Effects of quaternary ligands on the inhibition of acetylcholinesterase by arsenite

Arsenite inhibits acetylcholinesterase in a second-order reaction. The rate and equilibrium constants depend upon pH and have values on the order of 10(2) M-1 min-1 and 10(5) M (dissociation), respectively. Some quaternary ammonium ligands completely block the arsenite inhibition of the enzyme, others decrease the rate of the reaction and some, notably pyridine-2 aldoxime methiodide, greatly accelerate the rate of the reaction, up to 220-fold. Accelerators may bind at a separate enzyme site distinct form the anionic site involved in substrate binding. Although the kinetic data are consistent with a covalent reaction between arsenite and acetylcholinesterase, chemical evidence excludes the involvement of sulfhydryl groups which are usually implicated in arsenite inhibition.