Steady-state kinetic parameters were measured at 25 °C. Concentrated protein aliquots were thawed on ice and diluted with a buffer base mix (BBM) containing 50 mM sodium acetate, 100 mM Tris, and 100 mM bis-Tris from pH 4.0 to pH 7.5. This buffer system maintains constant ionic strength throughout the pH range examined. A 50 mM solution of pNPP was prepared in the buffer base mix. The reactions were run on a 96-well plate using substrate concentrations from 0.76 to 10.61 mM. Reactions were allowed to proceed for 2 -8 min for WT SHP-1 and the SHP-1 variants.