Effects of N-acetylimidazole on human erythrocyte ATPase activity. Evidence for a tyrosyl residue at the ATP binding site of the (Na+ plus K+)-dependent ATPase.

1. Acetylation of human erythrocytes by N-acetylimidazole alters the structure of stroma prepared from these cells and the degree of alteration appears to be dependent upon the level of the initial treatment. These changes do not occur when stroma are acetylated. 2. Deacetylation by hydroxylamine or mild alkaline treatment causes a complete recovery of the (Na+ plus K+)-dependent and the Ca2+ -stimulated ATPase activities and indicates that the inhibition is due to the acetylation of a tyrosyl residue. There is only partial recovery of the Mg2+ -dependent ATPase after deacetylation. 3. ATP or Mg-ATP completely protect the (Na+ plus K+)-dependent ATPase, but not the Ca2+ -stimulated system. 4. The results indicate that the (Na+ plus K+)-dependent and the Ca2+ -stimulated ATPase activities have separate substrate binding sites and most likely are separate enzyme systems. 5. Acetylation of human erythrocytes has no effect on D-glucose transport.