Description
Xanthine oxidase (XO), or xanthine oxidoreductase, is a complex molybdoflavoenzyme which, in humans, is recognized as the terminal enzyme of purine catabolism, catalyzing the hydroxylation of hypoxanthine to xanthine and then to uric acid. When acting as an NADH oxidase, XO is a generator of superoxide, a powerful reactive oxygen species (ROS). XO has also been noted to produce hydrogen peroxide (H2O2) and superoxide during ischemiareperfusion injury. Due to their highly reactive nature, these ROS affect various molecular components of the cell, with excess amounts leading to cell degeneration and death. XO is present in nearly all species. In mammalian tissues, XO is found predominantly in the liver and intestine. Human XO activity is almost exclusively limited to these tissues, with only trace levels found elsewhere in the body. However, in several disease states, levels of circulating XO have been seen to increase dramatically. This is especially true of liver disease, during which circulating levels of XO may be 1,000-fold greater