Kinetics of inhibition of soluble peripheral nerve esterases by PMSF: a non-stable compound that potentiates the organophosphorus-induced delayed neurotoxicity.

The kinetic analysis of esterase inhibition by acylating compounds (organophosphorus carbamates and sulfonyl fluorides) is sometimes unable to yield consistent results by fitting simple inhibition kinetic models to experimental data of complex systems. In this work, kinetic data were obtained for phenylmethylsulfonyl fluoride (PMSF) tested at different concentrations incubated for up to 3 h with soluble fraction of chicken peripheral nerve. PMSF is a protease and esterase inhibitor causing protection or potentiation of the organophosphorus-induced delayed neuropathy and is unstable in water solution. The target of the promotion effect was proposed to be a soluble esterase not yet identified. A kinetic model equation was deduced assuming a multienzymatic system with three different molecular phenomena occurring simultaneously: (1) inhibition, (2) spontaneous chemical hydrolysis of the inhibitor and (3) ongoing inhibition (inhibition during the substrate reaction). A three-dimensional fit of the model was applied for analyzing the experimental data. The best-fitting model is compatible with a resistant component (16.5-18%) and two sensitive enzymatic entities (both 41%). The corresponding second-order rate constants of inhibition (ki = 12.04 × 10⁻² and 0.54 × 10⁻² μM⁻¹ min⁻¹, respectively) and the chemical hydrolysis constant of PMSF (kh = 0.0919 min⁻¹) were simultaneously estimated. These parameters were similar to those deduced in fixed-time inhibition experiments. The consistency of results in both experiments was considered an internal validation of the methodology. The results were also consistent with a significant ongoing inhibition. The proportion of enzymatic components showed in this work is similar to those previously observed in inhibition experiments with mipafox, S9B and paraoxon, demonstrating that this kinetic approach gives consistent results in complex enzymatic systems.