Calorimetric study of microsomal membrane.

Differential scanning calorimetric measurements of normal rat liver microsomes reveal a single thermal transition at about 50 degrees. This transition is assigned to irreversible prorein denaturation. There is no evidence for a reversible lipid phase transition at any temperature above 0 degrees, indicating that the microsomal membrane is in teh fluid state under these conditions. Rats fed a fat-free diet which increases the degree of saturation of fatty acids in the membrane lipids do produce microsomal membranes exhibiting a reversible lipid phase transition. The NADH=dependent and NADPH-dependent enzymatic reductions of cytochrome c show linear Arrhenius behavior in the normal rat liver microsomes but reveal discontinuities and breaks in the Arrhenius plots at approximately the calorimetrically determined phase transition temperatures in microsomes from rats fed the fat-free diet. Hence, the fluidity of cell membranes can be altered by diet with consequent effects on membrane-supported functions. The data further show that the lipid organization of the membrane is not independent of the protein component and supports models of membrane structure where a separate class of lipids forms a boundary between the bulk phase and the proteins.