Calorimetric studies of the thermotropic phase behavior of cells, membranes and lipids from fatty acid-homogeneous Acholeplasma laidlawii B.

We have investigated the comparative lipid thermotropic phase behavior of intact cells, isolated membranes and total membrane lipid dispersions from fatty acid-homogeneous Acholeplasma laidlawii B by high-sensitivity differential scanning calorimetry. We find that the gel to liquid-crystalline phase transition temperature, enthalpy and degree of cooperativity are essentially identical in viable cells and isolated membranes and that these parameters are not affected by the thermal denaturation of the cellular or membrane proteins. In contrast, the extracted lipid dispersions, although having the same phase transition temperature, exhibit a greater enthalpy and a reduced cooperativity than do the lipids in cells or membranes. These results imply that lipid organization is identical in cells and isolated membranes, but different in the derived model membrane, probably due to the influence of membrane proteins and/or the presence of lipid bilayer compositional asymmetry in cells and membranes. We also demonstrate for the first time the existence of gel state polymorphism in a biological membrane.