Phosphatidylinositol as the endogenous activator of the (Na+ + K+)-ATPase in microsomes of rabbit kidney.

Incubation of rabbit kidney microsomes with pig pancreatic phospholipase A2 produced residual membrane preparations with very low (Na+ + K+)-ATPase activity. The activity could be restored by recombination with lipid vesicles of negatively-charged glycerophospholipids. Vesicles of pure phosphatidylcholine and phosphatidylethanolamine were virtually inactive in this respect, but could reactivate in the presence of cholate. Incubation of the microsomes with a combination of phospholipase C (Bacillus cereus) and spingomyelinase C (Staphylococcus aureus) resulted in 90--95% release of the phospholipids. The residual membrane contained only phosphatidylinositol and still showed 50--100% of the (Na+ + K+)-ATPase activity.