Inhibition of cholinesterases with cationic phosphonyl oximes highlights distinctive properties of the charged pyridine groups of quaternary oxime reactivators

Abstract

Oxime-induced reactivation of phosphonylated cholinesterases (ChEs) produces charged phosphonyl pyridine oxime intermediates (POXs) that are most potent organophosphate (OP) inhibitors of ChEs. To understand the role of cationic pyridine oxime leaving groups in the enhanced anti-ChE activity of POXs, the bimolecular rate constants for the inhibition ( k i ) of acetylcholinesterases (AChE) and butyrylcholinesterases (BChE), and the rate of decomposition ( k d ) of authentic O-alkyl methylphosphonyl pyridine oximes (AlkMeP-POXs) and N, N-dimethylamidophosphoryl pyridine oximes (EDMP-POXs), were studied. Stability ranking order in aqueous solutions correlated well with the electronic features and optimized geometries that were obtained by ab initio calculations at 6-31G ∗∗ basis set level. AlkMeP-POXs of the 2-pyridine oxime series were found to be 4- to 8-fold more stable ( t 1/2=0.7 to 1.5 min) than the homologous O, O-diethylphosphoryl (DEP) oxime. Results suggest that re-inhibition of enzyme activity by POX is less likely during the reactivation of DEP–ChEs (obtained by use of DEP-containing pesticides) by certain oximes, compared to nerve agent-inhibited ChEs. The greatest inhibition was observed for the O-cyclohexyl methylphosphonyl-2PAM derivative (4.0×10 9 M −1 min −1; mouse AChE) and is 10-fold higher than the k i of cyclosarin. Increasing the size of the O-alkyl substituent of AlkMeP-POXs had only a small to moderate effect on the k i of ChEs, signifying a major role for the cationic pyridine oxime leaving group in the inhibition reaction. The shape of plots of log k i vs. p K a of the leaving groups for AlkMeP-PAMs and DEP-PAMs, could be used as a diagnostic tool to highlight and rationalize the unique properties of the cationic moiety of pyridine oxime reactivators.