Hersh L B, Barker L A, Rush B
J Biol Chem. 1978 Jul 25;253(14):4966-70.
The kinetic constants, Km and Vmax, for the choline acetyltransferase reaction were determined for choline and eight choline analogs under conditions of high (0.3 M) and low (approximately 0.01 M) sodium chloride. At high sodium chloride, the maximal velocities of the different substrates varied over 27-fold, while at low sodium chloride, less than a 5-fold variation was observed. Dead-end inhibition studies using acetylaminocholine as the inhibitor showed that under conditions of high sodium chloride, inhibition changes from noncompetitive to competitive as the reactivity of the substrate decreases. Under conditions of low sodium chloride, acetylaminocholine inhibition is nonlinear and noncompetitive with respect to all substrates tested. These results suggest that increased ionic strength increases the rate of coenzyme A dissociation from the enzyme. The rate-determining step of the reaction can be ternary complex interconversion, coenzyme A release, or both, depending on the ionic strength and the substrate employed.
在高（0.3 M）和低（约0.01 M）氯化钠条件下，测定了胆碱乙酰转移酶反应中胆碱和八种胆碱类似物的动力学常数Km和Vmax。在高氯化钠浓度下，不同底物的最大反应速度变化超过27倍，而在低氯化钠浓度下，观察到的变化小于5倍。以乙酰氨基胆碱作为抑制剂的终产物抑制研究表明，在高氯化钠条件下，随着底物反应性降低，抑制作用从非竞争性转变为竞争性。在低氯化钠条件下，乙酰氨基胆碱对所有测试底物的抑制作用是非线性且非竞争性的。这些结果表明，离子强度增加会提高辅酶A从酶上解离的速率。反应的限速步骤可能是三元复合物的相互转化、辅酶A的释放，或两者兼有，这取决于离子强度和所使用的底物。
