Selectivity, acetylcholinesterase inhibition kinetics, and quantitative structure-activity relationships of a series of N-(2-oxido-1,3,2-benzodioxa-phosphol-2-yl) amino acid ethyl or diethyl esters

The inhibitory effects of a recently introduced series of the titled compounds on insect and mammalian acetylcholinesterase (AChE) activity were examined, where the median inhibition concentration (I50) and the inhibition kinetic parameters, bimolecular inhibition rate constant (ki), affinity constant (Ka), and phosphorylation rate constant (kp), were determined for each compound. Results indicated that all examined dioxaphospholenes had less inhibitory effects on mammalian AChE than fenitrothion, a commercial pesticide with moderate mammalian toxicity. The highest selectivity was obtained with compounds containing glutamic and leucine moieties (2.70 and 2.18, respectively) while selectivity of fenitrothion was 0.93. The low inhibitory effects of the examined dioxaphospholenes on mammalian AChE were attributed to their low phosphorylation rates (kp < 2.2 min−1) compared to that of fenitrothion (kp = 4.84 min−1). QSAR equations indicated that the inhibition process is controlled mainly by both the phosphorylation rate (direct effect) and the affinity of compounds toward the enzyme (inverse effect). Although the compounds' hydrophobicity had no effects on the inhibition process, it affects the compounds' toxicity since it affects the ability of compounds to penetrate insects to reach the enzyme active site.