Acceleration of the rate of reaction of methanesulfonyl fluoride and acetylcholinesterase by substituted ammonium ions.

Methanesulfonyl fluoride is an oxydiaphoric (acid-transferring) inhibitor of acetylcholinesterase. It reacts with the enzyme to produce a methanesulfonyl enzyme derivative. The group in the enzyme that is sulfonylated lies near the anionic site and is possibly the same group that is acetylated during the normal catalytic activity of the enzyme, i.e. the basic group of the esteratic site (1, 2). Other anhydrides of methanesulfonic acid-in particular, those containing suitable quaternary ammonium functions-are also oxydiaphoric inhibitors and produce the same methanesulfonyl enzyme. The rate of reaction of the enzyme with these inhibitors is slowed by simple inhibitors such as tetramethylammonium ion (1,2). In the case of methanesulfonyl fluoride, the situation is quite different, possibly in part because the small size of the leaving group-fluorine-may allow the reaction to proceed even if the anionic site is occupied by tetramethylammonium ion. The situation is far more complicated, however, because the experimental fact is that the rate of reaction is increased by a factor of 6. With tetraethylammonium ion, the rate of reaction is increased by a factor of more than 30. In this paper we describe the effect of a number of substituted ammonium ions on the rate of this reaction.