Acceleration of the Rate of Reaction of Dimethylcarbamyl Fluoride and Acetylcholinesterase by Substituted Ammonium Ions.

The inhibition of acetylcholinesterase and other hydrolytic enzymes by methanesulfonyl fluoride, other sulfonyl fluorides, and anhydrides of methanesulfonic acid has been studied by several workers (2-5). Certain alkyl ammonium ions greatly increase the rate of irreversible inhibition of acetylcholinesterase by methanesulfonyl fluoride (6). The inhibition arises from the formation of a specific methanesulfonyl enzyme derivative (7). Although the ions which accelerate the sulfonylation act as reversible inhibitors for the hydrolysis of substrates and even for the sulfonylation of the enzyme by other methanesulfonates, the situation is very different in the case of inhibition by methanesulfonyl fluoride. This difference may arise in part from the small size of the fluorine leaving group, which may make it possible for the reaction to proceed at the esteratic site even though the anionic site is occupied by an ammonium ion. Tetraethylammonium ion was the most potent of the ions tested, and it accelerated the reaction more than 30 times. To see whether the phenomenon of acceleration is more general, we investigated the effect of some of these ions on the rate of inhibition of acetylcholinestcrase by dimethylcarbamyl fluoride. Similarly to methanesulfonyl fluoride, dimethylcarbamyl fluoride reacts with acetylcholinesterasc (2, 3) to produce a specific dimethylcarbamyl enzyme which is inactive (8). Unlike the methanesulfonyl enzyme, the enzyme derivative thus formed hydrolyzes readily in water to regrnerate the free and active enzyme. The concurrent operation of these two reactions attains a steady state wherein the rate of formation of the dimethylcarbamyl enzyme equals its rate of recovery. After the steady state is reached, the fraction of enzyme carbamylated does not change with time if the circumstances are such that the concentration of dimethylcarbamyl fluoride is not sensibly decreased. This is the case when the concentration of enzyme is low. Dimethylcarbamyl fluoride is therefore both a very poor sub-