title: 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.
abstract: 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.
mesh_terms: D000107:Acetylation; D000109:Acetylcholine / Q000378:metabolism*; D001665:Binding Sites; D002800:Cholinesterase Inhibitors; D006863:Hydrogen-Ion Concentration; D006868:Hydrolysis; D007700:Kinetics; D009994:Osmolar Concentration; D012964:Sodium / Q000378:metabolism*
chemical_list: D002800:Cholinesterase Inhibitors; D012964:Sodium; D000109:Acetylcholine