Takamiya Shinzaburo, Hashimoto Muneaki, Kazuno Saiko, Kikkawa Mika, Yamakura Fumiyuki
Department of Molecular and Cellular Parasitology, Juntendo University School of Medicine, 2-1-1 Hongo, Bunkyo-ku, Tokyo 113-8421, Japan.
Parasitol Int. 2009 Sep;58(3):278-84. doi: 10.1016/j.parint.2009.03.003. Epub 2009 Apr 5.
We reported previously that Ascaris suum cytochrome b5, specifically expressed in this nematode at the adult stage and dually localized in extracellular perienteric fluid and hypodermis, is involved in both perienteric NADH-methemoglobin and cytosolic NADH-metmyoglobin reduction, where cytochrome b5 functions as an electron carrier between NADH-mediated cytochrome b5 reductase and substrates, methemo(myo)globins to reduce the nonfunctional globins back to functional ferrous hemo(myo)globins. To further characterize NADH-methemo(myo)globin reductase systems, the midpoint potentials of A. suum perienteric hemoglobin and body wall myoglobin, as well as the affinities of Ascaris methemoglobin and metmyoglobin toward cytochrome b5, were evaluated using potentiometric titration and surface plasmon resonance techniques, respectively. Midpoint potentials of +7.2 mV and +19.5 mV were obtained for Ascaris perienteric hemoglobin and body wall myoglobin, respectively. The affinities of Ascaris perienteric methemoglobin and body wall metmyoglobin toward the nematode cytochrome b5 were comparable to that for mammalian hemoglobin and cytochrome b5; association constants were 0.585 x 10(3) M(-1) and 2.32 x 10(3) M(-1), respectively, with rapid equilibration kinetics. These observations highlight the physiological importance of A. suum perienteric NADH-methemoglobin and cytosolic metmyoglobin reductase systems. Differential roles of A. suum perienteric hemoglobin and body wall myoglobin are also discussed from the viewpoint of oxygen homeostasis under hypoxic conditions.
我们之前报道过，猪蛔虫细胞色素b5在该线虫成虫阶段特异性表达，并且双重定位于细胞外的围脏液和皮下组织，它参与围脏液中NADH-高铁血红蛋白和胞质中NADH-高铁肌红蛋白的还原过程，在这个过程中细胞色素b5作为NADH介导的细胞色素b5还原酶与底物高铁（肌）红蛋白之间的电子载体，将无功能的球蛋白还原为有功能的亚铁血（肌）红蛋白。为了进一步表征NADH-高铁（肌）红蛋白还原酶系统，分别使用电位滴定法和表面等离子体共振技术评估了猪蛔虫围脏血红蛋白和体壁肌红蛋白的中点电位，以及猪蛔虫高铁血红蛋白和高铁肌红蛋白对细胞色素b5的亲和力。猪蛔虫围脏血红蛋白和体壁肌红蛋白的中点电位分别为+7.2 mV和+19.5 mV。猪蛔虫围脏高铁血红蛋白和体壁高铁肌红蛋白对该线虫细胞色素b 的亲和力与哺乳动物血红蛋白和细胞色素b5的亲和力相当；结合常数分别为0.585×10³ M⁻¹和2.32×10³ M⁻¹，具有快速平衡动力学。这些观察结果突出了猪蛔虫围脏NADH-高铁血红蛋白和胞质高铁肌红蛋白还原酶系统的生理重要性。还从缺氧条件下氧稳态的角度讨论了猪蛔虫围脏血红蛋白和体壁肌红蛋白的不同作用。
