Ishikawa Takahiro, Tamaki Shun, Maruta Takanori, Shigeoka Shigeru
Faculty of Life and Environmental Science, Shimane University, 1060 Nishikawatsu, Matsue, Shimane, 690-8504, Japan.
Faculty of Agriculture, Kindai University, 3327-204 Nakamachi, Nara, 631-8505, Japan.
Adv Exp Med Biol. 2017;979:47-64. doi: 10.1007/978-3-319-54910-1_4.
Reactive oxygen species (ROS) such as superoxide and hydrogen peroxide are by-products of various metabolic processes in aerobic organisms including Euglena. Chloroplasts and mitochondria are the main sites of ROS generation by photosynthesis and respiration, respectively, through the active electron transport chain. An efficient antioxidant network is required to maintain intracellular ROS pools at optimal conditions for redox homeostasis. A comparison with the networks of plants and animals revealed that Euglena has acquired some aspects of ROS metabolic process. Euglena lacks catalase and a typical selenocysteine containing animal-type glutathione peroxidase for hydrogen peroxide scavenging, but contains enzymes involved in ascorbate-glutathione cycle solely in the cytosol. Ascorbate peroxidase in Euglena, which plays a central role in the ascorbate-glutathione cycle, forms a unique intra-molecular dimer structure that is related to the recognition of peroxides. We recently identified peroxiredoxin and NADPH-dependent thioredoxin reductase isoforms in cellular compartments including chloroplasts and mitochondria, indicating the physiological significance of the thioredoxin system in metabolism of ROS. Besides glutathione, Euglena contains the unusual thiol compound trypanothione, an unusual form of glutathione involving two molecules of glutathione joined by a spermidine linker, which has been identified in pathogenic protists such as Trypanosomatida and Schizopyrenida. Furthermore, in contrast to plants, photosynthesis by Euglena is not susceptible to hydrogen peroxide because of resistance of the Calvin cycle enzymes fructose-1,6-bisphosphatse, NADP-glyceraldehyde-3-phosphatase, sedoheptulose-1,7-bisphosphatase, and phosphoribulokinase to hydrogen peroxide. Consequently, these characteristics of Euglena appear to exemplify a strategy for survival and adaptation to various environmental conditions during the evolutionary process of euglenoids.
活性氧（ROS），如超氧化物和过氧化氢，是包括眼虫在内的需氧生物各种代谢过程的副产物。叶绿体和线粒体分别是光合作用和呼吸作用通过活跃的电子传递链产生ROS的主要场所。需要一个有效的抗氧化网络来将细胞内ROS池维持在氧化还原稳态的最佳条件下。与植物和动物的网络比较表明，眼虫已经获得了ROS代谢过程的某些方面。眼虫缺乏过氧化氢酶和典型的含硒半胱氨酸的动物型谷胱甘肽过氧化物酶来清除过氧化氢，但仅在细胞质中含有参与抗坏血酸-谷胱甘肽循环的酶。眼虫中的抗坏血酸过氧化物酶在抗坏血酸-谷胱甘肽循环中起核心作用，形成与过氧化物识别相关的独特分子内二聚体结构。我们最近在包括叶绿体和线粒体在内的细胞区室中鉴定出了过氧化物还原酶和NADPH依赖性硫氧还蛋白还原酶同工型，表明硫氧还蛋白系统在ROS代谢中的生理意义。除了谷胱甘肽外，眼虫还含有不寻常的硫醇化合物锥虫硫醇，这是一种不寻常形式的谷胱甘肽，由两个谷胱甘肽分子通过亚精胺连接体连接而成，已在诸如锥虫目和裂殖酵母目等致病原生生物中被鉴定出来。此外，与植物不同，眼虫的光合作用对过氧化氢不敏感，这是因为卡尔文循环酶果糖-1,6-二磷酸酶、NADP-甘油醛-3-磷酸酶、景天庚酮糖-1,7-二磷酸酶和磷酸核酮糖激酶对过氧化氢具有抗性。因此，眼虫的这些特征似乎例证了眼虫类在进化过程中生存和适应各种环境条件的一种策略。
