Phosphylation kinetic constants and oxime‐induced reactivation in acetylcholinesterase from fetal bovine serum, bovine caudate nucleus, and electric eel

Abstract

Kinetic constants for selected phosphonate and phosphinate inhibitors of fetal bovine serum acetylcholinesterase (FBS AChE; EC 3.1.1.7), bovine caudate nucleus AChE (BCN AChE), and eel AChE have been determined. Oxime reactivation of the phosphylated enzymes has also been evaluated. In general a rank order with respect to organophosphorus compound (OP) inhibition of the enzymes was observed: soman (pinacolyl methylphosphonofluoridate) was found to be the most potent inhibitor, and 4‐nitrophenyl methyl(phenyl)phosphinate (PMP) the least potent. On average the bimolecular rate constant for soman inhibition of eel AChE was nearly twofold greater (9.3 × 10⁷ M ⁻¹ s⁻¹) than that for FBS AChE (5.5 × 10⁷ M ⁻¹ s⁻¹) and nearly fourfold greater than that for BCN AChE (2.2 × 10⁷ M ⁻¹ s⁻¹). In addition, 4‐nitrophenyl chloro‐methyl(phenyl)phosphinate (CPMP) inhibition of eel AChE on average was nearly 10‐fold greater than FBS AChE and three orders of magnitude greater than BCN AChE. The oxime HI‐6 reactivated soman phosphonylated enzymes to a considerably greater extent than other oximes, and FBS AChE was notably more responsive to HI‐6 than to other oximes. The individual mean values of the k_i for each inhibitor in each class (phosphonate or phosphinate) were different with respect to each AChE, which may be a reflection of differences in enzyme configuration, whereas the general rank order of inhibitor potency within each class, reflected by the k_i, was similar with respect to each AChE, which may be related to similar active centers. In general, oxime potency and rank order varied with each inhibitor and with each AChE, although there was some similarity in oxime rank order between the two mammalian AChEs. Overall, the data support the selection of FBS AChE as the enzyme of choice for in vitro testing of OP inhibitors and reactivators.