Phosphoryl oxime inhibition of acetylcholinesterase during grime reactivation is prevented by edrophonium

Reactivation of organophosphate (OP)-inhibited acetylcholinesterase (AChE) is a key objective in the treatment of OP poisoning. This study with native , wild-type, and mutant recombinant DNA-expressed AChEs, each inhibited by representative OP compounds, establishes a relationship between edrophonium acceleration of oxime-induced reactivation of OP-AChE conjugates and phosp horyl oxime inhibition of the reactivated enzyme that occurs during reactiv ation by pyridinium oximes LuH6 and TMB4. No such recurring inhibition coul d be observed with HI-6 as the reactivator due to the extreme lability of t he phosphoryl oximes formed by this oxime. Phosphoryl oximes formed during reactivation of the ethoxy methylphosphonyl-AChE conjugate by LuH6 and TMB4 were isolated for the first time and their structures confirmed by P-31 NM R. However, phosphoryl oximes formed during the reactivation of the diethyl phosphoryl-AChE conjugate were not sufficiently stable to be detected by 31 P NMR. The purified ethoxy methylphosphonyl oximes formed during the reacti vation of ethoxy methylphosphonyl-AChE conjugate with LuH6 and TMB4 are 10- to 22-fold more potent than MEPQ as inhibitors of AChE and stable for seve ral hours at pH 7.2 in HEPES buffer. Reactivation of both ethoxy methylphos phonyl- and diethylphosphoryl-AChE by these two oximes was accelerated in t he presence of rabbit serum paraoxonase, suggesting that organophosphorus h ydrolase can hydrolyze phosphoryl oxime formed during the reactivation. Our results emphasize that certain oximes, such as LuH6 and TMB4 if used in th e treatment of OP pesticide poisoning may cause prolonged inhibition of ACh E due to formation of phosphoryl oximes.