Thiazopyr [methyl 2-(difluoromethyl)-5-(4,5-dihydro-2-thiazolyl)-4-(2-methylpropyl)-6-(trifluoromethyl)-3-pyridinecarboxylate] is an experimental herbicide being developed for weed control in cotton, citrus, and other crops. The in vitro metabolic fate of key thiazopyr crop metabolites in animals was investigated using rat liver homogenates. The thiazopyr crop metabolites studied were sulfoxide ester, sulfone ester, nitrile ester, amide acid, and sulfonic diacid. Our results showed that the ester crop metabolites were extensively metabolized by rat liver homogenates to products previously observed during in vivo rat metabolism studies. Metabolites were identified using several techniques including high-performance liquid chromatography coelution with standards, direct identification by mass spectrometry and comparison with standards, and derivatization followed by mass spectrometry. In contrast, the acidic crop metabolites (amide acid and sulfonic diacid) were not metabolized, likely due to their water solubility and high polarity. Our study showed that these crop metabolites, if absorbed by rats, are further transformed to polar products that are likely to be eliminated rapidly from the body.

1. Thiazopyr was metabolized by liver microsomes from male Sprague-Dawley rats to a previously unidentified metabolite.2. The new metabolite was identified by coelution with an authentic standard in hplc and by electrospray lc/ms as the corresponding carboxylic acid.3. Formation of the carboxylic acid metabolite was inhibited in the presence of mono-oxygenase inhibitors including piperonyl butoxide, 1-aminobenzotriazole, metyrapone and tetcyclacis.4. Transformation of thiazopyr to its carboxylic acid by rat liver microsomes is mediated by mono-oxygenases and not hydrolases.

Thiazopyr is an experimental herbicide from the pyridine family that shows excellent preemergent activity against narrow-leafed weeds. Biotransformation of thiazopyr was examined in vitro using rat liver microsomes to assess the extent of its metabolism in vivo . Thiazopyr was rapidly and extensively oxidized exclusively in the thiazoline ring. The sulfur atom in the thiazoline ring was sequentially oxidized to the corresponding sulfoxide and sulfone metabolites. Oxidation of the carbon atoms in the thiazoline ring produced carbinolimine intermediates, which underwent dehydration or arrangements leading to the opening of thiazoline ring. A total of six metabolites were identified based on electrospray LC/MS and/or cochromatography with standards.

This study demonstrates a useful application of on-line microbore high-performance liquid chromatography (HPLC) fast atom bombardment (FAB) and thermospray (TSP) mass spectrometry techniques for identification of metabolites from the in vitro metabolism of an experimental Monsanto herbicide: 2-chloro-N-(ethoxymethyl-N-[2-methyl-6- (trifluoromethyl)phenyl]acetamide, 'chloroacetanilide'. The microbore HPLC FAB technique on a high-resolution sector mass spectrometer accelerated identification of polar metabolites from the in vitro metabolism study of the herbicide. It provided good chromatographic resolution and excellent FAB sensitivity with strong protonated molecular ions. Scanning high-resolution LC/FAB mass spectrometry also provided molecular formulae for structural elucidation of unknown metabolites. Sample purification and concentration were minimized. Identification of less polar metabolites was carried out using LC/TSP mass spectrometry with a quadrupole mass spectrometer. LC/TSP mass spectrometry provided useful structural information for both polar and less polar metabolites because their spectra showed more fragmentation than FAB spectra. Glutathione conjugation was the major reaction observed during in vitro incubation of the herbicide. Oxidation of the chloroacetanilide by rat liver enzymes was also a significant metabolic reaction. Seven metabolites were identified, of which four were glutathione conjugates.

1. Transformation of dithiopyr by rat liver enzymes in vitro produced the corresponding monoacids as the predominant metabolites. 2. Transformations of the methylthioester functional groups in dithiopyr to the monoacids were mediated via rat liver microsomal oxygenases, and not via esterases. 3. Based on the formation of a dithiopyr-glutathione conjugate, the mechanism of monoacid formation is believed to proceed through an initial sulphur oxidation of the methylthioester group and a subsequent nucleophilic displacement reaction.

The combination of electron capture ionization with liquid chromatography/mass spectrometry produces a valuable technique for metabolism studies of fluorinated compounds. Selective ionization of halogenated components simplifies sample purification since most biological constituents are transparent to detection. This report details the application of this technique to in vitro and in vivo metabolism studies of fluorinated herbicides.

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTProperties and decomposition of 2,6-diethylnitrosobenzeneStephen J. Wratten, Hideji Fujiwara, and R. Thomas SolstenCite this: J. Agric. Food Chem. 1987, 35, 4, 484–491Publication Date (Print):July 1, 1987Publication History Published online1 May 2002Published inissue 1 July 1987https://pubs.acs.org/doi/10.1021/jf00076a011https://doi.org/10.1021/jf00076a011research-articleACS PublicationsRequest reuse permissionsArticle Views138Altmetric-Citations7LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InRedditEmail Other access optionsGet e-Alertsclose Get e-Alerts