Mechanism of sonication of aqueous egg yolk lecithin dispersions and nature of the resultant particles

Sonochemistry-Assembled Stimuli-Responsive Polymer Microcapsules for Drug Delivery

Stimuli-responsive polymer microcapsules (PMs) fabricated by the sonochemical method have emerged for developing useful drug delivery systems, and the latest developments are mainly focusing on the synthetic strategies and properties such as structure, size, stability, loading capacity, drug delivery, and release. There, the primary attribution of sonochemistry is to offer a simple and practical approach for the preparation of PMs. Structure, size, stability, and properties of PMs are designed mainly according to synthetic materials, implementation schemes, or specific demands. Numerous functionalities of PMs based on different stimuli are demonstrated: targeting motion in a magnetic field or adhering to the living cells with sensitive sites through molecular recognition, and stimuli-triggered release including enzymatic catalysis, chemical reaction as well as physical or mechanical process. The current review discusses the basic principles and mechanisms of stimuli effects, and describes the progress in the application such as targeted drug systems and controlled drug systems, and also gives an outlook on the future challenges and opportunities for drug delivery and theranostics.

Repurposing doxycycline for synucleinopathies: remodelling of α-synuclein oligomers towards non-toxic parallel beta-sheet structured species

Synucleinophaties are progressive neurodegenerative disorders with no cure to date. An attractive strategy to tackle this problem is repurposing already tested safe drugs against novel targets. In this way, doxycycline prevents neurodegeneration in Parkinson models by modulating neuroinflammation. However, anti-inflammatory therapy per se is insufficient to account for neuroprotection. Herein we characterise novel targets of doxycycline describing the structural background supporting its effectiveness as a neuroprotector at subantibiotic doses. Our results show that doxycycline reshapes α-synuclein oligomers into off-pathway, high-molecular-weight species that do not evolve into fibrils. Off-pathway species present less hydrophobic surface than on-pathway oligomers and display different β-sheet structural arrangement. These structural changes affect the α-synuclein ability to destabilize biological membranes, cell viability, and formation of additional toxic species. Altogether, these mechanisms could act synergically giving novel targets for repurposing this drug.

Structural characterization of heparin-induced glyceraldehyde-3-phosphate dehydrogenase protofibrils preventing α-synuclein oligomeric species toxicity

Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) is a multifunctional enzyme that has been associated with neurodegenerative diseases. GAPDH colocalizes with α-synuclein in amyloid aggregates in post-mortem tissue of patients with sporadic Parkinson disease and promotes the formation of Lewy body-like inclusions in cell culture. In a previous work, we showed that glycosaminoglycan-induced GAPDH prefibrillar species accelerate the conversion of α-synuclein to fibrils. However, it remains to be determined whether the interplay among glycosaminoglycans, GAPDH, and α-synuclein has a role in pathological states. Here, we demonstrate that the toxic effect exerted by α-synuclein oligomers in dopaminergic cell culture is abolished in the presence of GAPDH prefibrillar species. Structural analysis of prefibrillar GAPDH performed by small angle x-ray scattering showed a particle compatible with a protofibril. This protofibril is shaped as a cylinder 22 nm long and a cross-section diameter of 12 nm. Using biocomputational techniques, we obtained the first all-atom model of the GAPDH protofibril, which was validated by cross-linking coupled to mass spectrometry experiments. Because GAPDH can be secreted outside the cell where glycosaminoglycans are present, it seems plausible that GAPDH protofibrils could be assembled in the extracellular space kidnapping α-synuclein toxic oligomers. Thus, the role of GAPDH protofibrils in neuronal proteostasis must be considered. The data reported here could open alternative ways in the development of therapeutic strategies against synucleinopathies like Parkinson disease.

Visualizing digestive organ morphology and function using differential fatty acid metabolism in live zebrafish

Lipids are essential for cellular function as sources of fuel, critical signaling molecules and membrane components. Deficiencies in lipid processing and transport underlie many metabolic diseases. To better understand metabolic function as it relates to disease etiology, a whole animal approach is advantageous, one in which multiple organs and cell types can be assessed simultaneously in vivo. Towards this end, we have developed an assay to visualize fatty acid (FA) metabolism in larval zebrafish (Danio rerio). The method utilizes egg yolk liposomes to deliver different chain length FA analogs (BODIPY-FL) to six day-old larvae. Following liposome incubation, larvae accumulate the analogs throughout their digestive organs, providing a comprehensive readout of organ structure and physiology. Using this assay we have observed that different chain length FAs are differentially transported and metabolized by the larval digestive system. We show that this assay can also reveal structural and metabolic defects in digestive mutants. Because this labeling technique can be used to investigate digestive organ morphology and function, we foresee its application in diverse studies of organ development and physiology.

The influence of phospholipids on the transport of antibacterial agents across non-aqueous barriers, and the methods used to quantitate the rate processes

Penicillin G and ampicillin are known to bind with phospholipids, which may influence antibiotic transport across the bacterial cell wall. This influence has been investigated in vitro in a three-phase cell containing 1-octanol as the non-aqueous barrier. The data have been analysed by an iterative technique using digital computation, in order to quantitate the rate processes. The results indicate that binding of the antibiotics with phospholipids occurs at the oil-water interface which influences the further transport of the antibiotics through the non-aqueous phase. The possible biological significance is discussed.

Stabilization of lipid bilayer vesicles by sucrose during freezing

The freeze-induced fusion and leakage of small unilamellar vesicles (SUV) of natural and synthetic phosphatidylcholines and the suppression of these processes by sucrose was studied by electron microscopy, by high-resolution NMR, and by ESR techniques. During slow freezing of SUV suspensions in water, the lipid was compressed into a small interstitial volume and transformed into a multilamellar aggregate without vesicular structure. When frozen in sucrose solution, the lipid also was compressed between the ice crystals but remained in the form of vesicles. The fractional amount of lipid remaining as SUV after freezing was found to increase significantly only at sucrose/lipid molar ratios above 0.4. Eu(3+) displaced sucrose from the lipid by competitive binding. During freezing in the absence of sucrose, the vesicles became transiently permeable to ions. ESR studies showed that fusion of vesicles in the absence of sucrose is far more extensive when they are frozen while above their phase-transition temperature (t(c)) than when frozen while below their t(c). It is concluded that the extent of membrane disruption depends on the membrane mobility at the moment of freezing and that sucrose exerts its protective effect by binding to the membrane interface and/or by affecting the water structure.

Biomimetic nanoparticles: preparation, characterization and biomedical applications

Mimicking nature is a powerful approach for developing novel lipid-based devices for drug and vaccine delivery. In this review, biomimetic assemblies based on natural or synthetic lipids by themselves or associated to silica, latex or drug particles will be discussed. In water, self-assembly of lipid molecules into supramolecular structures is fairly well understood. However, their self-assembly on a solid surface or at an interface remains poorly understood. In certain cases, hydrophobic drug granules can be dispersed in aqueous solution via lipid adsorption surrounding the drug particles as nanocapsules. In other instances, hydrophobic drug molecules attach as monomers to borders of lipid bilayer fragments providing drug formulations that are effective in vivo at low drug-to-lipid-molar ratio. Cationic biomimetic particles offer suitable interfacial environment for adsorption, presentation and targeting of biomolecules in vivo. Thereby antigens can effectively be presented by tailored biomimetic particles for development of vaccines over a range of defined and controllable particle sizes. Biomolecular recognition between receptor and ligand can be reconstituted by means of receptor immobilization into supported lipidic bilayers allowing isolation and characterization of signal transduction steps.

Glyceraldehyde-3-phosphate dehydrogenase tetramer dissociation and amyloid fibril formation induced by negatively charged membranes

Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) is a multifunctional enzyme related with Huntington's, Parkinson's and Alzheimer's diseases. The ability of negatively charged membranes to induce a rapid formation of GAPDH amyloid fibrils has been demonstrated, but the mechanisms by which GAPDH reaches the fibrillar state remains unclear. In this report, we describe the structural changes undergone by GAPDH at physiological pH and temperature conditions right from its interaction with acidic membranes until the amyloid fibril is formed. According to our results, the GAPDH-membrane binding induces a beta-structuring process along with a loss of quaternary structure in the enzyme. In this way, experimental evidences on the initial steps of GAPDH amyloid fibrils formation pathway are provided.

A possible cellular explanation for the NMR-visible mobile lipid (ML) changes in cultured C6 glioma cells with growth

The NMR-visible mobile lipid (ML) signals of C6 glioma cells have been monitored at 9.4 and 11.7 T (single pulse and 136 ms echo time) from cell pellets by (1)H NMR spectroscopy. A reproducible behavior with growth has been found. ML signals increase from log phase (4 days of culture) to postconfluence (7 days of culture). This ML behavior is paralleled by the percentage of cells containing epifluorescence detectable Nile Red stained cytosolic droplets (range 23%-60% of cells). The number of positive cells increases after seeding (days 0-1), decreases at log phase (days 2-4), increases again at confluence (day 5) and even further at post-confluence (day 7). C6 cells proliferation arrest induced by growth factors deprivation induces an even higher accumulation of cytosolic droplets (up to 100% of cells) and a large ML increase (up to 21-fold with respect to 4-day log phase cells). When neutral lipid content is quantified by thin-layer chromatography (TLC) on total lipid extracts of C6 cells, no statistically significant change can be detected (in microg/10(8) cells) with growth or growth arrest in major neutral lipid containing species (triacylglycerol, TAG, diacylglycerol, DAG, cholesteryl esters, ChoEst) except for DAG, which decreased in post-confluent, 7-day cells. The apparent discrepancy between NMR, optical microscopy and TLC results can be reconciled if possible biophysical changes in the neutral lipid pool with growth are taken into account. A cellular explanation for the observed results is proposed: the TAG-droplet-size-change hypothesis.

Molecular view by fourier transform infrared spectroscopy of the relationship between lactocin 705 and membranes: speculations on antimicrobial mechanism

Lactocin 705 is a bacteriocin whose activity depends upon the complementation of two peptides, termed Lac705alpha and Lac705beta. Neither Lac705alpha nor Lac705beta displayed bacteriocin activity by itself when the growth of sensitive cells was monitored. To obtain molecular insights into the lactocin 705 mechanism of action, Fourier transform infrared spectroscopy was used to investigate the interactions of each peptide (Lac705alpha and Lac705beta) with dipalmitoylphosphatidylcholine liposomal membranes. Both peptides show the ability to interact with the zwitterionic membrane but at different bilayer levels. While Lac705alpha interacts with the interfacial region inducing dehydration, Lac705beta peptide interacts with only the hydrophobic core. This paper presents the first experimental evidence that supports the hypothesis that Lac705alpha and Lac705beta peptides could form a transmembrane oligomer. From the obtained results, a mechanism of action of lactocin 705 on membrane systems is proposed. The component Lac705alpha could induce the dehydration of the bilayer interfacial region, and the Lac705beta peptide could insert in the hydrophobic region of the membrane where the peptide has adequate conditions to achieve the oligomerization.

Reducing liposome size with ultrasound: bimodal size distributions

Sonication is a simple method for reducing the size of liposomes. We report the size distributions of liposomes as a function of sonication time using three different techniques. Liposomes, mildly sonicated for just 30 sec, had bimodal distributions when surface-weighted with modes at about 140 and 750 nm. With extended sonication, the size distribution remains bimodal but the average diameter of each population decreases and the smaller population becomes more numerous. Independent measurements of liposome size using Dynamic Light Scattering (DLS), transmission electron microscopy (TEM), and the nystatin/ergosterol fusion assay all gave consistent results. The bimodal distribution (even when number-weighted) differs from the Weibull distribution commonly observed for liposomes sonicated at high powers over long periods of time and suggests that a different mechanism may be involved in mild sonication. The observations are consistent with the following mechanism for decreasing liposome size. During ultrasonic irradiation, cavitation, caused by oscillating microbubbles, produces shear fields. Large liposomes that enter these fields form long tube-like appendages that can pinch-off into smaller liposomes. This proposed mechanism is consistent with colloidal theory and the observed behavior of liposomes in shear fields.

Structure of fully hydrated fluid phase lipid bilayers with monounsaturated chains

Quantitative structures are obtained at 30 degrees C for the fully hydrated fluid phases of palmitoyloleoylphosphatidylcholine (POPC), with a double bond on the sn-2 hydrocarbon chain, and for dierucoylphosphatidylcholine (di22:1PC), with a double bond on each hydrocarbon chain. The form factors F(qz) for both lipids are obtained using a combination of three methods. (1) Volumetric measurements provide F(0). (2) X-ray scattering from extruded unilamellar vesicles provides /F(qz)/ for low q(z). (3) Diffuse X-ray scattering from oriented stacks of bilayers provides /F(qz)/ for high q(z). Also, data using method (2) are added to our recent data for dioleoylphosphatidylcholine (DOPC) using methods (1) and (3); the new DOPC data agree very well with the recent data and with (4) our older data obtained using a liquid crystallographic X-ray method. We used hybrid electron density models to obtain structural results from these form factors. The result for area per lipid (A) for DOPC 72.4 +/- 0.5 A(2) agrees well with our earlier publications, and we find A = 69.3 +/- 0.5 A2 for di22:1PC and A = 68.3 +/- 1.5 A2 for POPC. We obtain the values for five different average thicknesses: hydrophobic, steric, head-head, phosphate-phosphate and Luzzati. Comparison of the results for these three lipids and for our recent dimyristoylphosphatidylcholine (DMPC) determination provides quantitative measures of the effect of unsaturation on bilayer structure. Our results suggest that lipids with one monounsaturated chain have quantitative bilayer structures closer to lipids with two monounsaturated chains than to lipids with two completely saturated chains.

Influence of the long-chain/short-chain amphiphile ratio on lateral diffusion of PEG-lipid in magnetically aligned lipid bilayers as measured via pulsed-field-gradient NMR

Lateral diffusion measurements of polyethylene glycol(PEG)-lipid incorporated into magnetically aligned lipid bilayers, composed of dimyristoyl phosphatidylcholine (DMPC) plus dihexanoyl phosphatidylcholine (DHPC) plus 1 mol % (relative to DMPC) dimyristoyl phosphatidylethanolamine-n-[methoxy(polyethylene glycol)-2000] (DMPE-PEG 2000), were performed using stimulated-echo pulsed-field-gradient proton ((1)H) nuclear magnetic resonance spectroscopy. The DMPE-PEG 2000 (1 mol %, 35 degrees C) lateral diffusion coefficient D varied directly with the mole fraction of DMPC, X(DMPC) = q/(1+q) where q = DMPC/DHPC molar ratio, decreasing progressively from D = 1.65 x 10(-11) m(2) s(-1) at q approximately 4.7 to D = 0.65 x 10(-11) m(2) s(-1) at q approximately 2.5. Possible sources of this dependence, including orientational disorder, obstruction, and PEG-lipid sequestration, were simulated using, respectively, a diffusion-in-a-cone model, percolation theory, and a two-phase PEG distribution model. Orientational disorder alone was not capable of reproducing the observations, but in combination with either obstruction or PEG-lipid two-phase distribution models did so satisfactorily. A combination of all three models yielded the most reasonable fit to the observed dependence of lateral diffusion on q. These same effects would be expected to influence lateral diffusion of any bilayer-associating species in such systems.

Lateral diffusion of PEG-Lipid in magnetically aligned bicelles measured using stimulated echo pulsed field gradient 1H NMR

Lateral diffusion measurements of PEG-lipid incorporated into magnetically aligned bicelles are demonstrated using stimulated echo (STE) pulsed field gradient (PFG) proton (1H) nuclear magnetic resonance (NMR) spectroscopy. Bicelles were composed of dimyristoyl phosphatidylcholine (DMPC) plus dihexanoyl phosphatidylcholine (DHPC) (q = DMPC/DHPC molar ratio = 4.5) plus 1 mol % (relative to DMPC) dimyristoyl phosphatidylethanolamine-N-[methoxy(polyethylene glycol)-2000] (DMPE-PEG 2000) at 25 wt % lipid. 1H NMR STE spectra of perpendicular aligned bicelles contained only resonances assigned to residual HDO and to overlapping contributions from a DMPE-PEG 2000 ethoxy headgroup plus DHPC choline methyl protons. Decay of the latter's STE intensity in the STE PFG 1H NMR experiment (g(z) = 244 G cm(-1)) yielded a DMPE-PEG 2000 (1 mol %, 35 degrees C) lateral diffusion coefficient D = 1.35 x 10(-11) m2 s(-1). Hence, below the "mushroom-to-brush" transition, DMPE-PEG 2000 lateral diffusion is dictated by its DMPE hydrophobic anchor. D was independent of the diffusion time, indicating unrestricted lateral diffusion over root mean-square diffusion distances of microns, supporting the "perforated lamellae" model of bicelle structure under these conditions. Overall, the results demonstrate the feasibility of lateral diffusion measurements in magnetically aligned bicelles using the STE PFG NMR technique.

Zeaxanthin (dihydroxy-beta-carotene) but not beta-carotene rigidifies lipid membranes: a 1H-NMR study of carotenoid-egg phosphatidylcholine liposomes

1H-NMR technique was applied to study liposomes formed with egg-yolk phosphatidylcholine containing as an additional component two carotenoid pigments: beta-carotene or zeaxanthin (dihydrohy-beta-carotene). A strong rigidifying effect of zeaxanthin but not of beta-carotene with respect to hydrophobic core of lipid bilayer was concluded from the carotenoid-dependent broadening of the NMR lines assigned to -CH2- groups and terminal -CH3 groups of lipid alkyl chains. A similar effect of zeaxanthin with respect to polar headgroups was concluded on the basis of the effect of the pigment on the shape of NMR lines attributed to -N+(CH3)3 groups. In contrast, beta-carotene increases motional freedom of lipid polar headgroups. The inclusion of both carotenoids to liposomes resulted in the enhanced penetration of Pr3+ ions to the polar zone of the external layer of a membrane monitored by the splitting of the -N+(CH3)3 signal, the effect of beta-carotene being much more pronounced. Differences in the effect on membrane structure and molecular dynamics observed for beta-carotene and its polar derivative are discussed in terms of organization of a carotenoid-containing lipid membrane.

Structure and phase behavior of lipid suspensions containing phospholipids with covalently attached poly(ethylene glycol)

Liposomes containing phospholipids with covalently attached poly(ethylene glycol) (PEG-lipids) are being developed for in vivo drug delivery. In this paper we determine the structure and phase behavior of fully hydrated distearoylphosphatidylcholine (DSPC) suspensions containing PEG-lipids composed of distearoylphosphatidylethanolamine with attached PEGs of molecular weights ranging from 350 to 5000. For DSPC:PEG-lipid suspensions containing 0-60 mol % PEG-lipid, differential scanning calorimetry shows main endothermic transitions ranging from 55 to 64 degrees C, depending on the size of the PEG and concentration of PEG-lipid. The enthalpy of this main transition remains constant for all PEG-350 concentrations but decreases with increasing amounts of PEG-750, PEG-2000, or PEG-5000, ultimately disappearing at PEG-lipid concentrations greater than about 60 mol %. Low-angle and wide-angle x-ray diffraction show that tilted gel (L beta') phase bilayers are formed for all PEG-lipid molecular weights at concentrations of about 10 mol % or less, with the distance between bilayers depending on PEG molecular weight and PEG-lipid concentration. At PEG-lipid concentrations greater than 10 mol %, the lipid structure depends on the size of the PEG moiety. X-ray diffraction analysis shows that untilted interdigitated (L beta I) gel phase bilayers form with the incorporation of 40-100 mol % PEG-350 or 20-70 mol % PEG-750, and untilted gel (L beta) phase bilayers are formed in the presence of about 20-60 mol % PEG-2000 and PEG-5000. Light microscopy, turbidity measurements, x-ray diffraction, and 1H-NMR indicate that a pure micellar phase forms in the presence of greater than about 60% PEG-750, PEG-2000, or PEG-5000.

Effect of vesicle composition and curvature on the dissociation of phosphatidic acid in small unilamellar vesicles--a 31P-NMR study

Sonicated small unilamellar vesicles (SUVs) containing phosphatidic acid (PA) give two PA 31P-NMR resonances corresponding to PA molecules in the inner and outer leaflets of the bilayer. This NMR differentiation between the two monolayers is not due to a pH gradient across the membrane but instead reflects differential packing in the inner and outer leaflets imposed by the highly curved SUV surface. The apparent pKa of the outer-leaflet PA increases with decreasing surface curvature and with increasing PA content. The estimated relationship between the apparent pKa of the outer-leaflet PA headgroup and vesicle curvature may provide a qualitative probe for effects related to surface curvature in these model-membrane systems.

Acyl chain orientational order in large unilamellar vesicles: comparison with multilamellar liposomes: a 2H and 31P nuclear magnetic resonance study

Large unilamellar vesicles (LUVs) composed of 1-[2H31]palmitoyl-2-oleoyl phosphatidylcholine (POPC-d31), with diameters of approximately 117 +/- 31 and 180 +/- 44 nm, were prepared by extrusion through polycarbonate filters with pore sizes of 0.1 and 0.2 microns, respectively. The 2H nuclear magnetic resonance (NMR) spectra obtained at 21 degrees C contain two components: a broad component (approximately 17 kHz linewidth) corresponding to the methylene groups and a narrower component originating from the methyl groups. Spectra with increasing powder pattern characteristics were obtained by reducing the rate of phospholipid reorientations by addition of glycerol (to increase the solvent viscosity) and by lowering the temperature. Full powder spectra, characteristic of liquid-crystalline bilayers, were obtained for both LUV samples at 0 degrees C in the presence of 50 wt% glycerol. Individual quadrupolar splittings were not resolved in these spectra, due to broader linewidths in the LUVs, which have significantly shorter values for spin-spin relaxation time T2 measured from the decay of the quadrupolar echo (90 microseconds) than the multilmellar vesicles (MLVs; 540 microseconds). Smoothed order parameter profiles (OPPs) were obtained for these samples by integration of the dePaked spectra. The OPPs were very similar to the OPP of POPC-d31 MLVs in 50 wt% glycerol at the same temperature, indicating that orientational order in MLVs and LUVs with a diameter of > or = 100 nm is essentially the same. The presence of 80 wt% glycerol was found to have a disordering effect on the vesicles.

The effect of positive and negative pH-gradients on the stability of small unilamellar vesicles of negatively charged phospholipids

The stability of small unilamellar vesicles (SUV) made from negatively-charged phosphatidate by ultrasonication or pH-jump has been investigated. As criteria for the vesicle stability are used: (I) the bilayer integrity as judged from the permeability of the fluorescent probe carboxyfluorescein (CF) and (II) the susceptibility of the phospholipid vesicles to fusion as judged by gel filtration and freeze-fracture electron microscopy. Egg phosphatidate SUV (PA-SUV) whose internal cavity is in equilibrium with the dispersion medium are strictly speaking thermodynamically unstable by these criteria. They may, however, be regarded as stable from a practical point of view. CF-release is observed with a half-time of 14 days and also some vesicle fusion, particularly at low temperature (4 degrees C). The small effects observed, e.g., the small tendency of the vesicles to undergo fusion is probably due to the high surface charge density of PA bilayers. A main finding of this work is that the same positive pH-gradient which is used in the pH-jump method to drive the formation of SUV from large phosphatidic acid bilayer sheets has a stabilizing effect on the resulting PA-SUV. Stabilization is achieved by positive pH-gradients of about two pH-units or more with the pH of the external medium exceeding the pH of the vesicle cavity. Under these conditions, up to about 8 weeks no significant loss of entrapped CF and no fusion of SUV was observed both at 4 degrees C and room temperature. In contrast, a reverse or negative pH-gradient of several pH units applied to PA-SUV (with the external pH being lower than that of the vesicle cavity) destabilizes PA-SUV. Such a gradient can be shown to lead to a dramatic perturbation of the lipid bilayer packing as evident from a significant increase in CF permeability. The local perturbation of the phospholipid bilayer is accompanied by massive vesicle fusion which is prominent at low temperature (4 degrees C).

Effect of the asymmetric Ca2+ distribution on the bilayer properties of phosphatidylcholine-sonicated vesicles

The incorporation of Ca2+ in the inner volume of egg phosphatidylcholine vesicles increases the fluorescence anisotropy of a diphenylhexatriene probe. This increase is higher than for Na+ at the same normality. An effect of the same magnitude is induced by Ca2+ when using binary lipid mixture (dioleoylphosphatidylcholine and dipalmitoylphosphatidylcholine) as long as the mixture is maintained below the phase-transition temperature of the saturated species. The influence of Ca2+ may be explained by an asymmetric distribution of the saturated and unsaturated lipids between the internal and the external monolayers.

Ovine ceroid-lipofuscinosis II: Pathologic changes interpreted in light of biochemical observations

Ceroid-lipofuscinosis is studied in a breed of sheep and the histochemical and ultrastructural changes are interpreted in light of the known chemical composition of cytosomes. It is concluded that the ovine disease is a lysosomal proteinosis. It is postulated that the multilamellar arrays reflect a complex three-dimensional structure of alternating lipid and protein with the proteinaceous component being specific polypeptides. This is supported by powder X-ray diffractions at 10A implying a repeating proteinaceous unit in the structures. The coherent size of diffracting units was remarkably similar to the periodicity of lamellae observed in electron micrographs of lipopigment cytosomes. The fluorescent and staining characteristics of lipopigment can be explained by the above conclusion and postulate.

Spontaneous vesiculation of uncharged phospholipid dispersions consisting of lecithin and lysolecithin

The work presented here demonstrates that the phenomenon of spontaneous vesiculation is not restricted to charged lipids and lipid mixtures, but occurs also in isoelectric phospholipid mixtures consisting of egg phosphatidylcholine (EPC) and egg lysophosphatidylcholine (lyso-EPC). 1H high-resolution NMR and freeze-fracture electron microscopy have been used to characterize the mixed EPC/lyso EPC dispersions in excess H2O. The predominant phase in these mixed phospholipid dispersions is smectic (lamellar) at least up to approximately 70% lysophosphatidylcholine. The type of phospholipid aggregate formed in excess H2O depends on the mole ratio diacyl to monoacyl phosphatidylcholine. The dispersive (lytic) action of lysophosphatidylcholine on phosphatidylcholine bilayers becomes effective at lysophospholipid contents in excess of approximately 10%. Large multilamellar liposomes are disrupted and replaced by smaller particles, mainly unilamellar vesicles. Between 30 and 70% lysophosphatidylcholine a significant proportion of the total phospholipid is present as small unilamellar vesicles (SUV) of a diameter of 23 nm (range: 20-70 nm). At even higher lysophosphatidylcholine contents the fraction of phospholipid present as small mixed micelles with a diameter smaller than about 14 nm grows at the expense of the vesicular structures. There is a second effect of increasing the quantity of lysophosphatidylcholine in phosphatidylcholine bilayers: the presence of lysophosphatidylcholine in excess of 10% renders the phospholipid bilayer more permeable to ions as compared to pure phosphatidylcholine bilayers. The key factor in inducing spontaneous vesiculation is probably not the charge but the wedge-like shape of the lysophospholipid molecule. The molecular shape may give rise to an asymmetric distribution of lysophosphatidylcholine between the two halves of the bilayer, thus stabilizing highly curved bilayers as present in SUV.

The interaction of saccharides with lipid bilayer vesicles: stabilization during freeze-thawing and freeze-drying

The fusion of small unilamellar vesicles of phosphatidylcholines during freeze-thawing and freeze-drying/rehydration, and the suppression of fusion under these conditions by various saccharides, was investigated by gel filtration on Sepharose 4B, quasielastic light scattering, high-resolution 1H-NMR, ESR spin labeling, and differential scanning calorimetry. Freeze-thawing and freeze-drying of aqueous small unilamellar vesicle suspensions in the presence of sufficient sucrose had no significant effect on the average size and size distribution of small unilamellar vesicles. In the presence of sucrose the structural integrity and the permeability properties of the phosphatidylcholine bilayers were retained during freeze-thawing and freeze-drying. A comparison of the stabilizing effect of sucrose with that of trehalose and glucose showed that the stabilization is not sugar-specific but is a general property of saccharides. The fraction of small unilamellar vesicles recovered after freeze-thawing depended on the saccharide/phosphatidylcholine molar ratio. The mechanism of the cryoprotective effect involves binding of the sugar to the phospholipid polar group, probably through hydrogen bonding.

Measurement of the glucose permeation rate across phospholipid bilayers using small unilamellar vesicles. Effect of membrane composition and temperature

Small unilamellar vesicles were used to measure the permeability of saturated phosphatidylcholine bilayers to glucose. The presented method circumvents most of the common restriction of classical permeability experiments. Increasing the fatty acid chain length of the lipids reduced the permeation rate significantly. Raising the temperature above that of the lipid phase transition drastically increased membrane permeability. Arrhenius plots demonstrated the activation energy to be independent of membrane composition and the phase-state of the lipids. The permeation process is discussed in terms of a constant energy to disrupt all hydrogen bonds between permeant and aqueous solvent prior to penetrating the membrane. The magnitude of the permeability coefficient is partly determined by a unfavourable change in entropy of activation on crossing the water/lipid interface. All results indicate that the penetration of the dehydrated permeant into the hydrophobic barrier is the rate-limiting step in the permeation of glucose.

Spontaneous vesiculation of phospholipids: a simple and quick method of forming unilamellar vesicles

Phosphatidic acid dispersions in H2O vesiculate when the pH is increased transiently (less than 2 min) from approximately 3 to 10.5-11. The same phenomenon is observed in mixed dispersions of phosphatidylcholine and phosphatidic acid in H2O when the pH is increased from approximately 3 to 7 to 8. With phosphatidic acid, this treatment produces small closed unilamellar vesicles (200-600 A) from 50-60% of the total phospholipid, with the remainder present as large unilamellar vesicles and multilamellar structures. The extent of vesiculation depends on the pH that the dispersion is exposed to. With mixed phospholipid dispersions, similar vesicles are obtained; in addition to pH, the extent of vesiculation depends on the phosphatidylcholine/phosphatidic acid (wt/wt) ratio; as this ratio increases, the percentage of small unilamellar vesicles formed decreases. The short exposure of the phospholipids to high pH does not cause lipid degradation, as assessed by thin-layer chromatography; hence, the formation of degradation products can be ruled out as being responsible for the spontaneous vesiculation. Ionization of the phosphate group, resulting in a high surface charge density, may be an important factor in the spontaneous vesiculation. The proportion of phospholipid present as small unilamellar vesicles was determined by gel filtration on Sepharose 4B and by 1H NMR. The small vesicles gave rise to a reasonably well resolved high-resolution NMR spectrum. A good correlation was found between the proportion of phospholipid giving rise to the high resolution spectrum, as derived from spectral intensity measurements, and the proportion present as small unilamellar vesicles, as derived from gel filtration.

The kinetics of ionophore X-537A-mediated transport of manganese through dipalmitoylphosphatidylcholine vesicles. A 1H-NMR study

We have studied the kinetics of ionophore X-537A-mediated transport of manganese ions into small unilamellar vesicles formed from dipalmitoylphosphatidylcholine. To follow the transport we used the paramagnetic effect of manganese on the 1H-NMR signal from choline trimethylammonium groups on the inner phospholipid monolayer. The transport of only one manganese ion produces an intravesicular concentration which is high enough (approx. 1 mM) to substantially broaden this signal. The observed signal thus arises predominantly from those vesicles which contain no manganese. Therefore, as manganese is transported into the vesicles the observed signal decreases in intensity, but does not broaden. The initial time-dependence of the intensity of the signal, S(t), can be approximated by the simple first-order rate law: S(t) = S(0) exp(-k't), where k' is the probability per unit time for the transport of a manganese ion from the external medium to the intravesicular space. From the dependence of k' on the ionophore X-537A concentration we conclude that manganese is transported into the vesicles via both 1 : 1 and 2 : 1 complexes with ionophore X-537A. At low ratios of ionophore X-537A to vesicles transport via the 1 : 1 complex predominates; at high ratios transport via the 2 : 1 complex predominates. From the dependence of k' on manganese concentration we determined that under our conditions the equilibration of ionophore X-537A between vesicles is much faster than the transport of manganese through the vesicles. Lastly, from the dependence of k' on temperature, we conclude that the ionophore X-537A-mediated transport of manganese into the dipalmitoylphosphatidylcholine vesicles is very sensitive to the gel-liquid crystalline phase transition.

The adsorption of divalent cations to phosphatidylglycerol bilayer membranes

The ability of Stern equation to describe the adsorption of divalent cations to phosphatidylglycerol membranes was tested by combining 31P-NMR and electrophoretic mobility measurements. In 0.1 M sodium chloride both the 31P-NMR and the zeta potential data are well described by the Stern equation. 31P-NMR and 13C-NMR results indicate that cobalt forms inner-sphere complexes only with the phosphate group of phosphatidylglycerol molecules and that a substantial fraction of the adsorbed cobalt ions form outer-sphere complexes. Evidence is presented that suggests the alkaline earth cations also bind to phospholipids mainly by forming outer sphere complexes. Electrophoretic mobility measurements were performed with several different divalent cations. In all cases the zeta potentials in 0.1 M sodium chloride were well described by the Stern equation. The intrinsic 1:1 association constants (M-1) for the phosphatidylglycerol complexes decreased in the sequence: Mn2+, 11.5; Ca2+, 8.5; Ni2+, 7.5; Co2+, 6.5; Mg2+, 6.0; Ba2+, 5.5 and Sr2+, 5.0.

1H-NMR study of the effect of synthetic polymers on the fluidity, transition temperature and fusion of dipalmitoyl phosphatidylcholine small vesicles

The interaction of water-soluble polymers with dipalmitoyl phosphatidylcholine small vesicles and the effect on vesicle fusion were studied by means of 1H-NMR spectrometry. The motion of dipalmitoyl phosphatidylcholine molecules decreased on interaction with the polymers and was detected as a change in the signal intensity. The interaction behavior of polymers is very sensitive to the chemical structure of the applied polymers. Poly(styrene sulfonic acid) and poly(ethylene glycol) decreased the motion of the choline methyl group, predominantly through coulombic and hydrophobic interaction forces, respectively. For example, in the case of the poly(styrene sulfonic acid)-containing system, the signal intensity of the choline methyl group was decreased about 15% while those of the hydrophobic methylene and terminal methyl groups were scarcely decreased by the addition of polymer to a final concentration of 4.0 x 10(-2) unit mol/l. These polymers are considered to interact with the surface of the vesicle membrane. On the other hand, poly(L-glutamic acid) and poly(N-vinyl-2-pyrrolidone) decreased the signal intensities of not only the choline methyl group, but also those of the hydrophobic methylene and terminal methyl groups. This result suggests that part of these polymers might be incorporated into the hydrophobic region of the vesicle membrane. Addition of the non-ionic polymers inhibited vesicle fusion considerably. This effect was explained by the stabilization of dipalmitoyl phosphatidylcholine vesicles by complexation with these polymers.