Kinetic investigations into the interactions of aprophen with cholinesterases and a carboxylesterase

Acetylcholinesterases, butyrylcholinesterases, and carboxylesterases appear to form kinetically a homologous enzyme series with respect to many substrates and inhibitors. The present paper evaluates the interaction of aprophen with acetylcholinesterases, butyrylcholinesterases, and carboxylesterases with respect to (1) protecting the enzyme from organophosphate and carbamate inhibition, (2) accelerating pralidoxime iodide (2-PAM) regeneration of the diisopropylphospho-enzyme, and (3) comparing the inhibition and regeneration kinetics of a soluble mammalian acetylcholinesterase with that of bovine erythrocyte acetylcholinesterase. The irreversible inhibition kinetics of diisopropyl fluorophosphate (DFP) and eserine inhibition of fetal bovine serum acetylcholinesterase were typical of other acetylcholinesterases as indicated by the bimolecular inhibition rate constants, k i , of 7.7 ± 1.3 × 10 4 M −1 and 2.9 ± 1.7 × 10 6 M −1, respectively. Similarly, the bimolecular regeneration rate constant, k r , for 2-PAM regeneration of the diisopropylphospho-acetylcholinesterase was 14.7 M −1 min −1. The bimolecular rate constants, k i and k r , were not statisticaly perturbed when the reaction was monitored in the presence of aprophen with the fetal bovine serum acetylcholinesterase. Human serum butyrylcholinesterase was partially protected from DFP inhibition by aprophen with no detectable change in the bimolecular inhibition rate constant, k i . The regeneration of the diisopropylphospho-butyrylcholinesterase by 2-PAM was accelerated in the presence of aprophen by a factor of 2.7 over that of 2-PAM alone (8.4 ± 2.2 M −1 to 23.1 ± 2.6 M −1 min −1respectively). Neither the inhibition (DFP) nor the regeneration (2-PAM) kinetics observed for the carboxylesterase was perturbed by the presence of aprophen.