differential scanning calorimetry of milk fat globule membranes differential scanning calorimetry was employed as an aid in examining the structure of the bovine milk fat globule membrane at least six major endotherms are observed between 10 and 90 degree c corresponding to orderdisorder transitions of discrete structural domains of the membrane these endothermic transitions occur at 16 28 43 58 68 and 75 degrees c the transitions occurring between 10 and 50 degrees c were reversible suggesting the involvement of lipid however the high temperature transitions were irreversible the calorimetric c transition center at 43 degree c was shown to involve neutral lipid since the endotherm was reversible insensitive to proteolysis and similar to the endotherm of the isolated neutral lipid fraction of the milk fat globule membrane the glycolipid and phospholipid fractions of the milk fat globule membrane yielded endotherms outside of the temperature range of the c transition another endotherm the d transition 58 degree c was found to involve the denaturation of the major membrane coat protein butyrophilin band 12 evidence for this assignment included the following observations i the nearly selective proteolysis of butyrophilin resulted in the complete removal of the d transition ii the butyrophilinenriched triton x100insoluble pellet of milk fat globule membrane yielded a relatively normal d transition and iii the irreversible disulfidestabilized aggregation of butyrophilin occurred in the membrane solely at the temperature of the d transition furthermore no other prominent milk fat globule membrane polypeptide formed these nonnative disulfide crossbridges during the d transition the sources of the other major endotherms of the milk fat globule membrane have not yet been assigned