XO catalyzes the hydroxylation of hypoxanthine to xanthine and then further catalyzes the oxidation of xanthine to uric acid. When oxidizing NADH, XO generates superoxide, a powerful ROS. XO is found predominantly in the liver and intestine. However, levels of circulating XO can increase dramatically with disease. Cayman’s XO Assay provides a simple and accurate method for quantifying xanthine oxidase activity. The assay is based on a multistep enzymatic reaction whose end product resorufin, a highly fluorescent compound, can be easily analyzed using an excitation wavelength between 520-550 nm and an emission wavelength between 585-595 nm.