Molecular Pharmacology, Vol 7, 129-135, Copyright © 1971 by the American Society for Pharmacology and Experimental Therapeutics

The Influence of Tetraethylammonium Ion on the Reaction between Acetylcholinesterase and Selected Inhibitors

¹ Department of Biochemistry, North Carolina State University, Raleigh, North Carolina 27607

The values of the affinity constant (K_a) and the first-order inhibition rate constant (k₂) of selected inhibitors of acetylcholinesterase (acetylcholine hydrolase, EC 3.1.1.7) were determined in the presence and absence of tetraethylammonium ion (TEA). Although TEA is believed to act as a purely competitive inhibitor toward cationic compounds, the present study reveals that this is not a general case. TEA did reduce the rate of inhibition by the organophosphate diisopropylphosphorylthiocholine in a purely competitive manner, but its effect on the rates of inhibition by two cationic carbamates was more complex. TEA at 0.5 mM reduced the apparent value of K_a, but increased the apparent k₂ value of neostigmine. At higher TEA concentrations anomalous curving was observed in the inhibition plots of dimethylcarbamylcholine. These results indicate that the inhibitors are bound simultaneously with TEA to the enzyme, and this suggests that TEA binds to an allosteric site.

Similar studies on inhibitors that contained the smaller fluoride leaving group revealed that TEA acted as a purely competitive inhibitor toward diisopropyl phosphorofluoridate. On the other hand, it is estimated that TEA may increase the first-order sulfonation rate by methanesulfonyl fluoride from 72.5 min^-1 to 2376 min^-1.