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Xanthine oxidase (XO, sometimes 'XAO') is a form of xanthine oxidoreductase, a type of enzyme that generates reactive oxygen species. These enzymes catalyze the oxidation of hypoxanthine to xanthine and can further catalyze the oxidation of xanthine to uric acid. These enzymes play an important role in the catabolism of purines in some species, including humans. Xanthine oxidase is defined as an ''enzyme activity'' (EC 1.17.3.2). The same protein, which in humans has the HGNC approved gene symbol ''XDH'', can also have xanthine dehydrogenase activity (EC 1.17.1.4). Most of the protein in the liver exists in a form with xanthine dehydrogenase activity, but it can be converted to xanthine oxidase by reversible sulfhydryl oxidation or by irreversible proteolytic modification.〔(【引用サイトリンク】 url =http://www.ncbi.nlm.nih.gov/sites/entrez?Db=gene&Cmd=ShowDetailView&TermToSearch=7498 )〕 ==Reaction== The following chemical reactions are catalyzed by xanthine oxidase: * hypoxanthine + H2O + O2 xanthine + H2O2 * xanthine + H2O + O2 uric acid + H2O2 * Xanthine oxidase can also act on certain other purines, pterins, and aldehydes. For example, it efficiently converts 1-methylxanthine (a metabolite of caffeine) to 1-methyluric acid, but has little activity on 3-methylxanthine. * Under some circumstances it can produce superoxide ion RH + H2O + 2 O2 ROH + 2 O2− + 2 H+.〔 Image:Hypoxanthin - Hypoxanthine.svg|hypoxanthine (one oxygen atom) Image:Xanthin - Xanthine.svg|xanthine (two oxygens) Image:Harnsäure Ketoform.svg|uric acid (three oxygens) 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「xanthine oxidase」の詳細全文を読む スポンサード リンク
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