CHOLESTEROL MODULATION OF MOLECULAR ACTIVITY OF RECONSTITUTED SHARK NA+,K+-ATPASE

The cholesterol content of liposome bilayers has been varied between 0-40 mol% to study the effects on reconstituted Na+,K+-ATPase. The maximum hydrolytic activity of reconstituted Na+,K+-ATPase was increased by cholesterol at concentrations above 10 mol% for both the physiological Na+/K+-exchange reactions, as well as for the partial reactions Na+/Na+-exchange and uncoupled Na+ efflux. Omission of cholesterol from the liposome bilayer modified the activation by cytoplasmic Na+, indicating effects on both V-max and on the Na+-affinity. Several other kinetic parameters were found to be strongly influenced as well, most notable the steady-state phosphorylation level, and the characteristics of the phosphorylation/dephosphorylation reactions. These results indicate that cholesterol interacts directly with the Na+,K+-ATPase as an essential effector perhaps by affecting its conformational mobility or monomer interaction.