The effect of cholesterol and epicholesterol incorporation on the permeability and on the phase transition of intact Acholeplasma laidlawii cell membranes and derived liposomes

1. 1. The effect of incorporated cholesterol and epicholesterol upon the glycerol and erythritol permeability through the membrane of Acholeplasma laidlawii (previously denoted as Mycoplasma laidlawii) is studied. Both sterols, when present in the growth medium, are incorporated to the same extent in the A. laidlawii membrane. Only the cholesterol-containing A. laidlawii membrane shows a reduced permeability towards glycerol and erythritol as compared to the sterol-free cells. The 3α-hydroxy isomer, epicholesterol, does not affect the membrane permeability. 2. 2. Liposomes prepared from lipids isolated from cells grown on cholesterol-rich media also show a reduced glycerol and erythritol permeability as compared to liposomes prepared from lipids isolated from sterol-free control cells. This permeability-lowering effect can be correlated with a condensing effect of cholesterol upon a monolayer of total A. laidlawii lipids. 3. 3. The phase transitions occurring in membranes and extracted lipids of A. laidlawii have been studied by differential scanning calorimetry. Incorporated cholesterol causes a considerable reduction of the energy content of this phase transition. This reduction in energy is the same for the intact A. laidlawii cell membrane as for the liposomal bilayer system of the extracted lipids dispersed in water. 4. 4. Using the synthetic lecithin 1-oleoyl-2-stearoyl-sn-glycero-3-phosphorylcholine the same phenomenon is observed. 32 mole % cholesterol completely eliminated the phase transition of the lecithin. Epicholesterol 5α-androstan-3β-ol and cholest-4,6-dien-3-one are unable to show such an effect, also suggesting the importance of the 3β-OH group of the sterol molecule for the specific sterol-lecithin interaction

The distribution of cholesterol in bilayers of phosphatidylcholines as visualized by freeze fracturing

The crystallization behavior of bilayers of synthetic phosphatidylcholines in the presence of cholesterol was investigated by freeze fracturing. 1. 1. Below the lipid-phase transition, cholesterol is randomly distributed over the lateral plane of a bilayer of the single species dimyristoylphosphatidylcholine, as far as can be detected within the lateral resolution of freeze fracturing. 2. 2. In an equimolar mixture of dilauroyl- and dimyristoylphosphatidylcholine, which shows cocrystallization, cholesterol is distributed randomly in the bilayers below the phase-transition temperature. 3. 3. In an equimolar mixture of dipalmitoyl and 1-palmitoyl-2-oleoylphosphatidylcholine, which shows monotectic behaviour, cholesterol interacts preferentially with the liquid crystalline species 1-palmitoyl-2-oleoylphosphatidylcholine when the other component dipalmitoylphosphatidylcholine passes from the liquid crystalline to the gel state upon cooling the mixture

Differential scanning calorimetry on mixtures of lecithin, lysolecithin and cholesterol

The effect of increasing concentrations of lysolecithin (1-palmitoyl-sn-glycerol-3-phosphorylcholine) on the gel → liquid crystal thermal transition of lecithin (1,2-dipalmitoyl-sn-glycerol-3-phosphorylcholine) in the aqueous phase was studied by differential scanning calorimetry (DSC). Lysolecithin showed an endothermic transition at 3.4°C whereas the transition of the lecithin occurred at 42°C. No phase separation could be observed calorimetrically at lysolecithin concentrations up to 60 mol%. Freeze etch electron microscopy showed that mixtures containing as much as 50 mol% lysolecithin exist in a lamellar phase. The lysolecithin was found to cause an initial slight increase in the enthalpy of transition followed by a gradual decrease. The enthalpy increased again at very high lysolecithin concentrations. The lysolecithin also caused a non-linear decrease in the temperature at which the lecithin transition took place. Cholesterol was found to decrease the enthalpy of transition of the lysolecithin, eliminating it at a concentration of 50 mol%. Cholesterol caused an increase in the temperature at which the lysolecithin transition took place

Studies on the biological properties of polyene antibiotics: Comparison of other polyenes with filipin in their ability to interact specifically with sterol

1.1. The interaction fo the five polyene antibiotics filipin, etruscomycin, pimaricin, nystatin and amphotericin B with sterol, primarily free, liposomal and membrane bound cholesterol has been examined. Each of these antibiotics has a characteristic ultraviolet absorption spectrum in aqueous or organic solvents with three or four ultraviolet absorption maxima.Addition of free cholesterol to aqueous solutions of these antibiotics results in a change of the ratio of the ultraviolet absorbance maxima. The order of effectiveness of interaction with cholesterol as judged by this criterion was filipin, amphotericin B, etruscomycin and pimaricin. 2. 2. The same alteration in ultraviolet absorption spectra of filipin etruscomycin and amphotericin B observed with addition of free cholesterol to aqueous solutions of these antibiotics also occurs upon addition of these antibiotics to liposomal, erythrocyte or Acholeplasma membrane bound cholesterol. No spectral change was found in membranes devoid of cholesterol. This spectra alteration of the antibiotic was accompanied by a binding of the antibiotic to the membrane. Both nystatin and pimaricin showed little change in spectrum with sterol in these systems, either the artificial or natural membranes which contained cholesterol. 3. 3. The structural requirements of the sterol for the spectral change with filipin, etruscomycin and amphotericin B include a planar sterol nucleus, and intact side chain at C-17 and 3β-hydroxyl group. The spectral change was not affected by the pH except in the case of amphotericin B. 4. 4. Measurements by differential scanning calorimetry of the effect of these polyene antibiotics on the phase transition of lecithin and lecithin-cholesterol showed that all polyenes can reduce the ecithin cholesterol interaction

Calorimetric and freeze-etch study of the influence of Mg2+ on the thermotropic behaviour of phosphatidylglycerol

In the presence of Mg2+ ions phosphatidylglycerol shows supercooling which leads to the formation of a metastable gel phase. This contrasts with the behaviour of this negatively charged phospholipid in the presence of Ca2+ ions (Biochim. Biophys. Acta 339 (1974) 432). It is demonstrated that the heat content of this phospholipid is dependent on the ionic environment

Non-random distribution of cholesterol in phosphatidyl-choline bilaye

1. 1. The effect of cholesterol upon the phase transition occurring in various mixtures of synthetic disaturated phosphatidylcholines with fatty acid constituents of 12, 14, 16, and 18 carbon atoms was measured by differential scanning calorimetry. 2. 2. Mixtures which differ 2 carbon atoms show cocrystallisation of the paraffin chains. In these mixtures cholesterol interacts randomly with the various phosphatidylcholine species. 3. 3. Mixtures which differ 4 or more carbon atoms show monotectic behaviour (phase separation). In these mixtures cholesterol interacts preferentially with the phosphatidylcholine species with the lowest transition temperature. This results in a non-random distribution of cholesterol at temperatures at which phase separation occurs. The implications of these findings for cholesterol-containing biological membranes are discussed