A differential scanning calorimetry study of acetylcholine receptor-rich membranes from Torpedo californica.

Various acetylcholine receptor-rich membrane preparations from Torpedo californica electroplax tissue were examined using the techniques of differential scanning calorimetry coupled with gel electrophoretic analysis of heat-denaturing material and functional assays following passage through discrete transitions. In unfractionated membranes, four irreversible calorimetric transitions were observed, one of which (Td = 59 degrees C) could be assigned to a complete loss of acetylcholine receptor function. A second lower temperature transition apparently corresponds to loss of certain peripheral membrane proteins including the Mr = 43,000 polypeptide and the acetylcholinesterase activity. Membrane preparations highly enriched in acetylcholine receptor polypeptides contained a major transition at 59 degrees C which could be shown to be sensitive to the presence of added ligands of the acetylcholine receptor, supporting its assignment to structural alterations of the receptor protein or its arrangement in the membrane.