Carbonic Anhydrase (CA) catalyzes the reversible reaction of CO2? H2O = HCO3-? H , which is fundamental to many processes such as respiration, renal tubular acidification and bone resorption. CA13 is a cytosolic enzyme with a unique and widespread distribution pattern as compared to the other cytosolic CAs, indicating that it may play important physiological roles in several organs. The activity of recombinant mouse CA13 was measured by its ability to hydrolyze 4-Nitrophenyl acetate (4-NPA) to 4-Nitrophenol. The reaction was performed in 12.5 mM Tris, 75 mM NaCl, pH 7.5 (assay buffer), initiated by addition 50 μL of various concentrations of CA13 (diluted by assay buffer) to 50 μL of 2 mM substrate 4-NPA (100 mM stock in Acetone, diluted by assay buffer). Incubated at 37℃ for 5min, then read at a wavelength of 400 nm.