Inhibition of Acetylcholinesterases by Stereoisomeric Organophosphorus Compounds Containing Both Thioester and -Nitrophenyl Leaving Groups.

Studies with acetylcholinesterase (AChE) inhibited by organophosphorus (OP) compounds with two chiral centers can serve as models or surrogates for understanding the rate, orientation, and postinhibitory mechanisms by the nerve agent soman that possesses dual phosphorus and carbon chiral centers. In the current approach, stereoisomers of -methyl, [-(succinic acid, diethyl ester), -(4-nitrophenyl) phosphorothiolate (MSNPs) were synthesized, and the inhibition, reactivation, and aging mechanisms were studied with electric eel AChE (eeAChE) and recombinant mouse brain AChE (rmAChE). The MSNP isomer was the strongest inhibitor of both eeAChE and rmAChE at 8- and 24-fold greater potency, respectively, than the weakest isomer. eeAChE inhibited by the - or -MSNP isomer underwent spontaneous reactivation ∼10- to 20-fold faster than the enzyme inhibited by - and -MSNP, and only 4% spontaneous reactivation was observed from the -eeAChE adduct. Using 2-pyridine aldoxime methiodide (2-PAM) or trimedoxime (TMB-4), eeAChE inhibited by - or -MSNP reactivated up to 90% and 3- to 4-fold faster than eeAChE inhibited by the - or -MSNP isomer. Spontaneous reactivation rates for rmAChE were 1.5- to 10-fold higher following inhibition by - and -MSNPs than inhibition by either isomer, a trend opposite to that found for eeAChE. Oxime reactivation of rmAChE following inhibition by - and -MSNPs was 2.5- to 5-fold faster than inhibition by - or -MSNPs. Due to structural similarities, MSNPs that phosphylate AChE with the loss of the -nitrophenoxy (PNP) group form identical, nonreactivatable adducts to those formed from -isomalathion; however, all the MSNP isomers inhibited AChE to form adducts that reactivated. Thus, MSNPs inactivate AChE via the ejection of either PNP or thiosuccinyl groups to form a combination of reactivatable and nonreactivatable adducts, and this differs from the mechanism of AChE inhibition by isomalathion.