Superoxide dismutase (SOD), as one of the body's most important defense mechanisms against freeradical damage, catalyzes the dismutation of the superoxide radical (O2-) into hydrogen peroxide (H2O2) and elemental oxygen (O2). In ScienCell's SOD Assay, the superoxide anions, generated from the conversion of xanthine to uric acid and hydrogen peroxide by xanthine oxidase (XOD), reduce WST-1 (4-[3-(4-iodophenyl)-2-(4-nitrophenyl)-2H-5-tetrazolio]-1,3-benzene disulfonate sodium salt) to water-soluble formazans, which can be measured by absorbance at 438 nm. SODs lower the rate of the reduction reaction by reducing superoxide anion concentrations. Therefore, the % Inhibition of the reduction reaction can be determined as a measurement of SOD activity.