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Oxime Reactivation Mechanisms for Organophosphate Conjugated Human Acetylcholinesterase
Author(s) -
Hou William C.,
Radic Zoran,
Sharpless K. Barry,
Taylor Palmer
Publication year - 2016
Publication title -
the faseb journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.709
H-Index - 277
eISSN - 1530-6860
pISSN - 0892-6638
DOI - 10.1096/fasebj.30.1_supplement.1191.4
Subject(s) - oxime , chemistry , acetylcholinesterase , nucleophile , reaction rate constant , stereochemistry , medicinal chemistry , catalysis , organic chemistry , kinetics , enzyme , physics , quantum mechanics
Organophosphates (OP) interacting with acetylcholinesterase (AChE) carry narrowly prescribed therapeutic benefits and are effective, biodegradable pesticides when judiciously used. However, certain volatile OP's have been employed insidiously in terrorism. To develop efficacious OP antidotes and scavenging agents, we have adopted a new chemistry with zwitterionic oximes in order for the antidote to cross the blood‐brain barrier and enable reactivation of AChE in the central nervous system. Such compounds also exhibit appropriate oral bioavailability, eliminating parenteral administration in field situations. Studies of oxime catalysis of low molecular weight OP esters show that the oximate is the general base, nucleophile in solution. To investigate whether a similar mechanism occurs in the active center of AChE, we examined the pH dependency of oxime reactivation on the OP conjugated human AChE. Quaternary pyridinium aldoximes, such as pralidoxime (2‐PAM), and a series of zwitterionic oxime species, such as RS 41A and RS194B, were compared at various oxime concentrations to deconstruct the overall kinetic constant for reactivation, k ov, into an apparent dissociation constant, K ox and a maximal rate, k 2 , of reactivation. General base catalysis predicts that, since the anionic oximate is the attacking species, rate constants would be expected to increase with pH, but oximolysis of the conjugated OP in the human AChE shows a broad pH dependence with a peak reactivation rate constant at pH values around 7.5 at both partial and full saturation of the active center. These findings suggest the possibility of transient electric fields and hydrogen bonding allow for proton tunneling of the oxime hydrogen for oximate formation in the active center gorge of AChE. Support or Funding Information Supported by USPHS grants GM 18360 and NS 058046.