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

Three new mono-pyridinium compounds were prepared: 1-phenacyl-2-methylpyrid inium chloride (1), 1-benzoylethylpyridinium chloride (2) and 1-benzoylethy lpyridinium-4-aldoxime chloride (3) and assayed in vitro for their inhibito ry effect on human blood acetylcholinesterase (EC 3.1.1.7, AChE). All the t hree compounds inhibited AChE reversibly; their binding affinity for the en zyme 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 th e allosteric (substrate inhibition) sites of the enzyme with estimated diss ociation constants of 6.9 mu M (K-cat) and 27 mu M (K-all), respectively. C ompound 2 bound to the catalytic site with K-cat = 59 mu M and compound 3 o nly to the allosteric site with K-all = 328 mu M. PI was evaluated from pho sphonylation measured in the absence and in presence of the compounds appli ed in a concentration corresponding to their K-cat or K-all value, and was also calculated from theoretical equation's deduced from the reversible inh ibition of the enzyme. Compounds 1 and 3 protected the enzyme from phosphon ylation 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 wi th PI calculated from reversible inhibition. Compound 3 was found to be a w eak reactivator of methylphosphonylated AChE with k(r),= 1.1 x 10(2) Lmol(- 1) min(-1).