Phosphotyrosine phosphatase activity in human placenta.

A phosphotyrosine phosphatase was enriched 9-fold from human placenta homogenates using ammonium sulfate fractionation and chromatographies on P-cellulose and Phenyl Sepharose. The enzyme preparation hydrolyzed phosphotyrosine, p-nitrophenyl phosphate, as well as phosphoenol-pyruvate. The enzyme was unstable, and its Km value for phosphotyrosine increased during the purification procedure. The optimum pH for phosphotyrosine was found to be pH 7.0. The phosphotyrosine phosphatase activity was completely inhibited at 2.6 mM Cu2+. Zn2+ was slightly inhibitory to the phosphatase activity. The effects of several inhibitors on the phosphotyrosine phosphatase activity were different from those on the p-nitrophenyl phosphatase activity. These results suggest that the phosphotyrosine phosphatase activity from human placenta is different from previously reported acid and alkaline phosphatases.