Why is the hydrolytic activity of acetylcholinesterase pH dependent? Kinetic study of acetylcholine and acetylthiocholine hydrolysis catalyzed by acetylcholinesterase from electric eel.

The dependence of the activity of acetylcholinesterase from electric eel at a pH value range of 4.8-9.8 (phosphate buffer), regarding acetylcholine and acetylthiocholine hydrolysis, was determined at 25 degrees C, ionic strength of 0.11 M, and initial substrate concentration of 4 mM. At a pH range of 4.8-9.8, the dependences A(pH) form a sigmoid increasing curve with the maximum catalytic activity at a pH range 8 9.5. For acetylcholine hydrolysis, the kinetic reason for such an increase in A consists mainly of an increase in the rate constant k(2) (Michaelis-Menten) model with increasing pH of the reaction mixture. For acetylthiocholine hydrolysis, the kinetic explication of the determined dependence A(pH) is more complicated.