The pH dependence of dealkylation in soman-inhibited cholinesterases and their mutants: Further evidence for a push-pull mechanism

Bimolecular rate constants for the inactivation of recombinant (r) human (Hu) butyrylcholinesterase (BChE) with P(S)C(S)- and P(S)C(R)-2-(3,3- dimethylbutyl) methylphosphonofluoridate (soman) are (92 ± 7) x 10⁶ M^-1 min^-1 and (13.7 ± 0.8) x 10⁶ M¹-¹ min^-1 at pH 7.4, μ = 0.1 M and 25 °C. Mutations of E197(199) to D or Q and W82(84) to A result in reductions in the rate constants for reactivation with P(S)C(S)-soman 4.3-, 11.8-, and 263-fold and with P(S)C(R)-soman by 6.5-, 47.3-, and 685-fold, respectively. The pH dependence of dealkylation (aging) in r mouse (Mo) acetylcholinesterase (AChE) and rHu BChE and their mutants inactivated with P(S)C(S)- and P(S)C(R)-soman was compared. Best-fit parameters for the asymmetric bell curves for the adducts of wild-type Mo AChE are pK₁ = pK₂ = 4.0-4.9 and pK₃ = 5.2-6.6. These pKs are consistent with the involvement of two carboxylic acids, possibly E202(199) and either E334(327) or E450(443), and H447(440)H⁺ in the dealkylation of AChE. E202Q MoAChE inactivated with the soman diastereomers yielded pK₃ = 5.5-5.8. Nearly symmetric pH curves for soman-inhibited wild-type and E197D Hu BChE gave pK₂ = 3.7-4.6 and pK₃ = 7.3-8.0, but much lower, pK₃ ~5, for the corresponding adduct of the E197Q mutant. Dealkylation in soman-inhibited BChE is consistent with the participation of one carboxylic acid side chain and H438(440)H⁺ Maximal rate constants for dealkylation (k(max)) are 1-6 min^-1 for AChE and 2 min^-1 for BChE at 25 °C. The W82 to A mutation in BChE results in the largest reduction, 2500-6000-fold, in the rate constant for dealkylation. The reduction in the rate constants for dealkylation in the E197 mutants is highly pH dependent. The solvent isotope effects at the pH maxima are 1.3- 1.4, indicating unlikely preprotonation or proton in 'flight' at the enzymic transition states. The new results support the push-pull mechanism of dealkylation in soman-inhibited cholinesterases proposed previously.