Nishino T
Department of Biochemistry and Molecular Biology, Nippon Medical School, Tokyo.
J Biochem. 1994 Jul;116(1):1-6. doi: 10.1093/oxfordjournals.jbchem.a124480.
Although mammalian xanthine oxidase exists originally as a dehydrogenase form in freshly prepared samples, it is converted to an oxidase form during purification, either irreversibly by proteolysis or reversibly by sulfhydryl oxidation of the protein molecule. However, avoiding proteolysis the mammalian enzyme can be purified as an interconvertible form and thus can be used to compare directly the properties of xanthine dehydrogenase and the oxidase derived from the same enzyme molecule. The cDNAs encoding the enzyme have been cloned from several sources, and structural information is becoming available. The most significant difference between the two forms is the protein conformation around FAD, which changes the redox potential of the flavin and the reactivity of FAD with the electron acceptors, NAD and molecular oxygen. The flavin semiquinone is thermodynamically stable in xanthine dehydrogenase, but is unstable in xanthine oxidase. Detailed analyses by stopped-flow techniques suggest that the flavin semiquinone reacts with oxygen to form superoxide anion while the fully reduced flavin reacts to form hydrogen peroxide. Although xanthine dehydrogenase can produce greater amounts of superoxide anion than xanthine oxidase during xanthine-oxygen turnover, it seems to be physiologically insignificant because NAD inhibits almost completely the formation of superoxide anion. Although the involvement of this enzyme in reperfusion injury has been proposed, this seems to be more complex than originally envisaged and still remains to be established.
尽管哺乳动物的黄嘌呤氧化酶在新鲜制备的样品中最初以脱氢酶形式存在，但在纯化过程中会转变为氧化酶形式，要么通过蛋白水解不可逆地转变，要么通过蛋白质分子的巯基氧化可逆地转变。然而，避免蛋白水解的情况下，哺乳动物的这种酶可以作为一种可相互转换的形式进行纯化，因此可用于直接比较黄嘌呤脱氢酶和源自同一酶分子的氧化酶的特性。编码该酶的cDNA已从多个来源克隆出来，结构信息也日益可得。这两种形式之间最显著的差异在于黄素腺嘌呤二核苷酸（FAD）周围的蛋白质构象，这改变了黄素的氧化还原电位以及FAD与电子受体烟酰胺腺嘌呤二核苷酸（NAD）和分子氧的反应性。黄素半醌在黄嘌呤脱氢酶中热力学稳定，但在黄嘌呤氧化酶中不稳定。通过停流技术进行的详细分析表明，黄素半醌与氧反应形成超氧阴离子，而完全还原型黄素则反应形成过氧化氢。尽管在黄嘌呤 - 氧周转过程中，黄嘌呤脱氢酶比黄嘌呤氧化酶能产生更多的超氧阴离子，但由于NAD几乎完全抑制超氧阴离子的形成，所以这在生理上似乎并不重要。尽管有人提出这种酶与再灌注损伤有关，但这似乎比最初设想的更为复杂，仍有待确定。
