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