Reactivation of ethyl methylphosphonylated eel acetylcholinesterase in vitro by 2PAM, H16, and a series of nonquaternary α-ketothiohydroximates

Kenley, Richard A.; Bedford, Clifford D.; Howd, Robert A.; Jackson, Stuart E.

doi:10.1016/0006-2952(85)90743-9

Cited by 6 publications

(6 citation statements)

References 10 publications

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“…2,4,11,13,17,22,26,48–50 2-PAM (pralidoxime) is the only FDA-approved countermeasure for OP chemical agent exposure for use in the US. The reaction of the ChE-1 inhibitor complex with 2-PAM was evaluated using the same concentrations of inhibitor 1 as that used for spontaneous reactivation.…”

Section: Resultsmentioning

confidence: 99%

“…Reactivation 14–16 may be further distinguished as a spontaneous process in which water acts as a nucleophile (hydrolysis) or more rapidly induced in an oxime-mediated nucleophilic process, both of which lead to scission of the AChE serine O–P bond. 8,11,13,17 Alternatively, aging is mechanistically aligned with a putative cationic process (protonation) leading to scission at an alkyl-oxygen bond affording a toxicologically deleterious AChE-phosphoanion (AChE-OP(O)(R)(O − ) species that is recalcitrant to reactivate even with oximes. 5,7,8,13,15,18 As a result, branched phosphoesters such as iPrO- and OCH(CH 3 )C-(CH 3 ) 3 that can better stabilize cations in the transition state favor aging and proceed via a path alternate to reactivation.…”

Section: Introductionmentioning

confidence: 99%

“…For example, OP-adducts containing small alkyl ester groups (R = Me, Et) are easily cleaved at the phospho-serine bond thereby reactivating AChE. 3,11 Conversely, OP-AChE adducts with highly branched alkyl ester groups (R = iPr, etc.) derived from OP nerve agents are less likely to be cleaved at the phospho- serine bond due to steric and electronic effects and are more susceptible to a dealkylation process known as "aging".…”

Section: ■ Introductionmentioning

confidence: 99%

“… − OP-AChE adducts are relatively stable, but many undergo secondary reactions by discrete mechanisms depending on the nature of the remaining groups attached to phosphorus. For example, OP-adducts containing small alkyl ester groups (R = Me, Et) are easily cleaved at the phospho-serine bond thereby reactivating AChE. , Conversely, OP-AChE adducts with highly branched alkyl ester groups (R = iPr, etc.) derived from OP nerve agents are less likely to be cleaved at the phospho-serine bond due to steric and electronic effects and are more susceptible to a dealkylation process known as “aging”. ,, Therefore, the differences between the two OP-adduct mechanistic fates are manifested in distinct kinetic processes that are predicated to a large extent on the OP ester substituents of the OP-AChE adduct.…”

Section: Introductionmentioning

confidence: 99%

“…derived from OP nerve agents are less likely to be cleaved at the phospho-serine bond due to steric and electronic effects and are more susceptible to a dealkylation process known as “aging”. ,, Therefore, the differences between the two OP-adduct mechanistic fates are manifested in distinct kinetic processes that are predicated to a large extent on the OP ester substituents of the OP-AChE adduct. Reactivation − may be further distinguished as a spontaneous process in which water acts as a nucleophile (hydrolysis) or more rapidly induced in an oxime-mediated nucleophilic process, both of which lead to scission of the AChE serine O–P bond. ,,, Alternatively, aging is mechanistically aligned with a putative cationic process (protonation) leading to scission at an alkyl-oxygen bond affording a toxicologically deleterious AChE-phosphoanion (AChE-OP(O)(R)(O – ) species that is recalcitrant to reactivate even with oximes. ,,,,, As a result, branched phosphoesters such as iPrO- and OCH(CH 3 )C(CH 3 ) 3 that can better stabilize cations in the transition state favor aging and proceed via a path alternate to reactivation. Clearly, reactivation and aging mechanisms are competing postinhibitory processes whose rates and outcomes are influenced by substituents with opposing electronic properties.…”

Section: Introductionmentioning

confidence: 99%

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Novel Organophosphate Ligand O-(2-Fluoroethyl)-O-(p-Nitrophenyl)Methylphosphonate: Synthesis, Hydrolytic Stability and Analysis of the Inhibition and Reactivation of Cholinesterases

Chao

Ahmed

Gerdes

et al. 2016

Chem. Res. Toxicol.

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The organophosphate O-(2-fluoroethyl)-O-(p-nitrophenyl) methyphosphonate 1 is the first-in-class, fluorine-18 radiolabeled organophosphate inhibitor ([18F]1) of acetylcholinesterase (AChE). In rats, [18F]1 localizes in AChE rich regions of the brain and other tissues where it likely exists as the (CH3)(18FCH2CH2O)P(O)-AChE adduct (ChE-1). Characterization of this adduct would define the inhibition mechanism and subsequent postinhibitory pathways and reactivation rates. To validate this adduct, the stability (hydrolysis) of 1 and ChE-1 reactivation rates were determined. Base hydrolysis of 1 yields p-nitrophenol and (CH3) (FCH2CH2O)P(O)OH with pseudo first order rate constants (kobsd) at pH 7.4 (PBS) of 3.25 × 10−4 min−1 (t1/2 = 35.5 h) at 25 °C and 8.70 × 10−4 min−1 (t1/2 = 13.3 h) at 37 °C. Compound 1 was a potent inhibitor of human acetylcholinesterase (HuAChE; ki = 7.5 × 105 M−1 min−1), electric eel acetylcholinesterase (EEAChE) (ki = 3.0 × 106 M−1 min−1), and human serum butyrylcholinesterase (HuBChE; 1.95 × 105 M−1 min−1). Spontaneous and oxime-mediated reactivation rates for the (CH3) (FCH2CH2O)P(O)-serine ChE adducts using 2-PAM (10 μM) were (a) HuAChE 8.8 × 10−5 min−1 (t1/2 = 131.2 h) and 2.41 × 10−2 min−1 (t1/2 = 0.48 h), (b) EEAChE 9.32 × 10−3 min−1 (t1/2 = 1.24 h) and 3.33 × 10−2 min−1 (t1/2 = 0.35 h), and (c) HuBChE 1.16 × 10−4 min−1 (t1/2 = 99.6 h) and 4.19 × 10−2 min−1 (t1/2 = 0.27 h). All ChE-1 adducts undergo rapid and near complete restoration of enzyme activity following addition of 2-PAM (30 min), and no aging was observed for either reactivation process. The fast reactivation rates and absence of aging of ChE-1 adducts are explained on the basis of the electron-withdrawing fluorine group that favors the nucleophilic reactivation processes but disfavors cation-based dealkylation aging mechanisms. Therefore, the likely fate of radiolabeled compound 1 in vivo is the formation of (CH3)(FCH2CH2O)P(O)-serine adducts and monoacid (CH3)(FCH2CH2O)P(O)OH from hydrolysis and reactivation.

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