Francis S E, Gluzman I Y, Oksman A, Banerjee D, Goldberg D E
Howard Hughes Medical Institute, Department of Molecular Microbiology, St. Louis, Missouri 63110, USA.
Mol Biochem Parasitol. 1996 Dec 20;83(2):189-200. doi: 10.1016/s0166-6851(96)02772-7.
In Plasmodium falciparum, a cysteine protease known as falcipain has been implicated in the essential metabolic process of hemoglobin degradation. Parallel lines of investigation, using native or recombinant enzyme, have led to differing conclusions about the specificity and role of this protease. We have now determined that (1) Native falcipain does not cleave hemoglobin unless this substrate has first been denatured by reducing agents, acid-acetone treatment or plasmepsin action. (2) Reducing agents such as glutathione cannot denature hemoglobin in the presence of catalase, which is accumulated in the digestive vacuole. (3) The purified native enzyme has kinetics similar to those obtained with trophozoite extract, but substantially different from those of recombinant enzyme. (4) Although there are numerous cysteine protease genes in the P. falciparum genome, the falcipain gene is the only one whose transcript can be detected in the early intraerythrocytic parasites. We conclude that falcipain likely works by degrading hemoglobin fragments after initial aspartic protease attack has denatured the substrate. We propose that falcipain inhibitors block the initial steps of degradation indirectly by promoting vacuolar accumulation of osmotically active hemoglobin peptides.
在恶性疟原虫中，一种名为疟原虫蛋白酶的半胱氨酸蛋白酶参与了血红蛋白降解这一重要代谢过程。使用天然酶或重组酶进行的平行研究得出了关于这种蛋白酶的特异性和作用的不同结论。我们现已确定：（1）天然疟原虫蛋白酶不会切割血红蛋白，除非该底物首先被还原剂、酸 - 丙酮处理或疟原虫天冬氨酸蛋白酶作用变性。（2）在消化液泡中积累的过氧化氢酶存在的情况下，如谷胱甘肽等还原剂不能使血红蛋白变性。（3）纯化的天然酶的动力学与滋养体提取物的动力学相似，但与重组酶的动力学有很大不同。（4）尽管恶性疟原虫基因组中有许多半胱氨酸蛋白酶基因，但疟原虫蛋白酶基因是唯一在红细胞内早期寄生虫中可检测到转录本的基因。我们得出结论，疟原虫蛋白酶可能在最初的天冬氨酸蛋白酶攻击使底物变性后，通过降解血红蛋白片段起作用。我们提出，疟原虫蛋白酶抑制剂通过促进具有渗透活性的血红蛋白肽在液泡中的积累，间接阻断降解的初始步骤。
