Degradation of Pyrethroid Insecticides in the Environment

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  • MIKAMI Nobuyoshi
    Laboratory of Biochemistry and Toxicology, Takarazuka Research Center, Sumitomo Chemical Co., Ltd.

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  • ピレスロイド系殺虫剤の環境における代謝・分解
  • ピレスロイド系殺虫剤の環境における代謝・分解(〔日本農薬〕学会賞受賞論文)
  • ピレスロイドケイ サッチュウザイ ノ カンキョウ ニ オケル タイシャ ブンカ

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Abstract

The environmental fates of fenvalerate and fenpropathrin have been studied in comparison with other agricultural pyrethroids such as permethrin, cypermethrin and deltamethrin. Al-though photostability of these pyrethroids has much improved, they are still susceptible to photolysis in the environment. The major photochemical reactions are isomerization, ester bond cleavage and decarboxylation. The mechanism of decarboxylation reaction, which is unique to fenvalerate photolysis, has been clarified by elucidation of the chemical structures of free radical intermediates, using a spin trap method combined with HPLC, ESR and GC-MS analyses. The metabolic profiles in plants are similar to those in mammals. The trans isomers are more rapidly hydrolyzed than the cis isomers, and oxidation occurs to a greater extent with the more stable cis isomers. The resultant products are converted to conjugates by coupling with glucose, gentiobiose, sophorose, glucosylxylose, malonylglucose, and two types of tri-glucose. Hydrogen cyanide, which would be released via ester hydrolysis, is rapidly detoxified in plants by metabolic conversion to β-cyanoalanine, asparagine, aspartic acid, and γ-glutamyl-β-cyanoalanine. The ester hydrolysis is also a main degradation route in soils. A slight modification in chemical structures significantly alters the degradation rate. Either replacement of chlorine with bromine or introduction of nitrile group to benzyl group retards the degradation. Further, the traps and αS isomers are degraded more rapidly than the cis and αR antipodes, respectively. Most of the isolated soil bacteria possess different types of pyrethroid-hydrolyzing enzymes with high substrate specificity. The stereoselective degradation could be rationalized by involvement of these enzymes. These environmental fate studies are useful not only in evaluating the safety but to facilitate creation of the more bioacceptable, biorational pesticide.

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