| Abstract: | The Na-K ATPase found in sedimentable fractions of intestinal epithelium of rats hydrolyzed cytidine triphosphate nearly as well as ATP (25% to 50%); was active only in presence of divalent cations, with specificity for Mg (100%), Mn (50%) and Ca (10%); showed a plateau of activation when Mg concentrations were in excess of substrate; and was inhibited by a second divalent cation (Zn > Mn > Ca), and by 3 × 10?4 M ouabain (50%). Parallel assays of rat red cell ghosts showed differences in substrate specificity (CTP was not utilized), in activation kinetics (activation peak with Mg) and in greater specificity to Mg (Mn was a weaker activator and Zn was a weaker inhibitor). Stabilities also differed in the two preparations: Na? K ATPase of intestinal epithelium was activated by sucrose extraction and denatured during cytolysis at room temperature, while that of red cell fragments was denatured during sucrose extraction and preserved by hemolysis at room temperature. Other properties of Na? K ATPase studied in the two tissues included activation by monovalent cations (optimum at 160 mM Na, 15 mM K), specificity to monovalent cations, and sensitivity to lipid solvents and to some drugs. The data were discussed in terms of comparative properties of Na? K ATPases of various cells. Residual ATPase activities of intestinal epithelium and red cell ghosts were shown to differ in substrate specificity, inhibition and activation. “Residual ATPase” from intestinal epithelium was a zinc-activated nucleoside polyphosphate phosphohydrolase, while ghosts contained Mg? ATPase. Only the latter enzyme was specific to ATP and Mg, activated by Ca in presence of Mg, and sensitive to inhibition by PCMB and Zn. |
