Acetyl phosphate as a substitute for ATP in [formula omitted] ATPase

Abstract

1. 1. In the presence of Mg 2+ and Na +, acetyl phosphate replaced ATP as an agent for phosphorylating Na +- and K +-dependent ATPase (ATP phosphohydrolase, EC 3.6.1.3). 2. 2. The phospho-enzymes produced by both substrates were similar in the following respects: (a) electrophoretic mobility and chemical reactivity of phosphopeptides released by proteolytic digestion of the denatured enzyme, (b) the quantity of phospho-enzyme, (c) the concentration of Na + for half-maximal phosphorylation, (d) a requirement for Mg 2+, (e) the half-life at o°. Furthermore, each substrate inhibited phosphorylation by the other, and the inhibitor ouabain enhanced the inhibitory effect of acetyl phosphate. 3. 3. In the presence of Mg 2+ and K +, acetyl phosphate, unlike ATP, also phosphorylated the enzyme. The quantity of phospho-enzyme was less and its turnover was faster. K + appeared to accelerate both phosphorylation and dephosphorylation. 4. 4. The same active site may be an intermediate in both the Na +- and K +-dependent ATPase activity and the K +-dependent acetyl phosphatase activity of this enzyme system in functionally different conformations of an active center.