CA2 (Carbonic anhydrase 2) is an enzyme that catalyzes reversible hydration of carbon dioxide. It is essential for bone resorption and osteoclast differentiation and contributes to intracellular pH regulation in the duodenal upper villous epithelium during proton-coupled peptide absorption. It is widely accepted that CA2 also catalyzes hydrolysis of p-Nitrophenyl Acetate. Thus, a hydration assay was conducted to test the catalytic activity of CA2 using 4-Nitrophenyl Acetate (4-NPA) as substrate. Briefly, different concentrations of CA2 were incubated with 1mM 4-NPA in reaction buffer. The absorbance at the wavelength of 400nm was read per hour, and the result was shown in figure 1. It is obvious that CA2 catalyzes hydrolysis of p-Nitrophenyl Acetate.