Michaelis Constant (Km)
20 mM Tris-HCl
The enzyme activity was determined with BioTek Synergy microplate reader at 37 °C with 200 μL reaction volumes, including 20 mM Tris-HCl (pH 8.0), 125 μM methyl parathion and purified enzyme (1 μg/mL). One unit (U) of MPH means micromoles of methyl parathion or parathion hydrolyzed by MPH per minute under optimum conditions (30 to 125 μM) were used to measure the kinetic constants (Km and kcat) with a Michaelis–Menten plot [v0 = Vmax[E]kcat/(Km + [s])]. All the enzymatic assays were repeated three times and the average values were adopted.