Studies on (Na+ + K+)-activated ATPase. XXXVIII. A 100 000 molecular weight protein as the low-energy phosphorylated intermediate of the enzyme

Abstract

Phosphorylation of NaI-treated bovine brain cortex microsomes by inorganic phosphate in the presence of Mg2+ and ouabain has been studied at 0 degrees C (pH 7.4) and 20 degrees C (pH 7.0). Nearly maximal (90%) and half-maximal phosphorylation are achieved at 20 degrees C within 2 min with 50--155 and 5.6--17 muM 32Pi, respectively, and at 0 degrees C within 75 s with 300--600 and 33--66 muM 32Pi, respectively. Maximal phosphorylation yields 146 pmol 32P - mg-1 protein. Without ouabain (20 degrees C, pH 7.0) less than 25% of the incorporation observed in the presence of ouabain is reached. Preincubation of the native microsomes with Mg2+ and K+, in order to decompose possibly present high-energy phosphoryl-bonds prior to ouabain treatment, does not affect the maximal phosphate incorporation. This indicates that the inorganic phosphate incorporation is not due to an exchange with high-energy phosphoryl-bonds, which might have been preserved in the microsomal preparations. Phosphorylation of the native microsomes by ATP in the presence of Mg2+ and Na+ reaches 90 and 50% maximal levels within 15--30 s at 0 degrees C and pH 7.4 at concentrations of [gamma-32P]ATP of 5--32 and 0.5--3.5 muM, respectively. The maximal phosphorylation level is 149 pmol 32P-mg-1 protein, equal to that of ouabain-treated microsomes phosphorylated by inorganic phosphate. Both inorganic phosphate and ATP phosphorylate on site per active enzyme subunit of 135 000 molecular weight. From the equilibrium constants for the phosphorylation of ouabain-treated microsomes by inorganic phosphate at 0 degrees C and 20 degrees C standard free-energy changes of --5.4 and --6.8 kcal/mol, respectively, are calculated. These values yield a standard enthalpy change of 14 kcal/mol and an entropy change of 70 cal/mol - degree K. This characterizes the reaction as a process driven by an entropy change. The intermediate formed by phosphorylation with Pi has maximal stability at acidic pH, as is the case for the intermediate formed with ATP. Solubilization in sodium dodecyl sulfate stabilizes the phosphoryl-bond in the pH range of 4--7. The non-solubilized preparation has optimal stability at pH 2--4, the level of which is equal to that of detergent-solubilized intermediate. Sodium dodecyl sulfate gel electrophoresis of the microsomes at pH 3, following incorporation of 32Pi yields 11 protein bands, only one of which (mol. wt 100 000--106 000) carries the radioactive label. This protein has the same molecular weight as the protein, which is phosphorylated by ATP in the presence of Mg2+ and Na+.