Isofenphos and an in vitro activation assay for delayed neuropathic potential

Organophosphorus compounds (OPs) that cause organophosphorus ester-induced delayed neuropathy (OPIDN) generally inhibit neurotoxic esterase (NTE). However, the assay itself, when conducted in vitro, misses OPs that are activated into OPIDN-causing agents in the body. A preparation of liver mixed-function oxidases and brain NTE was used to rapidly detect activations of OPs. The compounds (0.1 mM or less) to be tested were incubated with microsomes isolated from livers of phenobarbital-treated chick embryos (P-450 content averaged 1.81 +/- 0.27 nmol/mg protein, means +/- SD, N = 5) and NTE (average of 13.8 nmol/min/mg protein) from untreated chick embryo brains. The NTE was separated by calcium precipitation and its activity assayed as usual. The low inhibitions of NTE of compounds that were not neurotoxic (parathion, Diazinon) did not increase in the presence of NADPH; inhibitions of NTE of compounds that required activation (leptophos, S,S,S-tri-n-butyl phosphorotrithioate, and tri-o-cresyl phosphate) greatly increased with NADPH. Both the recently identified neuropathic OP isofenphos (IFP) and its oxon required activation to inhibit NTE (inhibitions of 20 and 80%, respectively). Evidence is presented that the possible neuropathic metabolite is des-N-isopropyl IFP oxon.