A new phosphatase of human platelets

Human platelets contain a phosphatase, which has a pH optimum at 7.0. Ethylenediamine tetraacetate and citrate at 0.01 mol/l concentration and Mg2+ at 0.002 mol/l concentration respectively bring about 7.7-, 3.6- and 2.1-fold increase in its activity. It is thermolabile. About 70% of its activity is lost when held at 50 degrees C for 10 min at pH 7.5. On electrophoresis through polyacrylamide gel at pH 8.3, about 87.8% of the total activity remained at the origin and the rest migrated toward the anode. Sodium chloride, potassium chloride, sodium phosphate, ouabain in the absence or presence of K+, potassium tartarte, phenylalanine, sodium fluoride have no effect on its activity. Urea at 1 mol/l and 0.5 mol/l concentrations respectively inhibited about 76% and 28% of its activity. It has the highest preference for p-nitrophenyl phosphate. Its Km and Vmax respectively are 5.7 x 10(-4) mol/l and 2.9 mumol . min-1. Its activity on beta-glycerophosphate is about 0.7% of that on p-nitrophenylphosphate. Its activity on ribo- and deoxyribonucleotides ranges from 0.4% to 3.0% of that on p-nitrophenylphosphate. Its maximum stimulation by EDTA, its non-dependence on K+ and its insensitivity to inorganic phosphate and ouabain, make human platelet neutral phosphatase herein described, distinct from the membrane bound neutral phosphtases so far reported.