METABOLISM AND NEURAL TOXICITY OF ORGANOPHOSPHORUS COMPOUNDS (PESTICIDE, CYTOCHROME P-450, ISOFENPHOS, N-DEALKYLATION)

An in vitro assay has been developed to assess the potentials of organophosphorus compounds (OPs) that require metabolic activation to produce delayed neuropathy. The assay involves the use of the neurotoxic esterase (NTE) from chick embryo brain and mixed-function oxidases (MFO) from chick embryo liver microsomes induced by phenobarbital. NTE inhibition is taken as indication of the potential for a compound to bring about delayed neuropathy. The assay was validated by testing parathion, paraoxon, diazinon, tri-o-cresyl phosphate (TOCP), S,S,S-tri-n-butyl phosphoro-trithioate (DEF), leptophos, and leptophosoxon. A high level of NTE inhibition was observed when NADPH was present for the OPs capable of producing delayed neurotoxicity (TOCP, DEF, and leptophos). No such inhibition was detected with or without NADPH for the OPs incapable of producing delayed neurotoxicity (parathion, paraoxon, and diazinon).; The MFO-NTE assay was applied in the study of several organophosphoramidates: Ruelene, Nemacur, Monitor, and isofenphos. Nemacur and isofenphos tested positive and extensive effort was made to identify the active metabolites. An important metabolite of isofenphos, des-N-isopropyl isofenphosoxon (DNIO), was generated by oxidation and N-dealkylation. DNIO was a more potent inhibitor of AChE and NTE than its parent compound, with I50's of approximately 4 x 10('-7) M and 4 x 10('-6) M for the two enzymes respectively and caused delayed neuropathy in hens in vivo. DNIO and des-N-isopropyl isofenphos (DNI) caused neuropathy in chickens at doses half of that required for isofenphos, indicating that N-dealkylation is an important factor in the delayed neurotoxic effect of isofenphos.; Nemacur was probably a "false positive". Hens receiving three doses of 40 mg/kg in a period of seven days showed no NTE inhibition in vivo. On the other hand, Monitor was a "false negative". When tested in hens, three doses of 50 mg/kg in a period of seven days led to a 65% and 52% NTE inhibition at brain and spinal cord respectively while no NTE inhibition was detected with the MFO-NTE assay. Although it remains to be tested whether Monitor will cause delayed neuropathy in vivo, the discrepancies between the in vitro and in vivo NTE inhibition data for Nemacur and Monitor suggest that the MFO-NTE assay needs further improvement. It may be possible to induce a more complete array of microsomal enzymes and improve the system by using a combination of inducers, including B-naphthoflavone, isosafrole, 3-methylcholanthrin, and polychlorinated biphenyl (PCB) instead of phenobarbital alone.