Candida glabrata sensu stricto is the main member of the complex. Combination of fluconazole and polymyxin B can be considered a treatment option for fluconazole resistant C. glabrata complex infections. In vivo studies are needed to validate these results.

Antimicrobial Octapeptin C4 Analogues Active against Cryptococcus Species

Resistance to antimicrobials is a growing problem in both developed and developing countries. In nations where AIDS is most prevalent, the human fungal pathogen Cryptococcus neoformans is a significant contributor to mortality, and its growing resistance to current antifungals is an ever-expanding threat. We investigated octapeptin C4, from the cationic cyclic lipopeptide class of antimicrobials, as a potential new antifungal. Octapeptin C4 was a potent, selective inhibitor of this fungal pathogen with an MIC of 1.56 μg/ml. Further testing of octapeptin C4 against 40 clinical isolates of C. neoformans var. grubii or neoformans showed an MIC of 1.56 to 3.13 μg/ml, while 20 clinical isolates of C. neoformans var. gattii had an MIC of 0.78 to 12.5 μg/ml. In each case, the MIC values for octapeptin C4 were equivalent to, or better than, current antifungal drugs fluconazole and amphotericin B. The negatively charged polysaccharide capsule of C. neoformans influences the pathogen's sensitivity to octapeptin C4, whereas the degree of melanization had little effect. Testing synthetic octapeptin C4 derivatives provided insight into the structure activity relationships, revealing that the lipophilic amino acid moieties are more important to the activity than the cationic diaminobutyric acid groups. Octapeptins have promising potential for development as anticryptococcal therapeutic agents.

Polymyxin B, in combination with fluconazole, exerts a potent fungicidal effect

OBJECTIVES

The objective of this study was to identify existing clinical compounds that either possess a fungicidal activity alone or can act synergistically with fungistatic antifungals.

METHODS

We screened a clinical compound library for drugs that exhibited anti-Aspergillus activity. Among selected compounds, the cationic peptide antibiotic polymyxin B was chosen for further characterization because it can be used parenterally and topically. The fungicidal effect of polymyxin B and its synergistic interactions with azole antifungals were tested against a variety of fungal species. The toxicity of the drug combination of polymyxin B and fluconazole was compared with that of each drug alone in mammalian cell cultures.

RESULTS

We found that polymyxin B possesses a broad-spectrum antifungal activity at relatively high concentrations. However, because of its synergistic interactions with azole antifungals, polymyxin B at much lower concentrations exerts a potent fungicidal effect against Cryptococcus neoformans, Candida albicans and non-albicans Candida species and moulds when combined with azoles. The combination of polymyxin B and fluconazole at concentrations within susceptible breakpoints is particularly potent against C. neoformans isolates, including fluconazole-resistant strains. The drug combination displayed no additional toxicity compared with polymyxin B alone when tested in cell culture.

CONCLUSIONS

The combination of polymyxin B and fluconazole has the potential to be used in the clinic to treat systemic cryptococcosis. Our findings suggest that combining cationic peptide antibiotics with azole antifungals could provide a new direction for developing novel antifungal therapies.

Polymyxin B binds to anandamide and inhibits its cytotoxic effect

Anandamide (ANA), an endogenous cannabinoid, can be generated by activated macrophages during endotoxin shock and is thought to be a paracrine contributor to hypotension. We discovered that ANA in saline/ethanol solution and in serum was efficiently adsorbed in a polymyxin B (PMB)-immobilized beads column and eluted with ethanol. We confirmed the direct binding of PMB to ANA by using surface plasmon resonance. The adsorption of ANA by PMB may abolish the diverse effects of ANA such as hypotension, immunosuppression, and cytotoxicity, and may suggest a new therapeutic strategy for endotoxin shock.

Activities of LL-37, a cathelin-associated antimicrobial peptide of human neutrophils

Human neutrophils contain two structurally distinct types of antimicrobial peptides, beta-sheet defensins (HNP-1 to HNP-4) and the alpha-helical peptide LL-37. We used radial diffusion assays and an improved National Committee for Clinical Laboratory Standards-type broth microdilution assay to compare the antimicrobial properties of LL-37, HNP-1, and protegrin (PG-1). Although generally less potent than PG-1, LL-37 showed considerable activity (MIC, <10 microgram/ml) against Pseudomonas aeruginosa, Salmonella typhimurium, Escherichia coli, Listeria monocytogenes, Staphylococcus epidermidis, Staphylococcus aureus, and vancomycin-resistant enterococci, even in media that contained 100 mM NaCl. Certain organisms (methicillin-resistant S. aureus, Proteus mirabilis, and Candida albicans) were resistant to LL-37 in media that contained 100 mM NaCl but were susceptible in low-salt media. Burkholderia cepacia was resistant to LL-37, PG-1, and HNP-1 in low- or high-salt media. LL-37 caused outer and inner membrane permeabilization of E. coli ML-35p. Chromogenic Limulus assays revealed that LL-37 bound to E. coli O111:B4 lipopolysaccharide (LPS) with a high affinity and that this binding showed positive cooperativity (Hill coefficient = 2.02). Circular dichroism spectrometry disclosed that LL-37 underwent conformational change in the presence of lipid A, transitioning from a random coil to an alpha-helical structure. The broad-spectrum antimicrobial properties of LL-37, its presence in neutrophils, and its inducibility in keratinocytes all suggest that this peptide and its precursor (hCAP-18) may protect skin and other tissues from bacterial intrusions and LPS-induced toxicity. The potent activity of LL-37 against P. aeruginosa, including mucoid and antibiotic-resistant strains, suggests that it or related molecules might have utility as topical bronchopulmonary microbicides in cystic fibrosis.

Modulation of polymyxin B effects on mammalian urinary bladder

This report demonstrates that Ca2+, Mg2+, and protons alter the ability of polymyxin B (PX, a cationic antibiotic used clinically as a bactericidal agent) to increase the apical membrane conductance of the rabbit urinary bladder. Using electrophysiological methods, we determine that these alterations occur by two mechanisms. First, they blocked the PX-induced conductance in a rapid and reversible manner; second, they competed with PX for a membrane binding site. In addition, Ca2+ (but not Mg2+ or protons) altered the rate at which the induced conductance could be reversed. When solution pH was greater than 8.8, PX was not able to induce a conductance. This ability of high pH to inhibit the action of PX was due to a decrease in the number of positive charges on PX. Further studies demonstrated that for maximal activity, PX required its fatty acid tail. These data were used to develop a model describing the mechanism by which PX can induce a conductance in the apical membrane of the rabbit urinary bladder.

Anionic phospholipids calcium binding sites in Duchenne and murine X-linked muscular dystrophy

Duchenne muscular dystrophy (DMD) and murine X-linked muscular dystrophy (mdx) are genetically homologous and both characterized by absence of dystrophin. The function of this protein is not defined nor is the pathogenesis of the severe muscle necrosis and progressive weakness found in DMD but not in mdx. Recently we found that anionic phospholipid (AP) calcium binding sites are lacking at the muscle cell surface in DMD and we correlated these data with dystrophin deficiency and muscle necrosis. In order to verify the role of AP lack in the pathogenesis of muscle necrosis in DMD we studied the ultrastructural localization of these Ca++ receptors in mdx muscle membrane showing that they are normally represented as they are in control mouse and normal human muscle. The absence of AP in DMD compared with a normal distribution in mdx suggests that these calcium binding site alterations play an important and specific role in muscle fiber necrosis.

Lack of anionic phospholipid calcium binding sites in Duchenne muscular dystrophy

We studied membrane ultrastructural localization of anionic phospholipids (AP) and sialic acid (SA) calcium binding sites in muscle biopsies from Duchenne muscular dystrophy (DMD) and 3 Becker's muscular dystrophy (BMD) patients using polymyxin B (PXB) and limulus polyphemus (LP) as cytochemical markers. We found that AP calcium binding sites are lacking at muscle cell surface in all DMD muscle tissues, in both intact and degenerating muscle fibers. In BMD, AP have an unusual distribution along plasma membrane. Sialic acid calcium binding sites have the same localization along plasma membrane and basal lamina in DMD, BMD, and control muscles. The absence or alterations of structures involved in calcium binding in DMD and BMD may alter membrane calcium permeability, leading to abnormal Ca2+ influx into cells causing muscle necrosis.

Ultrastructural localization of calcium binding sites on human muscle cell surface

Calcium (Ca2+) is mainly bound to anionic phospholipids and to sialic acid at the cell surface. We studied the ultrastructural localization of these Ca2+ binding sites in normal human muscle fibers, using Polymyxin B as a marker for anionic phospholipids and the lectin Limulus Polyphemus as a probe for sialic acid. We found that anionic phospholipids have a patchy distribution along the muscle sarcolemma, with a preferential localization at the I band level and at the junction between the I and A band. Sialic acid has an uniform distribution along the muscle plasma membrane and basal lamina. Our observations suggest that the plasma membrane, basal lamina, and transverse tubular system play an important role in providing the negative charge of the human muscle cell surface and that these structures may be involved in the binding of calcium.

Tritrichomonas foetus: freeze-fracture cytochemistry using polymyxin B

The general structure of Tritrichomonas foetus incubated in the presence of the peptide antibiotic polymyxin B, which interacts specifically with anionic phospholipids, was analyzed using transmission electron microscopy of thin sections, and freeze-fracture replicas. Polymyxin B induced morphological changes in the plasma membrane of the parasites with the formation of membrane blebs with a diameter varying from 65 nm to 1.5 micron. Freeze-fracture images of the membrane lining the blebs showed that their inner membrane half is smooth. However, membrane particles, with a density similar to that observed on the E face of the plasma membrane, were seen on the outer half of this membrane.

Alterations of phospholipid asymmetry in the membrane of spontaneously aggregated platelets in diabetes

The changes of asymmetric distribution of anionic phospholipids of human platelets in diabetic patients were studied by fluorescent and freeze fracture cytochemistry, using merocyanine 540 (MC 540) and polymyxin B (PxB) as specific markers. The membrane anionic phospholipids were detected with PxB, a membrane nonpermeant probe, used either in native form for freeze fracture electron microscopy or as dansylated or iodinated derivative for fluorescence microscopy or gamma counting, respectively. MC 540 is a naturally fluorescent probe which reportedly inserts into less packed bilayer domains. Both in platelet rich plasma and in washed platelets obtained from diabetic patients, some small platelet aggregates were observed, their number being generally dependent on the level of hyperglycemia. In contrast with single platelets, the aggregated ones bind PxB as revealed by all assay methods. The fluorescence microscopic studies with dansyl PxB and MC 540 displayed a strong binding of the fluorescent markers to aggregated platelets. The electron microscopic examination of freeze fracture replicas showed the appearance of characteristic PxB-induced deformations in the plasmalemma of aggregated platelets. The gamma counting of 125I-PxB incubated samples indicates significant differences on the platelets of diabetic patients as compared to those obtained from healthy subjects. Our data provide evidence that in diabetic patients, the spontaneous aggregated platelets are a result of the appearance of the anionic phospholipids in the outer half of plasmalemma. These changes may enhance the procoagulant activity and should represent a determinant of activated platelet recognition and their removal from circulation by splenic macrophages.

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)

Distribution of sterols and anionic lipids in human sperm plasma membrane: effects of in vitro capacitation

Filipin, a membrane beta-hydroxysterol probe, and polymyxin B (PXB), a probe for anionic lipids, were used to study human sperm plasma membrane (PM) with particular reference to changes induced by capacitation in vitro. In washed but noncapacitated spermatozoa the density of filipin/sterol complexes (FSC) was uniformly high in the PM overlying the acrosome, without any differences between its anterior and equatorial regions. The postacrosomal region was poor in FSC. The exclusion of FSC from small areas of the PM covering the two acrosomal regions and reduction of their number in the postacrosomal region were the main changes induced by sperm capacitation. Unlike filipin, PXB produced a capacitation-independent reaction pattern characterized by a high reactivity of the PM covering the anterior portion of the acrosome, probably conferring to this region its potential fusigenicity temporarily antagonized by the elevated sterol content. Only an exceptional and slight PXB binding was seen in the nonfusigenic equatorial and postacrosomal regions.

The effect of pH on cellular and membrane calcium binding and contraction of myocardium. A possible role for sarcolemmal phospholipid in EC coupling

Calcium binding to cultured whole cell monolayers, to their membranes made by "gas dissection," and to sarcolemmal vesicles from canine heart, is measured as a function of pH from 5.5-8.5. The effects on binding are compared to the contraction response of the cultured cells over the same pH range. All responses to pH, including calcium binding and contraction, are well described (r = 0.98-0.99) by a relation in which calcium binding depends primarily upon the extent of ionization of sarcolemmal binding sites. The effect of pH on calcium binding to phospholipid extracted from sarcolemmal vesicles suggests that the phospholipid accounts for at least 75% of the binding. The extent of ionization of the sites is pH-dependent and predicted by a form of the Henderson-Hasselbach relation with pK of the putative sites between 6.60 and 7.15. As pH increases from 5.5-8.5, membranous calcium binding, cellular calcium uptake, and contraction amplitude increase proportionally. Cellular calcium uptake increases by 4.4 mmol/kg dry weight cells (0.75 mmol/kg wet weight). Sixty percent to 65% of this increment is rapidly exchangeable and lanthanum displaceable, indicative of sarcolemmal localization. The remainder enters a slowly exchangeable (t1/2 = 26 minutes) compartment not directly related to support of contraction. The study supports the hypothesis that calcium bound to sarcolemmal sites plays a critical role in control of myocardial contraction.(ABSTRACT TRUNCATED AT 250 WORDS)

Mechanism of polymyxin B-mediated lysis of lipopolysaccharide-treated erythrocytes

A novel system was used previously to characterize the dynamic interaction of a polysaccharide-deficient, lipid-rich lipopolysaccharide (LPS) with rabbit erythrocytes (RaRBC). Exposure of the RaRBC to the LPS rendered them sensitive to induction of hemolysis by the cationic antibiotic polymyxin B (PB) in a time- and temperature-independent manner. Subsequent decay in the response of LPS-sensitized cells to PB was shown to be critically dependent on both the time and temperature of incubation of RaRBC with LPS and to be independent of a change in LPS binding (Carr and Morrison, Infect. Immun. 43:600-606, 1984). In the present study, we performed experiments designed to define the mechanism by which PB mediates hemolysis of LPS-sensitized RaRBC. Experiments were performed to examine the molecular requirements of the LPS and the PB that were essential for hemolytic activity. The capacity of various cations to mediate hemolysis of LPS-sensitized RaRBC or to block PB-mediated hemolysis and the temperature dependence of the PB lytic reaction were investigated. The results of these experiments suggest that PB-mediated hemolysis of LPS-treated erythrocytes is dependent upon an initial ionic association of PB with erythrocyte membrane-bound LPS, followed by hydrophobic insertion of the PB fatty acid into the erythrocyte membrane lipid bilayer.

Influence of polymeric effectors on binding of 3H-dihydroalprenolol to beta-adrenergic receptor of rat brain

The significance of anionic and cationic charges of glycocalyx, phospholipid or protein, etc. on the cell surface of the rat brain was examined for beta-adrenoceptors using the radioligand binding assay method. Thus, this experiment was designed to assess the effects of polymeric effectors, DNA, heparin, polymyxin B, histone, gelatin, colominic acid and bovine serum albumin (BSA), on the affinity of beta-adrenoceptors. The rat brain was used and the beta-adrenoceptor binding assay was carried out using 3H-dihydroalprenolol as a radioligand. Polymyxin B, DNA and heparin significantly caused a reduction in the maximum number of beta-adrenoceptors (Bmax), but only small changes were observed with histone, gelatin, BSA and colominic acid. Only DNA induced a decrease in the value of the dissociation constant (Kd) of beta-adrenoceptors. These results suggest that anionic or cationic charges in the environment of the receptor sites could have a crucial role in drug-receptor interaction.

Phospholipase D produces increased contractile force in rabbit ventricular muscle

Inclusion of phospholipase D (Streptomyces chromofuscus, 5 U/ml) in the medium perfusing rabbit papillary muscles increased peak force development by 78% and peak dF/dt by 89%. The maximal contractile response occurred 40-50 minutes after addition of the enzyme to the perfusate. As peak contractile response developed, aftercontractions appeared in most muscles. The inotropic response was slowly reversible, with disappearance of aftercontractions, upon discontinuation of the enzyme. Phospholipase D produces a specific increase of phosphatidic acid in the sarcolemma and, therefore, an increase in net anionic charge on the membrane. It has been shown previously that phospholipase D induces a large increase in sarcolemmal calcium binding. We propose, on the basis of the present study, that phospholipid-bound calcium may play a significant role in the control of contractile force in mammalian myocardium.

Ultrastructural localization of anionic phospholipids in skeletal muscle plasma membrane

Polymyxin B was used as a probe to label anionic phospholipids in skeletal muscle plasma membrane. This antibiotic produces muscle surface membrane lesions that can be identified in both thin sections and freeze-fracture replicas. The membrane perturbations assumed a patchy distribution with a preferential localization at the level of the I band and A-I bands junction. Intramembraneous particles were also observed within the lesions. We consider the possibility that microdomains of anionic phospholipids in muscle plasma membrane may function in the binding of Ca++.

A study of the structure and dynamics of complexes between polymyxin B and phosphatidylglycerol in monolayers by fluorescence

Interactions between the antibiotic polymyxin B and monolayers of dipalmitoylglycerophosphoglycerol have been reinvestigated through a study of the structure and dynamics of the complexes by means of an interface fluorimeter of our fabrication. A fluorescence technique has been developed where the use of linearly polarized incident beams gives the simultaneous determination of the orientation and the lateral diffusion rate of a fluorescent probe inserted in the film. The present investigation was carried out with 12-(9-anthroyloxy)-stearic acid, a fluorescent compound which forms non-fluorescent photodimers upon illumination. Orientation of the probe was studied by computing the ratio of the two dimerization constants KD and the ratio of the fluorescence intensities obtained with crossed linearly polarized incident lights. The lateral diffusion rate of the probe was obtained by measuring fluorescence recovery after photobleaching (photodimerization) of the probe. Control experiments, carried out with dimyristoylglycerophosphocholine, a lipid which does not interact with polymyxin B, show that the antibiotic does not significantly modify the behaviour of the probe. Both in terms of orientation and dynamics, with respect to dipalmitoylglycerophosphoglycerol, when the antibiotic is present in the subphase (1 microM, saturating conditions), data indicate that the lipid remains in a liquid-expanded state. This is true even at a high surface pressure (pi approximately equal to 37 mN X m-1), above the apparent 'transition' which can be observed at 30-35 mN X m-1 on its compression isotherm. Computation of the contribution of polymyxin B to the film expansion to the conclusion that this 'transition' would be a structural transition between two models of interaction: one, below the 'transition', where the polypeptide ring penetrates between the film-forming lipid molecules and another one, above the 'transition', were the antibiotic is adsorbed at the lipid-water interface with only its hydrocarbon chain penetrating the film.

Freeze-fracture cytochemistry with polymyxin B. A study on the plasma membrane of uterine epithelial cells

The peptide antibiotic polymyxin B, has been used as a label in a freeze-fracture cytochemical study of anionic phospholipids in the plasma membrane of uterine epithelial cells. The reagent produces mainly circular lesions visible on both P and E faces of fractured membranes. IMPs are found to be associated with lesions and we consider the possible association of integral membrane proteins with anionic phospholipids as well as the mechanisms of lesion formation.

Effects of calcium and neomycin on phase behavior of phospholipid bilayers

Calcium ion (Ca2+, 5 mM) caused a large upward shift in the transition temperature (Tc) of dipalmitoyl phosphatidylglycerol liposomes, apparently interacting with external and internal lipid in the bilayer. Neomycin (1 mM) caused only a small shift, apparently not penetrating the internal lipid domains. When liposomes were first incubated with Ca2+ followed by neomycin, the shift in Tc indicated displacement of Ca2+ by the drug. Liposomes of dipalmitoyl phosphatidylcholine or distearoyl phosphatidylcholine did not interact significantly with either cation. In mixed dipalmitoyl phosphatidylcholine-dipalmitoyl phosphatidylglycerol and distearoyl phosphatidylcholine-dipalmitoyl phosphatidylglycerol liposomes, addition of Ca2+ or neomycin resulted in phase separation.

Influence of polymyxin B, a probe for anionic phospholipids, on calcium binding and calcium and potassium fluxes of cultured cardiac cells

Polymyxin B, an amphiphilic, cationic peptidolipid, which is thought to bind to anionic phospholipids in cell membranes, is shown to interact with the cellular calcium of cultured neonatal rat myocardial cells in a dose-dependent, partially reversible manner. At concentrations of less than or equal to 0.1 mM, it has two distinct effects. First, it results in displacement of 1.4 +/- 0.3 mmol Ca/kg dry weight, which is equivalent to 18.1 +/- 3.4% of the total exchangeable cellular calcium. Total calcium displaced by polymyxin B and a nonspecific cationic probe, lanthanum, at its maximal displacing concentration (1 mM), was 5.9 +/- 1.3 mmol/kg dry weight. Thus, the total displaceable calcium represented 76.3 +/- 2.5% of the total exchangeable calcium. Second, polymyxin B (less than or equal to 0.1 mM) causes a reduction in net uptake of calcium, and slows the efflux of both calcium and potassium. Concentrations of polymyxin B higher than 0.1 mM result in an initial displacement of calcium, followed by an irreversible and sustained period of enhanced net calcium uptake. Efflux of calcium is slowed at the higher polymyxin B concentrations, whereas efflux of potassium is enhanced. Cellular contractile activity and electrical activity are irreversibly altered only by the higher concentrations. The results suggest that polymyxin B is a useful probe for the role of membrane phospholipids in control of ion binding and fluxes.

Polymyxin B-induced phase separation and acyl chain interdigitation in phosphatidylcholine/phosphatidylglycerol mixtures

Monolayers, fluorescence polarization, differential scanning calorimetry and X-ray diffraction experiments have been carried out to examine the effect of the polypeptide antibiotic polymyxin B on the phase behaviour of dipalmitoylphosphatidylglycerol (DPPG) either pure or mixed with dimyristoylphosphatidylcholine (DMPC) and dipalmitoylphosphatidylcholine (DPPC). It is shown that in both phosphatidylglycerol alone and phosphatidylglycerol/phosphatidylcholine mixtures, polymyxin B can induce either phase separation between lipid domains of various compositions or interdigitation of the acyl chains in the solid state, without segregation of the two lipids. Phase separation was observed by fluorescence and differential scanning calorimetry after addition of the antibiotic to vesicles composed of mixtures of DMPC and DPPG in conditions where polymyxin B did not saturate phosphatidylglycerol (DPPG to polymyxin B molar ratio, Ri, higher than 15). Phase separation was also observed in mixed monolayers of DPPC and of the 5:1 DPPG/polymyxin B complex, at high surface pressure. Acyl chain interdigitation was observed by X-ray diffraction in both 5:1 DPPG/polymyxin B mixtures and preformed 5:5:1 DMPC/DPPG/polymyxin B mixture, in which the antibiotic saturates phosphatidylglycerol (Ri 5). In both cases, raising the temperature gave rise to a complex double-peaked phase transition by differential scanning calorimetry, from the interdigitating phase to a normal L alpha lamellar phase. As it is known that polymyxin B does not interact with phosphatidylcholine, the data presented show that, when phosphatidylcholine and phosphatidylglycerol are mixed together, a phase perturbation such as acyl chain interdigitation, which normally affects only phosphatidylglycerol, is also felt by phosphatidylcholine.

Conversion of phospholipids to free fatty acids in response to acquisition of polymyxin resistance in Pseudomonas aeruginosa

The readily extractable lipids from a Pseudomonas aeruginosa isolate stepwise adapted to polymyxin resistance were compared with those of the susceptible parent and of a revertant strain which regained susceptibility. Significant qualitative and quantitative lipid alterations accompany the acquisition of resistance. Changes include the appearance of a major unidentified lipid (lipid X) unique to the readily extractable lipids of resistant cells. Comparative studies with parent and revertant strains indicated a significant decrease in the phospholipid content of resistant cells. Thin-layer chromatography of resistant-cell readily extractable lipids demonstrated: (i) the emergence of lipid X (36% of total readily extractable lipids), (ii) a decrease in phosphatidylethanolamine and phosphatidylglycerol, and (iii) an increase in diphosphatidylglycerol. Lipid X was purified by preparative silicic acid column chromatography and thin-layer chromatography and characterized by analytical thin-layer chromatography, column adsorption chromatography, and gas-liquid chromatography. Data from this study indicated that lipid X was a mixture of free fatty acids. The fatty acids present in lipid X were qualitatively and quantitatively the same as the fatty acids esterified to the phospholipids in the readily extractable lipids.

Ca2+ displacement by Polymyxin B from sarcolemma isolated by 'gas dissection' from cultured neonatal rat myocardial cells

Amphiphilic, cationic Polymyxin B is shown to displace Ca2+ from 'gas dissected' cardiac sarcolemma in a dose-dependent, saturable fashion. The Ca2+ displacement is only partially reversible, 57% and 63%, in the presence of 1 mM or 10 mM Ca2+, respectively. Total Ca2+ displaced by a non-specific cationic probe, lanthanum (La3+), at maximal displacing concentration (1 mM) was 0.172 +/- 0.02 nmol/microgram membrane protein. At 0.1 mM, Polymyxin B displaced 42% of the total La3+-displaceable Ca2+ or 0.072 +/- 0.01 nmol/microgram protein. 5 mM Polymyxin displaced Ca2+ in amounts equal to those displaced by 1 mM La3+. Pretreatment of the membranes with neuraminidase (removal of sialic acid) and protease leads to a decrease in La3+-displaceable Ca2+ but to an increase in the fraction displaced by 0.1 mM Polymyxin from 42% to 54%. Phospholipase D (cabbage) treatment significantly increased the La3+-displaceable Ca2+ to 0.227 +/- 0.02 nmol/microgram protein (P less than 0.05), a gain of 0.055 nmol. All of this phospholipid specific increment in bound Ca2+ was displaced by 0.1 mM Polymyxin B. The results suggest that Polymyxin B will be useful as a probe for phospholipid Ca2+-binding sites in natural membranes.

Antimicrobial action of compound 48/80--II mechanism of action

The mixture of compounds called compound 48/80 had been shown to have antimicrobial activity against a wide variety of microorganisms. In this paper it is shown that its primary site of attack appears to be on the membrane of the cell. In its presence, Tetrahymena became much more sensitive to osmotic stress, and alpha-methylglucose was rapidly released from preloaded Escherichia coli cells. The drug also had effects on cell viability, respiration, cell division, and the release of material absorbing at 260 nm. In general, its effects paralleled those of polymyxin B, although its structure is quite different except for the presence of amino groups and hydrophobic regions in both molecules. The activity of 48/80 was not due to detergent-like, surface-active properties and was antagonized by magnesium and other cations and by phosphatidylserine. Purification of the active principle might provide a relatively simple and readily modifiable probe of membrane function and possibly a new family of useful antimicrobial compounds.

Calorimetric investigation of polymyxin binding to phosphatidic acid bilayers

The cooperative binding process between the antibiotic peptide polymyxin-B and negatively-charged phosphatidic acid bilayers was investigated by differential thermal analysis and completed by fluorescence polarization measurements. The sigmoidal binding curves were analyzed in terms of the interaction energy within a domain formed by polymyxin and phosphatidic acid molecules. The formation of such a heterogeneous domain structure was favoured by high concentration of external monovalent ions. The cooperativity of the binding increased while a charge-induced decrease in the phase transition temperature of the pure lipid phase was observed with increasing ion concentration at a given pH. The reduced lateral coupling within the lipid bilayer in the presence of salt ions, as demonstrated by an increase in the lipid phase transition enthalpy, was considered to facilitate the cooperative domain formation. Moreover, an increase in the cooperativity of the polymyxin binding could be observed if phosphatidic acids of smaller chain length and thus of a lowered phase transition temperature were used. By the use of chemically-modified polymyxin we were able to demonstrate the effect of electrostatic and hydrophobic interaction. Acetylated polymyxin with a reduced positive charge was used to demonstrate the pure hydrophobic effect of polymyxin binding leading to a decrease in the phosphatidic acid phase transition temperature by about 20 degrees C. The cooperativity of the binding was strongly reduced. Cleavage of the hydrophobic polymyxin tail yielded a colistinnonapeptide which caused an electrostatically-induced increase in the phosphatidic acid phase transition temperature. With unmodified polymyxin we observed the combined effects of electrostatic as well as hydrophobic interaction making this model system interesting for the understanding of lipid-protein interactions. Evidence is presented that the formation of the polymyxin-phosphatidic acid complex is a lateral phase separation phenomenon.

Anionic lipid domains: correlation with functional topography in a mammalian cell membrane

Polymyxin B was used to explore distribution of anionic phospholipids in sperm plasma membranes by electron microscopy of freeze-fracture replicas. After exposure to Hepes/Tris-buffered polymyxin at 4 mM, phosphatidylcholine liposomes showed no perturbations nor did they fluoresce with dansylated incubation. When phosphatidylethanolamine was included in the liposomes, they became perturbed and fluoresced. Plasma membranes of Drosophila larval cells, containing or lacking cholesterol, were also disrupted by polymyxin. The cell membranes of guinea pig sperm were likewise disrupted but in specific functional areas. Fusional membrane domains showed protrusions; the stable membrane of the flagellum revealed diffuse bubbling. Regions of well-defined particle arrays and the postacrosomal segment maintained smooth contours. By fluorescence microscopy, we detected the same heterogeneous binding of the polymyxin dansyl derivative.

Effects of filipin, digitonin, and polymyxin b on plasma membrane of ram spermatozoa--an EM study

The effects of three membrane-active agents, filipin, digitonin, and polymyxin B on the plasma membrane of ram spermatozoa have been studied by freeze-etch electron microscopy. Filipin and digitonin both reacted with cholesterol and caused visible membrane modification in cholesterol-rich regions, with filipin being a more specific agent than digitonin. Polymyxin B, which is known to interact specifically with negatively charged phospholipids of bacterial membranes, exhibited a selective binding and subsequent modification of sperm plasma membranes. This binding was shown to be inhibited in the presence of 1 mmol/1 Ca2+. We hence propose that both filipin and polymyxin B are useful cytological markers for specific biological molecules in eukaryote membranes--filipin for cholesterol, and polymyxin B for negatively charged phospholipids.

Polymyxin B-phosphatidylglycerol interactions. A monolayer (pi, delta V) study

Through a monolayer investigation (pi, delta V), it is shown that the cationic antibiotic polymyxin B (or E) strongly interacts with films of acidic lipids, namely the didodecanoyl- and dihexadecanoylphosphatidylglycerol. The zwitterionic dihexadecanoylphosphatidylcholine was an unsuitable substrate. Interactions occurred at and above a polymyxin B concentration in the subphase of 2.5 . 10(-7) M, bringing about a considerable increase of both pi and delta V. These interactions proceeded in two steps, as revealed by a biphasic change of delta V with time. They were independent of the film molecular packing (fluid or gel states) and of the initial film pressure. Since it was possible to monitor the relative number of polymyxin B and didodecanoyl- or dihexadecanoylphosphatidylglycerol molecules in the monolayer, it is demonstrated that, at saturation, one polymyxin B molecule is bound to five phosphatidylglycerol molecules, a result which accounts for an exact neutralization of the charges. From competition experiments, it is shown that Na+ is ineffective in removing polymyxin B from the interface. Ca2+ appeared to be a stronger competitor but no complete antibiotic desorption was observed even at a Ca2+ concentration of 100 mM. As a working hypothesis, the antibiotic/lipid (1/5) system was assumed to constitute by itself one molecular species. The mixing of the polymyxin B/didodecanoylphosphatidylglycerol (1/5) system with an excess of lipid molecules in the monolayer was found to be ideal both in terms of pi and delta V. With dihexadecanoylphosphatidylglycerol, a small condensing effect could be detected only at intermediate surface pressures, in a region where the lipid phase transition occurred. The molecular area of polymyxin B interacting with didodecanoylphosphatidylglycerol can be calculated to be 1.23 +/- 0.05 nm2. It is proposed that the whole antibiotic molecule penetrates the film, the five bound lipid molecules being distributed aroung the peptide structure, at given positions imposed by the five 2,4-diaminobutyric acid residues.

Cooperative lipid-protein interaction. Effect of pH and ionic strength on polymyxin binding to phosphatidic acid membranes

The binding of polymyxin-B to charged dipalmitoyl phosphatidic acid membranes has been studied as function of the external pH and of the ionic strength of the buffer solution. The phase transition curves were obtained by measuring the fluorescence depolarization of diphenyl hexatriene incorporated into the membrane with temperature. The molecular process of polymyxin binding was elucidated: 1. At an ionic strength of I greater than or equal to 0.1 mol/l a three step phase transition curve is found. A high-temperature step corresponds to the non-bound lipid. A lowered phase transition concerns to protein-bound lipid domains. This again is splitted into two steps. An inner core of the domain is characterized by a lipid-protein complex which is stabilized through hydrophobic and electrostatic interactions between polymyxin and the charged lipid. This core is surrounded by an outer belt of only hydrophobically bound molecules. This part shows a lower phase transition temperature than the inner core. 2. The binding curves of polymyxin to phosphatidic acid membranes depend strongly on the ionic strength of the water phase. The cooperativity of the binding process increases with increasing ionic strength and reaches a constant value at I greater than 0.2 mol/l. The maximum fraction of bound lipid decreases with increasing ionic strength. 3. The pH of the water phase strongly influences the cooperative binding process. At pH 6 a loss of cooperativity is observed at low ionic strength. Increasing the ion concentration to I = 0.3 mol/l recuperates the cooperativity of the binding process. At pH 3.0 no cooperative binding is obtained even at high ionic strength.

Interactions of neomycin with monomolecular films of polyphosphoinositides and other lipids

The interactions of calcium and the aminoglycosidic antibiotic, neomycin, with various lipids were investigated in monomolecular films. Lipids were spread over a subphase of 0.05 M N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid, pH 7.0, and NaCl to give an ionic strength of 0.2. Measurements of surface pressure (pi) were taken with a Wilhelmy balance. In the absence of Ca2+, 1 muM--1 mM neomycin in the subphase decreased pi (i.e. condensed films) of all acidic lipids tested. In the presence of 1 mM Ca2+, neomycin did not change pi of films of phosphatidylserine, phosphatidylinositol and phosphatidic acid while it lowered pi of cardiolipin and cerebroside sulfate films. A unique pattern of interaction was observed with polyphosphoinositide monolayers. In the absence of Ca2+, 1 muM neomycin decreased pi followed by an increase of pi at higher neomycin concentrations. Ca2+ (1 mM) condensed the film significantly more than did neomycin. However, as little as 1 muM neomycin induced expansion of the calcium/lipid film which at 1 mM neomycin reached the same pi as in the absence of Ca2+. Such expansion was observed at all pressures of the film including the collapse pressure indicating a strong 'complex' between the drug and polyphosphoinositide not antagonized by Ca2+. In the absence of possible hydrophobic interactions, both the condensation and the expansion of the film should be mediated by ionic forces. Combined in vivo and in vitro evidence is discussed to suggest the polyphosphoinositides as the physiological receptors for aminoglycosides in the mammalian cell membrane.

Polymyxin binding to charged lipid membranes. An example of cooperative lipid-protein interaction

The binding of polymyxin-B to lipid bilayer vesicles of synthetic phosphatidic acid was studied using fluorescence, ESR spectroscopy and electron microscopy. 1,6-Diphenylhexatriene (which exhibits polarized fluorescence) and pyrene decanoic acid (which forms excimers) were used as fluorescence probes to study the lipid phase transition. The polymyxin binds strongly to negatively charged lipid layers. As a result of lipid/polymyxin chain-chain interactions, the transition temperature of the lipid. This can be explained in terms of a slight expansion of the crystalline lipid lattice (Lindeman's rule). Upon addition of polymyxin to phosphatidic acid vesicles two rather sharp phase transitions (width deltaT = 5 degrees C) are observed. The upper transition (at Tu) is that of the pure lipid and the lower transition (at T1) concerns the lipid bound to the peptide. The sharpness of these transitions strongly indicates that the bilayer is characterized by a heterogeneous lateral distribution of free and bound lipid regions, one in the crystalline and the other in the fluid state. Such a domain structure was directly observed by electron microscopy (freeze etching technique). In (1 : 1) mixtures of dipalmitoyl phosphatidic acid and egg lecithin, polymyxin induces the formation of domains of charged lipid within the fluid regions of egg lecithin. With both fluorescence methods the fraction of lipid bound to polymyxin-B as a function of the peptide concentration was determined. S-shaped binding curves were obtained. The same type of binding curve is obtained for the interaction of Ca2+ with phosphatidic acid lamellae, while the binding of polylysine to such membranes is characterized by a linear or Langmuir type binding curve. The S-shaped binding curve can be explained in terms of a cooperative lipid-ligand (Ca2+, polymyxin) interaction. A model is proposed which explains the association of polymyxin within the membrane plane in terms of elastic forces caused by the elastic distortion of the (liquid crystalline) lipid layer by this highly asymmetric peptide.