Proline 285 is integral for the reactivation of organophosphate-inhibited human butyrylcholinesterase by 2-PAM

Human butyrylcholinesterase (HuBChE) is a stoichiometric bioscavenger that protects from the toxicity of nerve agents. Non-human primates are suitable models for toxicity studies that cannot be performed in humans. We evaluated the biochemical properties of native macaque (MaBChE) tetramers, compared to recombinant MaBChE monomers, PEGylated recombinant MaBChE tetramers and monomers, and native HuBChE tetramers. K_m and k_cat values for butyrylthiocholine were independent of subunit assembly status. The K_m for all forms of MaBChE was about 70 μM, compared to 13 μM for HuBChE. The k_cat was about 100,000 min^-1 for MaBChE and 30,000 min^-1 for HuBChE. The reversible inhibitor ethopropazine had similar K_i values of 0.05 μM for all MaBChE forms and HuBChE. The bimolecular rate constant, ki, for inhibition by diisopropylfluorophosphate (DFP), an analog of sarin, was 2.2 to 2.5 × 10⁷ M^-1 min^-1 for all MaBChE forms and for HuBChE. A major difference between MaBChE and HuBChE was the rate of reactivation by 2-PAM. The second order rate constant for reactivation of DFP-inhibited MaBChE by 2-PAM was 1.4 M^-1 min^-1, but was 380 fold faster for DFP-inhibited HuBChE (k_r 531 M^-1 min^-1). The acyl pocket of MaBChE has Leu285 in place of Pro285 in HuBChE. The reactivation rate of DFP-inhibited HuBChE mutant P285L by 2-PAM was reduced 5.8-fold (k_r 92 M^-1 min^-1) indicating that P285 determines whether 2-PAM binds in an orientation that favors release of diisopropylphosphate. DFP-inhibited MaBChE treated with 0.2 M 2-PAM recovered 10% of its original activity, whereas DFP-inhibited HuBChE recovered 80% activity. It was concluded that the biochemical properties of MaBChE are similar to those of HuBChE except for the reactivation of DFP-inhibited BChE.