Mg2+- or Mn2+-dependent p-nitrophenylphosphatase activity is present in Ehrlich ascites tumor cells

The presence of a soluble, Mg2+- or Mn2+-dependent p-nitrophenylphosphatase activity in Ehrlich ascites tumor cell homogenates is reported. The crude homogenate was fractionated over Sephadex G-150 gel-filtration and DEAE-Sephacel anion-exchange columns, and two p-nitrophenylphosphatase activities were resolved. The most active fraction, Peak I, was characterized and found to be similar to phosphotyrosyl-protein phosphatases characterized elsewhere in that it has optimal activity at neutral pH; it is inhibited by phosphate, Zn2+, and vanadate; and it is not inhibited by levamisole. However, Peak I differs from phosphotyrosyl-protein phosphatases in that Mg2+ or Mn2+ is required for activity, fluoride is an inhibitor, and pyrophosphate is not inhibitory. Inhibition by the phosphorylated compounds phosphotyrosine, phosphoserine, phosphothreonine, ATP, CTP, GTP, ITP, NADP, fructose 6-phosphate, glucose 1-phosphate, galactose 1-phosphate, 2-phosphogluconic acid, and 6-phosphogluconic acid was also observed. Ehrlich ascites tumor cell p-nitrophenylphosphatase is shown to be sensitive to inactivation by trypsin, N-ethylmaleimide, or heat treatments.