(K+)-dependent acyl phosphatase as part of the (na+ + K+)-dependent ATPase of cell membranes.

Abstract

Summary 1. A (K + )-dependent acetyl phosphatase (acyl phosphate phosphohydrolase, EC 3.6.1.7) activity of a (Na + + K + )-dependent ATPase (ATP phosphohydrolase, EC 3.6.1.3) preparation from cell membranes of guinea-pig kidney cortex was compared with the (Na + (Na + + K + )-dependent ATPase activity. 2. Of a variety of phosphate compounds tested only acetyl phosphate and carbamyl phosphate showed a (K + )-dependent hydrolysis which was comparable with the (Na + + K + )-dependent hydrolysis of ATP. 3. A close relationship was shown between the (K + )-dependent hydrolysis of acetyl phosphate and the (K + )-dependent part of the hydrolysis of ATP by the (Na + + K + )-dependent ATPase preparations. Both activities showed the following characteristics: K + could be replaced by Rb + Cs + , NH 4 + or Li + but not by Na + . The K m 's for Mg 2+ , K + , Rb + , NH 4 + and Li + were similar. The optimal pH was 7.8 for both. Both activities were inhibited by ouabain, Ca 2+ , Hg 2+ , and F − with similar K i 's, They had similar activation energies. ATP and acetyl phosphate were each competitive inhibitors of the other as substrate. 4. These close relationships suggest that the (K + )-dependent acetyl phosphatase activity arises from the (Na + + K + )-dependent ATPase system and that an acyl phosphate may be a functional intermediate in the system.