The inactivation of acetylcholinesterase by trimethyloxonium ion, an active-site-directed methylating agent

Trimethyloxonium ion inactivates acetylcholinesterase from the electric eel and acetylcholinesterase on the surface of human red blood cells. Tetramethylammonium ion, which is a competitive inhibitor of acetylcholinesterase, protects against this inactivation. Trimethyloxonium ion does $\text{not}$ inactivate the system that transports choline into the red blood cell. We conclude that trimethyloxonium ion is an affinity-labeling reagent for acetylcholinesterase and that red blood cell acetylcholinesterase is probably not a component of the choline transport system.