Thermotropic lipid phase transition and the behavior of hydrolytic enzymes in the kidney cortex brush border membrane

Functional interactions of lipids and proteins were examined in brush-border membranes isolated from the kidney cortex by studying the temperature dependence of the hydrolytic enzyme activities. A close relationship was observed for the membrane proteins and the thermotropic lipid phase transitions. Three lines of evidences were provided for such dependence: a) Arrhenius relationship of the membrane-bound enzyme activities, and the effect of temperature in native and partially delipidated membranes, b) differential scanning calorimetric study of the membrane lipid phase transitions in the native and delipidated membranes, multilamellar vesicles prepared from the membrane extracted lipids, and in vesicles from dimyristoyl phosphatidylcholine, and c) the excimer (dimer)-formation studies of the membrane extrinsic fluorescent probe, pyrene, and the resultant membrane microviscosity. The brush-border membranes were partially delipidated with BuOH and 2,2,2-trifluoroethanol. The functional interactions of the delipidated membranes, which were greatly lost on lipid removal, were largely restored by the addition of exogenous lipids in the reconstitution process, which indicate the critical dependence of the membrane integral proteins on the neighboring lipid molecules in the bulk lipid phase.