Aoki T, Oya H
Mol Biochem Parasitol. 1987 Mar;23(2):173-81. doi: 10.1016/0166-6851(87)90153-8.
In Crithidia fasciculata, carbamoyl phosphate synthetase II, which catalyses the first step of de novo pyrimidine biosynthesis, was separated from aspartate carbamoyltransferase by ammonium sulfate fractionation. The antitumor drug acivicin competitively inhibited the synthetase II activity with respect to L-glutamine, yielding an apparent Ki of 2 microM. In the absence of L-glutamine, acivicin resulted in a selective, time-dependent inactivation of L-glutamine-dependent activity of the enzyme, with an inactivation constant (Kinact) of 100 microM and a minimum inactivation half-time (T) of 0.2 min. L-Glutamine protected the enzyme from inactivation. These results are consistent with a postulate that acivicin is an active site-directed affinity analogue of L-glutamine, achieving irreversible inactivation. The inactivated enzyme retained ammonia-dependent activity. Acivicin stimulated the ammonia-dependent activity by increasing the Vmax value of the enzyme; apparent Km values for ammonia and MgATP were not affected. Differential action of acivicin on the Crithidia and mammalian synthetase II is discussed.
在梨形鞭毛虫中，通过硫酸铵分级分离，将催化嘧啶从头生物合成第一步的氨甲酰磷酸合成酶II与天冬氨酸氨甲酰转移酶分离开来。抗肿瘤药物阿西维辛对L-谷氨酰胺竞争性抑制合成酶II的活性，表观抑制常数（Ki）为2微摩尔。在没有L-谷氨酰胺的情况下，阿西维辛导致该酶的L-谷氨酰胺依赖性活性出现选择性的、时间依赖性失活，失活常数（Kinact）为100微摩尔，最小失活半衰期（T）为0.2分钟。L-谷氨酰胺可保护该酶不被失活。这些结果与阿西维辛是L-谷氨酰胺的活性位点导向亲和类似物并导致不可逆失活的假设一致。失活后的酶保留了氨依赖性活性。阿西维辛通过增加该酶的最大反应速度（Vmax）来刺激氨依赖性活性；氨和MgATP的表观米氏常数（Km）未受影响。文中讨论了阿西维辛对梨形鞭毛虫和哺乳动物合成酶II的不同作用。
