Hersh L B, Peet M
J Biol Chem. 1977 Jul 25;252(14):4796-802.
Initial velocity patterns for human placental choline acetyltransferase show a series of converging lines for both the forward and reverse reaction; with Kacetyl coenzyme A = 11.9 micron, Kcholine = 0.41 mM, KCOA = 8.8 micron, and Kacetylcholine = 1.3 mM. The relative rates of acetylcholine synthesis (Vf) to acetylcholine breakdown (Vr) is 4.5. Product inhibition by acetylcholine is competitive with respect to choline and noncompetitive with respect to acetyl-CoA, while product inhibition by choline is competitive with respect to acetylcholine and noncompetitive with respect to coenzyme A. Chlorocholine, diethylaminoethanol, and acetylaminocholine were used as dead-end inhibitors and shown to inhibit competitively with respect to acetylcholine, and noncompetitively with respect to choline, acetyl-CoA, and CoA. At high choline concentrations, uncompetitive substrate inhibition is observed, and inhibition by acetylaminocholine changes from noncompetitive to competitive. Comparing the reactivity of dimethylaminoethanol to choline, and acetyldimethylaminoethanol to acetylcholine, the maximal velocities obtained with these analogues was approximately 25% of the natural substrates. These data are not consistent with the previously proposed ordered Theorell-Chance reaction mechanism, and have been interpreted in terms of a random binding mechanism.
人胎盘胆碱乙酰转移酶的初始速度模式显示，正向和反向反应均有一系列汇聚线；乙酰辅酶A的米氏常数（K）为11.9微摩尔，胆碱的K为0.41毫摩尔，辅酶A的K为8.8微摩尔，乙酰胆碱的K为1.3毫摩尔。乙酰胆碱合成（Vf）与乙酰胆碱分解（Vr）的相对速率为4.5。乙酰胆碱的产物抑制作用对胆碱而言是竞争性的，对乙酰辅酶A而言是非竞争性的，而胆碱的产物抑制作用对乙酰胆碱而言是竞争性的，对辅酶A而言是非竞争性的。氯胆碱、二乙氨基乙醇和乙酰氨基胆碱被用作终产物抑制剂，结果显示它们对乙酰胆碱具有竞争性抑制作用，对胆碱、乙酰辅酶A和辅酶A具有非竞争性抑制作用。在高胆碱浓度下，观察到反竞争性底物抑制作用，并且乙酰氨基胆碱的抑制作用从非竞争性变为竞争性。比较二甲基氨基乙醇与胆碱以及乙酰二甲基氨基乙醇与乙酰胆碱的反应活性，用这些类似物获得的最大速度约为天然底物的25%。这些数据与先前提出的有序Theorell-Chance反应机制不一致，已根据随机结合机制进行了解释。
