The fraction of phosphatidylinositol that activates the (Na+ + K+)-ATPase in rabbit kidney microsomes is closely associated with the enzyme protein

1. Extensive treatment of rabbit kidney microsomes with phosphatidylinositol-specific phospholipase C under various conditions never resulted in more than 75% hydrolysis of the substrate. 2. The non-degraded fraction of the phosphatidylinositol (10–12 nmol per mg microsomal protein) could be recovered only by an acidic extraction procedure. 3. The $\text{(}\text{Na}{}^{\mathrm{+}}\text{+}\text{K}{}^{\mathrm{+}}\text{)-ATPase}$ activity found in those membranes was not affected by this treatment. 4. Complete degradation of phosphatidylinositol could be easily achieved when the phospholipase was applied to rat liver microsomes which do not contain any detectable $\text{(}\text{Na}{}^{\mathrm{+}}\text{+}\text{K}{}^{\mathrm{+}}\text{)-ATPase}$ activity. 5. It is concluded that in rabbit kidney microsomes a close association exist between the $\text{(}\text{Na}{}^{\mathrm{+}}\text{+}\text{K}{}^{\mathrm{+}}\text{)-ATPase}$ and that fraction of the phosphatidylinositol that is directly involved in the maintenance of its activity.