Kinetics of the inhibitory interaction of organophosphorus neuropathy inducers and non-inducers in soluble esterases in the avian nervous system.

Some published studies suggest that low level exposure to organophosphorus esters (OPs) may cause neurological and neurobehavioral effects at long term exposure. These effects cannot be explained by action on known targets. In this work, the interactions (inhibition, spontaneous reactivation and "ongoing inhibition") of two model OPs (paraoxon, non neuropathy-inducer, and mipafox, neuropathy-inducer) with the chicken brain soluble esterases were evaluated. The best-fitting kinetic model with both inhibitors was compatible with three enzymatic components. The amplitudes (proportions) of the components detected with mipafox were similar to those obtained with paraoxon. These observations confirm the consistency of the results and the model applied and may be considered an external validation. The most sensitive component (Eα) for paraoxon (11-23% of activity, I(50) (30 min)=9-11 nM) is also the most sensitive for mipafox (I(50) (30 min)=4 nM). This component is spontaneously reactivated after inhibition with paraoxon. The second sensitive component to paraoxon (Eβ, 71-84% of activity; I(50) (30 min)=1216 nM) is practically resistant to mipafox. The third component (Eγ, 5-8% of activity) is paraoxon resistant and has I(50) (30 min) of 3.4 μM with mipafox, similar to NTE (neuropathy target esterase). The role of these esterases remains unknown. Their high sensitivity suggests that they may either play a role in toxicity in low-level long-term exposure of organophosphate compounds or have a protective effect related with the spontaneous reactivation. They will have to be considered in further metabolic and toxicological studies.