Stimulation of the ATP, Mg-dependent protein phosphatase by p-nitrophenyl phosphate.

The phosphorylase phosphatase activity of the ATP,Mg-dependent protein phosphatase is stimulated by p-nitrophenyl phosphate (pNPP). All the active forms of this type of enzyme show this property, which seems to be unrelated to any pNPP-hydrolyzing activity. The increase in activity is due to an increase in Vm, the Km being unchanged. The possibility that pNPP acts as a deinhibitor is excluded. pNPP acts as a competitive inhibitor on the phosphorylase phosphatase activity of the different polycation-stimulated protein phosphatases. Stimulation by pNPP can be used as a differential criterion in a specific assay of the active forms of the ATP,Mg-dependent phosphatase.