Identification of the major metabolites of quinocetone in swine urine using ultra-performance liquid chromatography/electrospray ionization quadrupole time-of-flight tandem mass spectrometry.

Quinocetone (QCT), 3-methyl-2-cinnamoylquinoxaline-1,4-dioxide, is a quinoxaline-N,N-dioxide used in veterinary medicine as a feed additive. QCT is broadly used in China to promote animal growth, but few studies have been performed to reveal the metabolism of QCT in animals until now. In the present study, the metabolites of QCT in swine urine were investigated using ultra-performance liquid chromatography/electrospray ionization quadrupole time-of-flight mass spectrometry (UPLC/ESI-QTOF-MS). Multiple scans of metabolites in MS and MS/MS modes and accurate mass measurements were performed simultaneously through data-dependent acquisition. Most measured mass errors were less than +/-5 mDa for both protonated molecules and product ions using external mass calibration. The structures of metabolites and their product ions were easily and reliably characterized based on the accurate MS(2) spectra and known structure of QCT. As expected, extensive metabolism was observed in swine urine. Thirty-one metabolites were identified in swine urine, most of which were reported for the first time. The results reveal that the N-O group reduction at position 1 and the hydroxylation reaction occurring at the methyl group, the side chain or on the benzene ring are the main metabolic pathways of quinocetone in swine urine. There was abundant production of 1-desoxyquinocetone and hydroxylation metabolites of 1-desoxyquinocetone. The proposed metabolic pathway of quinocetone in vivo can be expected to play a key role in food safety evaluations.