Selective phospholipid adsorption and atherosclerosis.

Disaturated (fully saturated) lecithins adsorb onto solid surfaces more readily than lecithins in which one or both fatty acids are unsaturated. If saturated lecithins adsorb to arterial walls as they do to glass and polystyrene surfaces, there may be increased probability of atherosclerosis when the disaturated lecithin content of plasma is elevated. Analyses of lecithins in plasma samples from patients with myocardial infarction, and from patients with premature atherosclerosis but with low concentrations of plasma cholesterol and triglycerides, are consistent with the hypothesis that a high concentration of disaturated lecithin in plasma may be a significant risk factor for atherosclerosis, independent of triglyceride and cholesterol concentrations.

The interaction between steroid hormones and lipid monolayers on water.

The interaction of progesterone, testosterone, androsterone, and etiocholanolone with insoluble lipid films (cholesterol and saturated hydrocarbons containing either alcohol, ester, acetamide, phosphate, amine, or carboxyl groups) was studied. In addition to surface pressure and surface potential measurements of the surface films, radioactive tracers were used to measure the concentration of adsorbed steroid in the lipid films. In general, steroids form mixed films with the insoluble lipid films. Compression of the insoluble lipid films to their most condensed state leads to complete ejection of adsorbed steroid from the surface in all cases except with the amine, for which a small amount of steroid is still retained in the surface. Interactions between the steroids and insoluble lipids are primarily due to van der Waals or dispersion forces; there were no significant contributions from dipole-dipole interactions (except possibly with the amine). Specific interactions between cholesterol and the soluble steroids were not observed. Evidence suggests that low steroid concentrations influence structure of lipid films by altering the hydration layer in the surface film. In contrast to a specific site of action, it is proposed that steroid hormones initiate structural changes in a variety of biological sites; this model of steroid action is consistent with the ubiquity of many steroid hormones.

Regional specificity of membrane instability in Alzheimer's disease brain.

We report an inherent tendency towards the destabilisation of cellular membranes in Alzheimer's disease (AD) brain. This tendency is a natural consequence of abnormal membrane lipid composition, which has previously been documented in AD. Membrane destabilisation may contribute to AD pathogenesis in its own right and may also facilitate amyloid beta-protein deposition, which is potentially neurotoxic. The instability was found to co-localise selectively with areas of neurodegeneration in AD brain, thereby possibly accounting for the focal pathology observed in this disorder.

Disease and anatomic specificity of ethanolamine plasmalogen deficiency in Alzheimer's disease brain.

A significant and selective deficiency of ethanolamine plasmalogen (PPE) relative to phosphatidylethanolamine was identified in post mortem brain samples from patients with Alzheimer's disease (AD). This lipid defect showed anatomic specificity, being more marked at a site of neurodegeneration in AD brain than in a region relatively spared by the disease (mid-temporal cortex vs. cerebellum) and disease specificity for AD: it was not observed at the primary site of neurodegeneration in Huntington's disease (caudate nucleus) nor Parkinson's disease (substantia nigra). PPE deficiency parallels an inherent tendency towards membrane bilayer instability previously detected in AD brain which is necessarily due to a change in membrane lipid composition, and which may contribute to AD pathogenesis.

Membrane bilayer instability and the pathogenesis of disorders of myelin.

We have previously shown that total lipid extracts from normal nervous tissues spontaneously form a structure in vitro resembling the cell membrane bilayer, but only at a critical temperature, T*, equal to the 'physiological' temperature of the original tissues. In the present study, we found T* for normal human myelin lipids was 37 degrees C, in agreement with the concept that lipid metabolic pools maintain a critical composition in vivo which permits spontaneous formation of the (myelin) membrane bilayer at normal body temperature. But T* for myelin lipids from a patient with metachromatic leukodystrophy was less than 30 degrees C. Thus, myelin lipid composition was inappropriate for normal bilayer stability at this patient's core temperature, suggesting a mechanism whereby defective lipid metabolism in this disease could produce pathological myelin. The shift in T* in this patient was unlikely to be simply secondary to myelin destruction, as myelin lipids from a patient with advanced multiple sclerosis yielded a normal value for T* of 37 degrees C, even when extracted from areas of extensive demyelination.

A differential heat-conduction microcalorimeter for heat-capacity measurements of fluids.

A heat-conduction calorimeter has been developed for measuring small changes in heat capacity of milligram samples of membrane lipid dispersed in water as a function of temperature. The operation of the instrument is based on the principle that the thermal response of the sample to a short (10 s), electrically generated heat burst is a function of the diffusivity of the sample. Modeling studies of the instrument's performance have revealed that the output response after the heat burst is a function of only the heat capacity, rho Cp. Calibration of the instrument experimentally confirmed this behavior. This feature obviated the need to measure the thermal conductivity in order to determine rho Cp from the diffusivity equation, eta = lambda/rho Cp. The calorimeter has the following characteristics: reproducibility of loading: +/- 400 microJ/C degrees.cm3; baseline stability: +/- 10 microJ/C degrees.cm3 per 36 h; resolution (+/- 1 S.D.): +/- 50 microJ/C degrees.cm3; sample size 600 microliters.

Phospholipid surface bilayers at the air-water interface. III. Relation between surface bilayer formation and lipid bilayer assembly in cell membranes.

Lipid bilayer assembly in cell membranes has been simulated with total lipid extracts from human red blood cells and from mesophilic and thermophilic bacteria grown at several temperatures. Aqueous dispersions of these natural lipid mixtures form surface bilayers, a single bimolecular lipid state, but only at the growth temperature of the source organism. Thus, a single isolated bilayer state forms spontaneously in vitro from lipids that are available in vivo at the growth temperature of the cell. Surface bilayers form at a specific temperature that is a function of hydrocarbon chain length and degree of fatty acid unsaturation of the phospholipids; this property is proposed as an essential element in the control of membrane lipid composition.

Thermodynamics of phospholipid bilayer assembly.

Thermodynamic properties of bilayer assembly have been obtained from measurements of the solubility of the sodium salt of dimyristoylphosphatidylglycerol (DMPG) in water. The standard free energy of bilayer assembly delta G degree a is shown to be RT 1n Xs + zF psi 0 where Xs is the mole fraction of dissolved lipid, F is the Faraday constant, z is the valence of the counterion (Na+), and psi 0 is the electrical double-layer potential of the ionized bilayer. The function d 1n Xs/dT was found to be discontinuous at 24 degrees C, the gel-liquid-crystal transition temperature (Tm) for DMPG. This function was unaffected when solubilities were measured in 0.001 M NaCl solutions; thus, psi 0 is constant in the experimental temperature interval (4-40 degrees C). Using a value of psi 0 = -180 mV [Eisenberg et al. (1979) Biochemistry 18, 5213-5223], and the temperature dependence of delta G degrees a, values for delta H degrees a and delta S degree a at 24 degrees C were calculated for the gel and liquid-crystal states of DMPG. For the gel, delta H degrees a and T delta S a are -26.2 and 12.7 kcal/mol, respectively; for the liquid-crystal, delta H degrees a and T delta S degrees a are -19.2 and -5.7 kcal/mol, respectively. The calculated value for the latent heat of the gel-liquid-crystal transition is 7 kcal/mol, in agreement with calorimetric measurements.

Equilibrium studies of lecithin-cholesterol interactions I. Stoichiometry of lecithin-cholesterol complexes in bulk systems.

The maximum molar ratio of lecithin:cholesterol in aqueous dispersions has been reported to be 2:1, 1:1, or 1:2. The source of the desparate results has been examined in this study by analyzing (a) the phase relations in anhydrous mixtures (from which most dispersions are prepared) and (b) various methods of preparing aqueous dispersions, with the purpose of avoiding the formation of metastable states that may be responsible for the variability of the lecithin-cholesterol stoichiometry. Temperature-composition phase diagrams for anhydrous mixtures of cholesterol (CHOL) with dimyristoyl (DML) and with dipalmitoyl (DPL) lecithin were obtained by differential scanning calorimetry (DSC). Complexes form with molar ratios for lecithin:CHOL of 2:1 and 1:2; they are stable up to 70 degrees C. When x(CHOL) < 0.33, two phases coexist: complex (2:1) plus pure lecithin; when 0.33 < x(CHOL) < 0.67 complexes (2:1) and (1:2) coexist as separate phases. The corresponding phase diagram in water for these mixtures was determined by DSC and isopycnic centrifugation in D(2)O-H(2)O gradients. Aqueous dispersions were prepared by various methods (vortexing, dialysis, sonication) yielding identical results except as noted below. The data presented supports the following phase relations. When x(CHOL) < 0.33, two lipid phases coexist: pure lecithin plus complex (2:1) where the properties of the lecithin phase are determined by whether the temperature is below or above T(c), the gel-liquid crystal transition temperature. Therefore, complex (2:1) will coexist with gel state below T(c) and with liquid crystal above T(c). The densities follow in the order gel > complex (2:1) > liquid crystal. The density of complex (2:1) is less sensitive to temperature in the range 5 degrees -45 degrees C compared to the temperature dependence for DML and DPL where large changes in density occur at T(c). When x(CHOL) > 0.33, CHOL phase coexists with complex (2:1); anhydrous complex (1:2) is apparently not stable in H(2)O. The results are independent of the method and temperature used for preparing the lipid dispersions. However, when dispersions are prepared by sonication or with solvents at T > T(c), an apparent 1:1 complex is formed. Evidence suggests the 1:1 complex is metastable.

Thermal instability of red blood cell membrane bilayers: temperature dependence of hemolysis.

Rates of human red blood cell hemolysis were measured as a function of temperature. Three distinct temperature intervals for hemolysis were noted: a) At temperatures equal to or less than 37 degrees C no hemolysis was observed for the duration of the incubation (30 hr). b) For temperatures exceeding 45 degrees C hemolysis rates are rapid and are accompanied by gross changes in cellular morphology. The activation energy for hemolysis is 80 kcal/mole; this value is characteristic of protein denaturation and enzyme inactivation suggesting that these processes contribute to hemolysis at these high temperatures. c) Between 38 and 45 degrees C the energy of activation is 29 kcal/mole, indicating that a fundamentally different process than protein inactivation is responsible for hemolysis at these relatively low temperatures. A mechanism based on the concept of the critical bilayer assembly temperature of cell membranes (N.L. Gershfeld, Biophys. J. 50:457-461, 1986) accounts for hemolysis at these relatively mild temperatures: The unilamellar state of the membrane is stable at 37 degrees C, but is transformed to a multibilayer when the temperature is raised; hemolysis results because formation of the multibilayer requires exposing lipid-free areas of the erythrocyte surface. An analysis of the activation energy for hemolysis is presented that is consistent with the proposed unilamellar-multibilayer transformation.

Phospholipid surface bilayers at the air-water interface. I. Thermodynamic properties.

Dispersions of dimyristoylphosphatidylcholine (DMPC) in water spontaneously form a surface bilayer at the equilibrium air/water surface (Gershfeld, N. L., and K. Tajima, 1979, Nature [Lond.]. 279: 708-709). This phenomenon has now been demonstrated with dispersions of dioleoylphosphatidylcholine (DOPC), dipalmitoylphosphatidylcholine (DPPC), and with a mixture of DMPC and DOPC. Each of these dispersions forms a surface bilayer at a singularity in temperature that is a characteristic of the phospholipid. The surface bilayer formed by the lipid mixture is shown to have the same composition as the bulk liquid-crystal phase of the dispersion, and the surface components have identical partial molar entropies as the bulk lipid components. These properties indicate that the surface bilayer has the same structure as the bilayer in the liquid-crystal phase of the bulk dispersion.

Phospholipid surface bilayers at the air-water interface. II. Water permeability of dimyristoylphosphatidylcholine surface bilayers.

Dispersions of dimyristoylphosphatidylcholine (DMPC) in water have been reported to form a structure at 29 degrees C at the equilibrium air/water surface with a molecular density equal to that of a typical bilayer. In this study, the water permeability of this structure has been evaluated by measuring the rate of water evaporation from DMPC dispersions in water in the temperature range where the surface film density exceeds that of a monolayer. Evaporation rates for the lipid dispersions did not deviate from those for lipid-free systems throughout the entire temperature range examined (20-35 degrees C) except at 29 degrees C, where a barrier to evaporation was detected. This strengthens the view that the structure that forms at this temperature has the properties of a typical bilayer.

Probing the critical unilamellar state of membranes.

Aqueous dispersions of membrane phospholipids comprised of multilamellar vesicles (MLVs) will spontaneously transform to a stable unilamellar structure equivalent to the membrane bilayer, but only at a critical temperature T*. Since much of the evidence for this transformation derives from equilibrium thermodynamic studies, a description of the molecular and topological events occurring as the critical unilamellar state assembles has not previously been possible. Here we report experiments that provide evidence of a spontaneous topological change from MLVs towards unilamellar vesicles (ULVs) at T*. By applying a shearing stress to vesicle suspensions we have observed a decrease of approximately 25% in the force required to cause bilayers to leak; this decrease is confined to temperatures near T*. The T* values observed agree with those previously obtained by equilibrium methods. Using the method with total lipid extracts from normal biological membranes confirms that T* equals the physiological temperature of the original membranes.