Characterization of a bovine brain magnesium-dependent phosphotyrosine protein phosphatase that is inhibited by micromolar concentrations of calcium

In this study a rho-nitrophenyl phosphate (PNPP) phosphatase was purified 476-fold from bovine brain cytosol. The molecular weight of the enzyme is 84,000 as determined by gel filtration. The PNPP phosphatase could also dephosphorylate 32P-Tyr]-casein and -poly (Glu, Tyr). 32P-ser]-casein and -histone were not substrates. The phosphatase activity was found to be totally dependent on divalent metal ions. Mg2+ was the most effective with Ka of 20 microM. Ca2+ was found to be a potent inhibitor of the phosphatase. Using PNPP as a substrate the IC50 for Ca2+ was 0.6 microM. Several known inhibitors of phosphotyrosyl protein phosphatases such as Zn2+, vanadate, and molybdate also inhibited the PNPP phosphatase. The very high sensitivity for inhibition by Ca2+ suggests that the activity of the phosphotyrosyl protein phosphatase may be regulated by fluctuations in the intracellular concentrations of Ca2+.