Neonicotinoids are the most important new class of insecticides of the last three decades, already accounting for about 10% of the world insecticide market and with major human exposure. They have the same nicotinic acetylcholine receptor (nAChR) target as nicotine but are much more potent and selective for insects. The long term objective is to define the mechanisms of selective toxicity for the major neonicotinoids, imidacloprid, thiacloprid, thiamethoxam and acetamiprid. The first specific aim is to establish the unique neonicotinoid and nicotinoid structural features for nAChR specificity and selective toxicity. The negatively-charged tip is proposed to confer potency and selectivity for insect nAChRs, prompting us to synthesize structural probes with this feature. The preferred conformation and configuration will be determined by X-ray crystallography and NMR and related to potency. Quantum mechanics studies will establish the electrostatic potential surface, molecular charge distribution and binding energy. The same approaches will be used to characterize the analogous nicotinoids selective for mammalian nAChRs.
The second aim i s to characterize nAChR subtypes, subunits and subsites that determine selective action. One goal is to establish the role(s) of alpha4beta2, alpha3beta2(and/or beta4) alpha5, or alpha7 and alpha1Gamma alpha1deltabeta1 subtypes in binding neonicotinoids and their metabolites relative to functional assays and selective toxicity. Drosophila nAChR and vertebrate nAChR subtypes are solubilized, purified by neonicotinoid- or nicotinoid-affinity chromatography, and labeled with potent azidoneonicotinoid and azidonicotinoid photoaffinity probes designed in this laboratory. The labeled subunits will be identified with particular attention to the proposed cationic subsite(s) in insects and pi-electron-rich subsite(s) in vertebrates. Molecular modeling of the detailed binding site architecture will then relate the structure of the Drosophila D-alpha subunits and the acetylcholine binding protein to the findings on photoaffinity labeling.
The third aim i s to define neonicotinoid metabolic activation and detoxification relative to selective toxicity. Metabolites of imidacloprid, thiacloprid, thiamethoxam and acetamiprid will be characterized and synthesized for receptor, toxicity and functional assays to clarify selective metabolic activation versus detoxification. Toxicokinetic studies with mice will relate the brain levels of neonicotinoids and their desnitro and descyano metabolites (toxic iminium derivatives) to the poisoning signs. Continuing investigations will define human cytochrome P450 isozyme specificity in metabolism of neonicotinoids and characterize human microsomal """"""""neonicotinoid nitroimine reductase"""""""" that generates unique hydrazone and triazolone derivatives of imidacloprid.
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