TY - JOUR T1 - Ouabain-Dependent Incorporation of <sup>32</sup>P from <em>p</em>-Nitrophenyl Phosphate into a Microsomal Phosphatase JF - Molecular Pharmacology JO - Mol Pharmacol SP - 99 LP - 107 VL - 6 IS - 2 AU - CHARLES E. INTURRISI AU - ELWOOD TITUS Y1 - 1970/03/01 UR - http://molpharm.aspetjournals.org/content/6/2/99.abstract N2 - The microsomal fraction of beef brain that contains both (Na+ + K+)-dependent ATPase and K+-dependent phosphatase activities can, in the presence of ouabain, be labeled with 32P from p-nitrophenyl phosphate-[32P]. The ouabain-dependent labeling requires Mg++, but not Na+ or K+. When a steady-state level of ouabain-dependent labeling is maintained with a saturating concentration of p-nitrophenyl phosphate-[32P], the specific activity of the phosphoprotein can be reduced b addition of unlabeled p-nitrophenyl phosphate or Pi. If, however, 32Pi of the same specific activity as the labeled p-nitrophenyl phosphate is added, there is no increase in the absolute amount of phosphoprotein labeled by the ouabain-dependent mechanism. The conditions that support ouabain-dependent labeling from p-nitrophenyl phosphate-[32P] also permit labeling by 32Pi. However, the amount of Pi generated by hydrolysis of p-nitrophenyl phosphate cannot account for the labeling observed. These studies suggest that ouabain places the phosphatase in a configuration that allows p-nitrophenyl phosphate and Pi, to label the same site on the enzyme. ER -