Stimulation by ATP of alkaline phosphatase in placental plasma membranes.

1. ATP stimulated the p-nitrophenyl phosphatase activity of placental plasma membranes, with an increase in activity of approximately 100% at 5 mM ATP. The stimulation was not dependent on the presence of Mg-2-+. 2. The K-m for p-nitrophenyl phosphate was not changed by the presence of 5 mM ATP. 3. ATP hydrolysis by the plasma membrane preparation under the same assay conditions as for alkaline phosphatase was not influenced by the presence of 5 mM p-nitrophenyl phosphate. 4. Extraction of the plasma membrane preparation with n-butanol abolished the stimulatory effect of ATP, as well as Ca-2-+-activated ATPase activity.