Identification of novel sugar-small-molecule conjugates in human urine based on multi-energy LC-MS/MS analysis

Sugar-mediated conjugation plays a central role in human biotransformation, yet many sugar-small-molecule conjugates remain unannotated due to structural complexity and limited spectral references. In this study, we re-examined LC-MS/MS data acquired from six NIST human urine reference materials to identify previously unrecognized sugar-containing conjugates. An analytical framework was implemented that integrated (A) diagnostic sugar fragments and neutral-loss validation at low collision energies, (B) extraction of non-sugar component (NSC) fragments acquired at higher collision energies to construct pseudo-spectra for library matching, and (C) MS1 feature reconstruction to validate species' mass, retention time, abundance, and relationships with other ions. Using this strategy, 23 previously unreported sugar-small-molecule conjugates were identified, including 8 N-acetylglucosamine (GlcNAc)-, 12 N-acetylgalactosamine (GalNAc)-, and 3 N-acetylneuraminic acid (NeuAc)-containing species. The detection of GalNAc and NeuAc in the conjugates expands the repertoire of sugars involved in small-molecule conjugation. Among the 23 conjugates, 15 contained glucuronic acid (GlcA), and in 14 cases GlcA functioned as a bridge linking an additional sugar moiety to the NSC. The NSCs include both endogenous and xenobiotic-derived compounds. Semi-quantitative analysis across six urine materials revealed both the high consistency of the analytical results and pronounced inter-sample variability. Collectively, these findings indicate the presence of previously unknown conjugates in human metabolism and show that multi-energy LC-MS/MS combined with sub-structural spectral interpretation provides an effective strategy for elucidating complex metabolites.