PROTEIN DERIVED RADICALS

过氧化氢和PUFA氢过氧化物对组织的氧化损伤已经成为一个反复出现的主题，作为诱导各种医疗条件的机制，包括心肌缺血、癌症和衰老。除了脂质，蛋白质也是这种损伤的目标。肌红蛋白是有氧肌肉组织中普遍存在的一种血红素蛋白，它具有过氧化物酶活性，可导致多种生物分子的氧化损伤，包括其自身和膜脂。我们现在正在总结一系列关于肌红蛋白衍生自由基的研究，这已经对肌红蛋白中形成的四种自由基及其相互关系有了相当全面的了解。由于线粒体是体内过氧化氢生成的主要部位，我们已经开始研究细胞色素c氧化酶（CcO）衍生的自由基形成。CcO是线粒体呼吸链的末端酶，是一种多组分膜蛋白，分子量为20万，由13种不同的多肽亚基组成。该酶的功能是在血红素a3-CuB的活性中心将二氧还原为H2O，并通过线粒体内膜将质子泵送，为ATP合成提供驱动能量。有报道称，在CcO的血红素a3-CuB中心可发生过氧化物酶样反应。过量H2O2的加入可使CcO氧化生成过氧中间体，即“化合物P”，而化合物P与亚铁氰化物或细胞色素c的单电子还原可生成含铁氧物质，即“化合物f”。最后，化合物F的单电子还原使CcO还原为氧化态，完成催化循环。化合物P的结构尚不清楚，还在争论中。化合物F被认为在结构上与一个类似酶解化合物ii的物质相关。如果H2O2/CcO体系的化学性质与过氧化物酶相似，那么化合物P和化合物F都具有高活性。然而，没有直接证据表明CcO的化合物P或化合物F可以氧化蛋白质并引发自由基介导的脂质过氧化，如在肌红蛋白和细胞色素c中所见。

Oxidative damage to tissues by hydrogen peroxide and PUFA hydroperoxides has become a recurring theme as a mechanism for the induction of a variety of medical conditions including myocardial ischemia, cancer, and aging. In addition to lipids, proteins are a target of such damage. Myoglobin, a heme protein that is ubiquitous in aerobic muscle tissues, has peroxidase activity that can result in oxidative damage to a variety of biological molecules, including itself and membrane lipids. We are now concluding a series of investigations of myoglobin-derived radicals, which has given a rather complete understanding of the four types of radicals formed in myoglobin and their interrelationships. Because mitochondria are the primary site of hydrogen peroxide generation in vivo, we have initiated investigations of cytochrome c oxidase (CcO)-derived radical formation. CcO, the terminal enzyme of the mitochondrial respiratory chain, is a multicomponent membrane protein with a molecular weight of 200,000 comprising 13 different polypeptide subunits. The enzyme functions to reduce dioxygen to H2O at the active center of heme a3-CuB and couples proton pumping across the mitochondrial inner membrane to provide the driving energy for ATP synthesis. It has been reported that a peroxidase-like reaction can occur at the heme a3-CuB center of CcO . The addition of excess H2O2 can oxidize CcO to yield peroxy intermediate, or "compound P," and one-electron reduction of compound P with either ferrocyanide or cytochrome c yields the ferryl-oxo species, or "compound F." Finally, one-electron reduction of compound F returns the CcO to the oxidized state and completes the catalytic cycle. The structure of compound P remains unclear and is under debate . Compound F was proposed to be structurally related to an HRP-compound II-like species. One would expect that both compound P and compound F are highly reactive if the chemistry of the H2O2/CcO system is similar to that of peroxidases. However, no direct evidence indicates that the compound P or compound F species of CcO can oxidize the protein and initiate free radical-mediated lipid peroxidation as seen with metmyoglobin and cytochrome c.