The Xanthine Oxidase Activity Assay Kit provides a simple and direct procedure for measuring XO activity in a variety of biological samples. XO activity is determined by a coupled enzyme assay, which results in a colorimetric (570 nm)/ fluorometric (λex = 535/λem = 587 nm) product, proportional to the hydrogen peroxide generated. One unit of XO is defined as the amount of enzyme that catalyzes the oxidation of xanthine, yielding 1.0 μmole of uric acid and hydrogen peroxide per minute at 25 °C.