CAT is a ubiquitous antioxidant enzyme involved in the detoxification of H2O2, a toxic product of both normal aerobic metabolism and pathogenic ROS production. It catalyzes the conversion of two molecules of H2O2 to molecular oxygen and two molecules of water (catalytic activity). CAT also demonstrates peroxidatic activity, in which low molecular weight alcohols can serve as electron donors. While aliphatic alcohols serve as specific substrates for CAT, other enzymes with peroxidatic activity do not utilize these substrates. Cayman’s Catalase Assay Kit utilizes the peroxidatic function of CAT for determination of enzyme activity in plasma, serum, erythrocyte lysates, tissue homogenates, and cell lysates. The method is based on the reaction of the enzyme with methanol in the presence of an optimal concentration of H2O2. The formaldehyde produced is measured colorimetrically with Purpald, a chromogen that specifically forms a bicyclic heterocycle with aldehydes, which upon oxidation changes from colorless to a purple color.