MRC Mitochondrial Biology Unit, Wellcome Trust/MRC Building, Cambridge, UK.
FEBS J. 2010 Mar;277(6):1465-80. doi: 10.1111/j.1742-4658.2010.07576.x. Epub 2010 Feb 9.
Cysteine plays a number of important roles in protecting the cell from oxidative damage through its thiol functional group. These defensive functions are generally considered to be carried out by the low molecular weight thiol glutathione and by cysteine residues in the active sites of proteins such as thioredoxin and peroxiredoxin. In addition, there are thiols exposed on protein surfaces that are not directly involved with protein function, although they can interact with the intracellular environment. In the present study, in subcellular fractions prepared from rat liver or heart, we show that the quantitatively dominant free thiols are those of cysteine residues exposed on protein surfaces and not those carried by glutathione. Within the mitochondrial matrix, the concentration of exposed protein thiols is 60-90 mm, which is approximately 26-fold higher than the glutathione concentration in that compartment. This suggests that exposed protein thiols are of greater importance than glutathione for nonenzyme catalysed reactions of thiols with reactive oxygen and nitrogen species and with electrophiles within the cell. One such antioxidant role for exposed protein thiols may be to prevent protein oxidative damage. In the present study, in mitochondrial membranes and in complex I, we show that exposed protein thiols protect against tyrosine nitration and protein dysfunction caused by peroxynitrite. Therefore, exposed protein thiols are the dominant free thiol within the cell and may play a critical role in intracellular antioxidant defences against oxidative damage.
半胱氨酸通过其巯基官能团在保护细胞免受氧化损伤方面发挥着许多重要作用。这些防御功能通常被认为是由低分子量巯基谷胱甘肽和蛋白质活性位点中的半胱氨酸残基（如硫氧还蛋白和过氧化物酶）来执行的。此外，还有一些暴露在蛋白质表面的巯基不直接参与蛋白质功能，但可以与细胞内环境相互作用。在本研究中，我们在从大鼠肝或心脏制备的亚细胞级分中表明，定量上占优势的游离巯基是暴露在蛋白质表面上的半胱氨酸残基的巯基，而不是谷胱甘肽所携带的巯基。在线粒体基质中，暴露的蛋白质巯基的浓度为 60-90mm，这大约是该隔室中谷胱甘肽浓度的 26 倍。这表明暴露的蛋白质巯基对于非酶催化的巯基与活性氧和氮物种以及细胞内的亲电试剂的反应比谷胱甘肽更为重要。暴露的蛋白质巯基的一种抗氧化作用可能是防止蛋白质氧化损伤。在本研究中，我们在线粒体膜和复合物 I 中表明，暴露的蛋白质巯基可防止酪氨酸硝化和过氧亚硝酸盐引起的蛋白质功能障碍。因此，暴露的蛋白质巯基是细胞内主要的游离巯基，可能在细胞内抗氧化防御对抗氧化损伤方面发挥关键作用。
