Carbonic anhydrase (CA) is an enzyme which catalyzes the interconversion of bicarbonate ions (HCO3-) and carbon dioxide (CO2). The activity levels of CA can be measured by taking advantage of the chemical reaction that it catalyzes. When CO2 is added to a tissue homogenate, the CA present converts the CO2 into HCO3- and a hydrogen ion (H+), which results in a decrease in pH. The rate at which the pH drops can be measured and converted into a CA activity mesaure. If there is no CA activity, the pH level decreases slowly and if there is a high CA activity, the pH decreases quickly.