Evoked effects of cholesterol binding on integral proteins and lipid fluidity of dog brain synaptosomal plasma membranes

Binding of cholesterol into dog brain synaptosomal plasma membranes (SPM) within the limits of concentration used (0.5-5 microM) follows an exponential curve described by the general formula y = a.ebx. This curve, which represents the total binding (specific and nonspecific), acquires sigmoid character in the presence of 100 microM cholesterol glucoside, with a Hill coefficient of h = 2.98 +/- 0.18. The specific activity of the Na+/K+-transporting ATPase and Ca2+-transporting ATPase rose after a 2-h preincubation of SPM with cholesterol (up to 5 microM) or its glucoside (up to 50 microM) to at least 50% above their original values. Fluorescence polarization of 1,6-diphenyl-1,3,5-hexatriene (DPH) increased with cholesterol glucoside (50 microM) incorporation. Cholesterol (5 microM) had no effect on the DPH fluorescence polarization. Arrhenius plots of Na+/K+-transporting ATPase activity exhibited a break point at 23.2 +/- 1.1 degrees C in control SPM, which was elevated to 29.5 +/- 1.4 degrees C in SPM treated with cholesterol glucoside (50 microM) and abolished in SPM treated with cholesterol (5 microM). The allosteric properties of SPM-bound Na+/K+-transporting ATPase inhibited by F- and Ca2+-transporting ATPase inhibited by Na+ (as reflected by changes in the Hill coefficient) were modulated by cholesterol. It could be stated that cholesterol glucoside (50 microM) produced an increased packing of the bulk lipids, while cholesterol (5 microM) increased the fluidity of the lipid microenvironment of both Na+/K+-transporting ATPase and Ca2+-transporting ATPase.