Nitroxides as Protectors Against Oxidative Stress

用作EPR自旋标记的氮氧化物（如tempol）已被证明具有超氧化物歧化酶（SOD）活性，并且在保护细胞免受各种氧化应激（包括过氧化氢、超氧化物、有机氢过氧化物、氧化还原循环化疗药物和电离辐射）方面非常有效。我们已经证明，Tempol可以保护体外细胞和小鼠免受电离辐射。因此，氮氧化物代表了一类新的辐射保护剂，可以广泛用于保护人类免受辐射。重要的是，我们已经证明tempol不能保护啮齿动物的肿瘤组织；我们认为其机制涉及正常组织与肿瘤组织的差异代谢减少特性。携带肿瘤的动物模型的体内电子顺磁共振成像研究表明，与正常组织相比，肿瘤中氮氧化物的减少速度更快。最近的研究表明，缺乏葡萄糖6磷酸脱氢酶（G6PD）的细胞将氮氧化物还原为羟胺的速度比对照细胞慢得多，这表明这一重要的生化途径在氮氧化物还原中的作用。我们最近也表明细胞内谷胱甘肽水平是组织中氮氧化物减少的主要决定因素。这些研究将使我们能够最好地确定氮氧化物作为正常组织的潜在选择性放射保护剂的适当给药途径。我们最近也表明，氮氧化物可以防止雌激素代谢物的细胞毒性和诱变性。这些研究将扩展到探索它们对体内肿瘤模型的影响。最后，初步研究表明，对p53基因敲除小鼠长期给予tempol（在食物或饮用水中）可以延长它们的寿命。此外，在对照小鼠中，我们观察到长期服用tempol导致体重急剧减轻，我们最近将其与骨骼肌中线粒体解偶联蛋白2的表达增加联系起来。由于氮氧化物很容易穿透细胞膜，是有效的抗氧化剂，它们可能用于其他医学研究领域，如缺血/再灌注损伤研究，预防白内障，炎症过程和衰老。

Nitroxides (such as tempol) which have been used as EPR spin labels have been shown to exhibit superoxide dismutase (SOD) activity and are quite effective agents in protecting cells against a wide variety of oxidative stresses including hydrogen peroxide, superoxide, organic hydroperoxides, redox-cycling chemotherapy drugs, and ionizing radiation. We have demonstrated that Tempol protects both cells in vitro and mice against ionizing radiation. Thus, the nitroxides represent a new class of radiation protectors that may have widespread use in protecting humans against radiation. Importantly, we have shown that tempol does not protect rodent tumor tissue; the mechanism of which we believe involves differential metabolic reduction properties of normal versus tumor tissue. In vivo electron paramagnetic resonance imaging studies in a tumor-bearing animal model has shown more rapid reduction of nitroxides in tumor compared to normal tissue. Recent studies have shown that cells deficient in glucose 6 phosphate dehydrogenase (G6PD) reduce the nitroxide to the hydroxylamine much slower than control cells suggesting a role for this important biochemical pathway in nitroxide reduction. We have also recently shown that intracellular glutathione levels are a major determinant of nitroxide reduction in tissues. These studies will enable us to best determine the appropriate routes of administration for nitroxides as potentially selective radioprotectors of normal tissues. We have also recently shown that nitroxides protect against the cytotoxicity and mutagenicity of estrogen metabolites. These studies will be expanded to explore their impact to in vivo tumor models. Lastly, preliminary studies have indicated that long-term administration of tempol (in the food or drinking water) to p53 knockout mice extends their life span. Additionally in control mice we have observed that long-term administration of tempol results in dramatic weight reduction, which we have recently correlated, with increased expression of mitochondrial uncoupling protein 2 in skeletal muscle. Since nitroxides readily penetrate cell membranes and are potent antioxidants, they may be of use in other areas of medical research such as ischemia/reperfusion injury studies, prevention of cataracts, inflammatory processes, and aging.