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A fluorescent probe study of the lipid mobility of membranes containing sodium- and potassium-dependent adenosine triphosphatase.

The temperature-activity relationship of a membrane (Na+ + K+)-ATPase preparation [Mg2+-dependent, ouabain-sensitive, (Na+ + K+)-activated ATP phosphohydrolase, EC 3.6.1.3] obtained from sheep kidney cortex and medulla was determined and found to be very similar to that previously reported for preparations of this enzyme from either rabbit kidney or ox brain. These temperature-activity relationships can be shown as Arrhenius plots which characteristically are nonlinear and have transition temperatures near 22°. Two noncovalently bound fluorescent probes, 12-(9-anthroyl)stearic acid (12-AS) and N-phenyl-1-naphthylamine (NPN), were used to label the hydrophobic core of the partially purified membranes rich in (Na+ + K+)-ATPase. The fluorescence polarization of these probes was determined between 10° and 40°. The rotational relaxation times (ρ) for each probe were then calculated, and secondary plots of reciprocal relaxation time vs. reciprocal temperature were constructed. The plots for membranes labeled with 12-AS and NPN were nonlinear and showed transition temperatures near 22°, in good agreement with the transition temperature of the hydrolytic activity of the enzyme. A similar transition temperature was detected by right-angle light scattering of an unlabeled microsomal preparation of (Na+ + K+)-ATPase and of an aqueous suspension of liposomes made from a total lipid extract of the enzyme-containing membranes, thus excluding any direct effect of addition of the fluorescent probes to the membranes. The transition temperatures observed under all experimental conditions were very similar. We conclude that the nonlinear temperature-activity relationship of (Na+ + K+-ATPase and the nonlinear fluorescence polarization—temperature profile both arise from a temperature-dependent change in the molecular mobility of the membrane lipids in the immediate environment of the probes and the "active center" of the (Na+ + K+)-ATPase subunits. These changes illustrate the strong cooperative effect between the physical state of the membrane lipids and the functional state of the enzyme protein in this particulate membrane enzyme system, and suggest a powerful modulating effect of membrane lipids in regulating enzyme activity, or drug-receptor interactions more generally.