Lipid dynamics and lipid-protein interactions in rat hepatocyte plasma membranes

Rat hepatocyte plasma membranes were isolated and examined by differential scanning calorimetry and by steady state fluorescence polarization of 1,6-diphenyl-1,3,5-hexatriene, 12-anthroyl stearate, and 2-anthroyl stearate. Calorimetry of the intact membranes revealed a reversible lipid thermotropic transition with lower and upper critical temperatures of 18 degrees and 31 degrees C, respectively. The transition was also observed in lipid extracts of the membrane, both dried and rehydrated. The transition enthalpies estimated for intact membranes, dried membrane lipids, and rehydrated lipids, respectively, were approximately 0.3, 1.2 to 1.4, and 0.5 to 0.7 cal/g of lipid. The transition observed in the native membranes is broad and of low enthalpy, owing in part to the relatively high cholesterol content and to protein-lipid interactions. Fluorescence polarization studies detect the lower critical temperature of the transition in the membranes and in sonicated dispersions of the membrane lipid. Arrhenius studies of membrane 5'-nucleotidase and alkaline phosphatase give two-slope plots with breakpoints, respectively, of approximately 17 degrees and 26 degrees C. These break points and others reported for a number of hepatocyte membrane protein activities cluster uniformly at either the lower or the upper critical temperature of the lipid transition observed by differential scanning calorimetry.