Acetylcholinesterase: evidence that sodium ion binding at the anionic site causes inhibition of the second-order hydrolysis of acetylcholine and a decrease of its pKa as well as of deacetylation.

For bovine erythrocyte acetylcholinesterase (acetylcholine hydrolase, EC 3.1.1.7), the Michaelis parameters Vmax., and Km for the natural substrate acetylcholine were estimated as a function of pH and sodium chloride concentration by the pH-stat method. A single dissociation constant for Na+ binding (K = 7 X 10(-3) M) suffices to explain the salt dependence of Vmax./Km and of Km as well as the pH dependence of Vmax./Km and Vmax., Km being pH independent. This finding provides evidence for a specific effect of Na+, presumably by binding at the anionic subsite of the active centre. Na+ binding causes a 50-fold decrease in kcat./Km as well as a decrease of one unit in the pKa of both kcat./Km and kcat.. The intrinsic pKa in the absence of salt at 25 degrees C is about 7.5. Comparison of the degree of fit of the data to the Debeye-Huckel equation, in accordance with an alternative general salt effect, as well as published data for sodium and potassium chlorides also favour a specific salt effect.