Inhibition of acetylcholinesterase by three new pyridinium compounds and their effect on phosphonylation of the enzyme

Abstract

Three new mono-pyridinium compounds were prepared: 1-phenacyl-2-methylpyridinium chloride (1), 1-benzoylethylpyridinium chloride (2) and 1-benzoylethylpyridinium-4-aldoxime chloride (3) and assayed in vitro for their inhibitory effect on human blood acetylcholinesterase (EC 3.1.1.7, AChE). All the three compounds inhibited AChE reversibly; their binding affinity for the enzyme was compared with their protective effect (PI) on AChE phosphonylation by soman and VX. Compound 1 was found to bind to both the catalytic and the allosteric (substrate inhibition) sites of the enzyme with estimated dissociation constants of 6.9 μM (K(cat)) and 27 μM (K(all)), respectively. Compound 2 bound to the catalytic site with K(cat) = 59 μM and compound 3 only to the allosteric site with K(all) = 328 μM. PI was evaluated from phosphonylation measured in the absence and in presence of the compounds applied in a concentration corresponding to their K(cat) or K(all) value, and was also calculated from theoretical equations deduced from the reversible inhibition of the enzyme. Compounds 1 and 3 protected the enzyme from phosphonylation by soman and VX, whereas no protection was observed in the presence of compound 2 under the same conditions. Irrespective of the binding sites to AChE, PI for compounds 1 and 3 evaluated from phosphonylation agreed with PI calculated from reversible inhibition. Compound 3 was found to be a weak reactivator of methylphosphonylated AChE with k(r) = 1.1 x 102 L mol-1 min-1.