Xanthine oxidase is an enzyme that oxidizes xanthine while using molecular oxygen as an electron acceptor. Its activity is classified as EC1.17.3.2. The IUBMB gives the overall reactionThe products are urate and hydrogen peroxide in this stated reaction. Other conditions can favor superoxide formation, so the equation does not specify every oxygen-reduction product in every assay.[ref]
Xanthine oxidase in IUBMB Enzyme Nomenclature. Reaction and Comments, peroxide and superoxide routes
For an idealized peroxide-route inventory, four micromoles of xanthine and ten micromoles of molecular oxygen, with sufficient water and no competing reactions, can yield four micromoles each of urate and hydrogen peroxide. Six micromoles of molecular oxygen remain. This is a stoichiometric limit; it does not predict the reaction rate or completion under a particular set of conditions.
Xanthine dehydrogenase instead couples the oxidation to reduction of nicotinamide adenine dinucleotide. Mammalian enzyme preparations can convert between the two forms. A 2000 comparison determined bovine dehydrogenase and oxidase structures at and , respectively. Changes near the flavin adenine dinucleotide region helped explain altered access of the NAD substrate and electron-acceptor preference.[ref]
Crystal structures of bovine milk xanthine dehydrogenase and xanthine oxidase: Structure-based mechanism of conversion by C. Enroth and others. Abstract and primary indexed structural comparison
Relating structure to both donor and acceptor chemistry is more informative than identifying the purine substrate alone. Articles by others on the same topic
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