Differential scanning calorimetry studies of rabbit cardiac sarcolemma

Thermal transitions were measured by differential scanning calorimetry for rabbit cardiac sarcolemma in 3-(N-morpholino)propanesulfonic acid buffer at pH 7.5, in glycerol–buffer and dimethyl sulfoxide – buffer mixtures, after heat denaturation, and after enzymatic degradation of the proteins. Specific solvent effects on the protein transitions were observed. Glycerol stabilized some of the four protein transitions, while dimethyl sulfoxide destabilized all protein transitions. The thermal transitions in the lower temperature range were studied for both the membranes and the lipid extracted from the membranes. A very small endotherm was observed for both the lipid extracted from the sarcolemma and the intact membrane (0.1–0.2 cal/g; 1 cal = 4.1868 J). A larger endotherm was observed in both the glycerol–buffer and dimethyl sulfoxide – buffer mixtures. Major perturbation of the protein by enzymatic degradation (papain or trypsin digestion), by heat denaturation, or by reaction with excess N-ethylmaleimide all produced larger endotherms near 20 °C. The very small magnitude of the endotherm near 20 °C suggests that it is not a typical gel – liquid crystalline transition of the bilayer. However, the occurrence of an endotherm in the extracted lipid suggests that some reorientation of lipid is involved.