Cholesterol-dependent interaction of polyunsaturated phospholipids with Na,K-ATPase

Polyunsaturated phospholipids such as 16:0-22:6 PC and 22:6 PC both stabilized the El conformation and inhibited turnover of Na,K-ATPase reconstituted into 18:1 PC or 18:1 PC/cholesterol liposomes. The inhibition increases in the order 22:6 PC > 16:0-22:6 PC both in the presence and in the absence of cholesterol, but is most pronounced in the absence of cholesterol. The inhibition of Na,K-ATPase turnover may thus correlate with the capability of polyunsaturated phospholipids and cholesterol to induce liquid-disordered and liquid-ordered lipid phases, respectively. In the presence of cholesterol 16:0-22:6 PC and 22:6 PC both increase the apparent Na+ affinity and change the K+ inhibition observed at low ATP concentration into activation. These effects on Na,K-ATPase kinetics can be explained by the ability of polyunsaturated phospholipids to induce lateral phase separation from cholesterol, which may be partially excluded from interaction with the Na,K-ATPase/lipid interface. Finally, inclusion of polyunsaturated phospholipids may induce changes in the bilayer hydrophobic thickness, which will increase the hydrophobic mismatch between lipids and protein.