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The best method for Arylesterase activity was determined spectrophotometrically at 270 nm with phenyl acetate used as the substrate. The assay mixture included 1.0 mmol/L of phenyl acetate and 0.9 mmol/L CaCl2 in 20 mmol/L Tris HCl, pH 8.0, at 25°C. Nonenzymatic hydrolysis of phenyl acetate was subtracted from the total rate of hydrolysis. The E270 for the reaction is 1310 mol/L−1 · cm−1 and 1 unit of arylesterase activity is equal to 1 micromole of phenyl acetate hydrolyzed per milliliter per minute.

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