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 $\text{(Na}{}^{\mathrm{+}}\text{+ K}{}^{\mathrm{+}}\text{)-ATPase}$ 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 sphingomyelinase C (Staphylococcus aureus) resulted in 90–95% release of the phospholipids. The residual membrane contained only phosphatidylinositol and still showed 50–100% of the $\text{(Na}{}^{\mathrm{+}}\text{+ K}{}^{\mathrm{+}}\text{)-ATPase}$ activity.