Rapid Detection and Characterization of in Vitro and Urinary N-Acetyl-L-cysteine Conjugates Using Quadrupole-Linear Ion Trap Mass Spectrometry and Polarity Switching

The present study describes a novel methodology for the rapid detection and structural characterization of unknown N-acetyl-L-cysteine (NAC) conjugates using polarity switching of triple quadrupole mass spectrometry. This method utilizes a negative neutral loss (NL) scan of 129 Da or multiple reaction monitoring (MRM) from predicted m/z values to product ions derived from the NIL of 129 Da as a survey scan to trigger the acquisition of enhanced product ion (EPI) spectra in the positive ion mode. Thus, selective detection of NAC conjugates and acquisition of fragment-rich MS/MS spectra were accomplished in a single LC/MS run. The utility of this methodology was evaluated through analysis of NAC conjugates of acetaminophen in human urine after an oral dose. The MRM-EPI approach, which showed better sensitivity than the NL-EPI approach in analyzing urine samples, revealed three NAC-acetaminophen conjugates in the human urine, including two minor NAC conjugates that were derived from hydroxyl acetaminophen and methoxy acetaminophen. In addition, the methodology was applied to screening for reactive metabolites of clozapine and diclofenac using NAC as a trapping agent. Results showed reactive metaholite profiles comparable to those obtained from glutathione (GSH) trapping experiments, while MS/MS spectra of NAC conjugates provided more valuable structural information than those of GSH adducts. The study demonstrates that NAC trapping followed by NL-EPI analysis is a useful approach for high-throughput screening of reactive metabolites and that the MRM-EPI method is well-suited for analysis of low levels of NAC conjugates in urine.