Department of Molecular and Cellular Biochemistry, University of California, Riverside, California 92521, USA.
J Biol Chem. 2009 Dec 18;284(51):35479-84. doi: 10.1074/jbc.M109.067355.
Vertebrate forms of the molybdenum-containing enzyme sulfite oxidase possess a b-type cytochrome prosthetic group that accepts reducing equivalents from the molybdenum center and passes them on to cytochrome c. The plant form of the enzyme, on the other hand, lacks a prosthetic group other than its molybdenum center and utilizes molecular oxygen as the physiological oxidant. Hydrogen peroxide is the ultimate product of the reaction. Here, we present data demonstrating that superoxide is produced essentially quantitatively both in the course of the reaction of reduced enzyme with O(2) and during steady-state turnover and only subsequently decays (presumably noncatalytically) to form hydrogen peroxide. Rapid-reaction kinetic studies directly following the reoxidation of reduced enzyme demonstrate a linear dependence of the rate constant for the reaction on [O(2)] with a second-order rate constant of k(ox) = 8.7 x 10(4) +/- 0.5 x 10(4) m(-1)s(-1). When the reaction is carried out in the presence of cytochrome c to follow superoxide generation, biphasic time courses are observed, indicating that a first equivalent of superoxide is generated in the oxidation of the fully reduced Mo(IV) state of the enzyme to Mo(V), followed by a slower oxidation of the Mo(V) state to Mo(VI). The physiological implications of plant sulfite oxidase as a copious generator of superoxide are discussed.
脊椎动物形式的含钼酶亚硫酸盐氧化酶拥有一个 b 型细胞色素辅基，它从钼中心接受还原当量，并将其传递给细胞色素 c。另一方面，植物形式的酶除了钼中心之外没有辅基，并且利用分子氧作为生理氧化剂。过氧化氢是反应的最终产物。在这里，我们提供的数据表明，在还原酶与 O(2)反应的过程中和在稳态周转期间，基本上定量地产生超氧化物，并且仅随后（可能是非催化地）衰减以形成过氧化氢。直接在还原酶的再氧化之后进行的快速反应动力学研究表明，该反应的速率常数与 [O(2)]呈线性关系，二级速率常数 k(ox) = 8.7 x 10(4) +/- 0.5 x 10(4) m(-1)s(-1)。当在存在细胞色素 c 的情况下进行该反应以跟踪超氧化物的产生时，观察到两相时间过程，表明在将酶的完全还原的 Mo(IV)状态氧化为 Mo(V)时，首先产生一个当量的超氧化物，然后 Mo(V)状态的氧化速度较慢。讨论了植物亚硫酸盐氧化酶作为超氧化物大量产生的生理意义。
