Sanders S A, Massey V
Department of Biological Chemistry, University of Michigan, Ann Arbor 48109-0606, USA.
Antioxid Redox Signal. 1999 Fall;1(3):371-9. doi: 10.1089/ars.1999.1.3-371.
Xanthine oxidoreductase is a complex enzyme found in a wide range of organisms. Recent interest in this enzyme stems from its ability to produce reactive oxygen species under a range of conditions. It is found as a homodimer, each unit containing a molybdopterin cofactor, two iron sulfur centers, and FAD. The enzyme can exist in two forms that differ primarily in their oxidizing substrate specificity. The dehydrogenase form preferentially utilizes NAD+ as an electron acceptor but is able to donate electrons to molecular oxygen. Xanthine dehydrogenase from mammalian sources can be converted to an oxidase form that readily donates electrons to molecular oxygen, but does not reduce NAD+. The catalytic mechanism of both forms of the enzyme can be described in terms of a rapid equilibrium model in which reducing equivalents are distributed rapidly between the different redox centers of the enzyme on the basis of their midpoint potentials. The present commentary gives a brief overview of the literature concerning the rapid equilibrium model and the differences between the two enzyme forms. NADH is also discussed in terms of an alternative to xanthine or hypoxanthine as an electron donor.
黄嘌呤氧化还原酶是一种存在于多种生物体中的复合酶。近期对该酶的关注源于其在一系列条件下产生活性氧的能力。它以同型二聚体形式存在，每个亚基包含一个钼蝶呤辅因子、两个铁硫中心和黄素腺嘌呤二核苷酸。该酶可以以两种形式存在，主要区别在于其氧化底物特异性。脱氢酶形式优先利用NAD⁺作为电子受体，但能够将电子传递给分子氧。来自哺乳动物的黄嘌呤脱氢酶可以转化为氧化酶形式，后者能轻易地将电子传递给分子氧，但不还原NAD⁺。两种形式的酶的催化机制都可以用快速平衡模型来描述，在该模型中，还原当量根据其中点电位在酶的不同氧化还原中心之间快速分布。本评论简要概述了有关快速平衡模型以及两种酶形式之间差异的文献。还讨论了NADH作为黄嘌呤或次黄嘌呤替代电子供体的情况。
