Superoxide dismutases (SODs) are metalloenzymes that catalyze the dismutation of the superoxide anion to molecular oxygen and hydrogen peroxide. SOD Activity Assay Kit bases on the reduction of WST-1, which produces a water-soluble coloured formazan dye. The rate of the reduction with a superoxide anion linearly relates to the xanthine oxidase (XO) activity. By contrast, SOD inhibits this reaction. Therefore, the inhibition activity of SOD can be determined by a colorimetric method. Specifically, one unit of SOD corresponds to the amount of enzyme needed to exhibit 50% dismutation of the superoxide radical.
The conversion of xanthine and O2 to uric acid and H2O2 by XOD generates superoxide radicals. As a result, the superoxide anions reduce a tetrazolium salt (WST-1) to a colored formazan product (WST-1 formazan) that absorbs light. SOD scavenges superoxide anions, thereby reducing the rate of formazan dye formation.