Identification of a novel animal metabolite of methomyl insecticide

In studies conducted to determine the fate of methomyl in goat, chicken, and monkey, an unknown highly polar, acidic metabolite was found in the urine/excreta of all three animals. The metabolite was purified by HPLC and analyzed by microcolumn LC/MS with electrospray ionization and by NMR. A Hypercarb porous graphitic carbon HPLC column was used for purification and LC/MS, since the unknown was unretained on conventional reversed phase HPLC columns. [C-14]Methomyl was used as radiotracer, and [C-13]methomyl was added at the 50% level to facilitate metabolite identification. The molecular weight of the metabolite was determined by LC/MS-electrospray, using positive and negative ionization, and structurally useful fragment ions were obtained in both modes by in-source collision-induced dissociation(CID). High-resolution accurate mass measurements of the MH+ doublet allowed assignment of the elemental composition. NMR data showed the presence of methyl, methylene, and methine protons with coupling observed between C-13 and the methyl and methine protons. From these data, the structure was determined to be an unusual double conjugate of acetonitrile (a degradate of methomyl), involving conjugation with both cysteine and sulfate.