Diphenyl ether herbicide metabolism in a spruce cell suspension culture: The identification of two novel metabolites derived from a glutathione conjugate

[CF₃-¹⁴C]Fluorodifen herbicide (2,4′-dinitro-4-trifluoromethyldiphenyl ether) was rapidly metabolized by a spruce cell suspension culture (Picea abies L. Karst). The primary route of metabolism involved cleavage of the diphenyl ether bond by glutathione (GSH). The resulting conjugate of fluorodifen (S-[4-trifluoromethyl-2-nitrophenyl]glutathione) appeared to be metabolized sequentially to the corresponding γ-glutamylcysteine conjugate, the cysteine conjugate, and two novel metabolites. These novel metabolites, identified by fast atom bombardment mass spectrometry and enzyme hydrolysis, were the S-glucoside and the S-(3-thio-2-O-glucosyl) lactic acid conjugate of 4-trifluoromethyl-2-nitrobenzene. This appears to be the first report on the plant metabolism of GSH conjugates to metabolites of these classes. During the metabolism of [¹⁴C]fluorodifen, a loss of water-soluble ¹⁴C-labeled metabolites from the spruce cells to the growth medium was observed. After 1 day, 63% of the applied ¹⁴C was present as metabolites in the cells and 30% was present as metabolites in the medium. However, after 10 days only 14% of the applied ¹⁴C was present as metabolites in the cells and 50% was present as metabolites in the medium. During this period, the recovery of ¹⁴C declined from 97% after 1 day to 64% after 10 days. The decrease in recovery may have been due to the loss of volatile metabolites such as 4-trifluoromethyl-2-nitrothiophenol. The major metabolites present after 1 day were the GSH conjugate (36%), the γ-glutamylcysteine conjugate (35%), and the cysteine conjugate (14%). After 10 days, the major metabolites were the S-glucoside (45%) and the S-(2-O-glucosyl)-3-thiolactic acid conjugate (12%).