Verham R, Meek T D, Hedstrom L, Wang C C
Mol Biochem Parasitol. 1987 May;24(1):1-12. doi: 10.1016/0166-6851(87)90110-1.
The IMP dehydrogenase of Tritrichomonas foetus, a parasitic protozoan incapable of de novo biosynthesis of purine nucleotides, has been purified about 1000-fold to apparent homogeneity. The purified enzyme demonstrated a 20-fold higher substrate turnover rate than the pure IMP dehydrogenase from sarcoma ascites tumor cells. It has a subunit molecular weight of 58,000, aggregates to a size of 380,000 at low ionic strength, and partly dissociates to a molecular weight of 270,000 in high salt concentrations. Unlike the IMP dehydrogenase of bacteria and mammals, the T. foetus enzyme does not require K+ for activity. The analysis of initial velocity and product inhibition data is consistent with a sequential, ordered bi bi kinetic mechanism for the parasite enzyme-catalyzed reaction, in which IMP binds before NAD+ and NADH is released before XMP. This is in contrast to the partially random mechanism of the bacterial enzyme which involves the formation of an enzyme-K+-(IMP) complex. Mycophenolic acid inhibits T. foetus IMP dehydrogenase uncompetitively versus both IMP and NAD+ with an apparent Ki of 9 microM. This value, which is several hundred-fold higher than that for mammalian IMP dehydrogenase, suggests significantly different binding properties of the mycophenolic acid site in T. foetus IMP dehydrogenase, which might be amenable to specific inhibitor design.
胎儿三毛滴虫是一种无法从头生物合成嘌呤核苷酸的寄生原生动物，其肌苷酸脱氢酶已被纯化至表观均一，纯化倍数约为1000倍。纯化后的酶显示出比肉瘤腹水肿瘤细胞中的纯肌苷酸脱氢酶高20倍的底物周转速率。它的亚基分子量为58,000，在低离子强度下聚合成380,000的大小，在高盐浓度下部分解离为270,000的分子量。与细菌和哺乳动物的肌苷酸脱氢酶不同，胎儿三毛滴虫的酶活性不需要K+。对初始速度和产物抑制数据的分析与寄生虫酶催化反应的顺序、有序双底物双产物动力学机制一致，其中IMP在NAD+之前结合，NADH在XMP之前释放。这与细菌酶的部分随机机制形成对比，细菌酶的机制涉及形成酶-K+-(IMP)复合物。霉酚酸对胎儿三毛滴虫肌苷酸脱氢酶对IMP和NAD+均表现出非竞争性抑制，表观Ki为9 microM。该值比哺乳动物肌苷酸脱氢酶的Ki值高数百倍，表明胎儿三毛滴虫肌苷酸脱氢酶中霉酚酸结合位点的结合特性存在显著差异，这可能适合设计特异性抑制剂。
