Isotope Labeling Strategies for Acylcarnitines Profile in Biological Samples by Liquid Chromatography-Mass Spectrometry.

Acylcarnitines are closely related to many metabolic diseases and likely to be good biomarkers for clinical diagnosis. Studies on acylcarnitines may help deepen the understanding of their functions associated with disease processes. However, the diversity of their structures and lack of commercial standards make them difficult to be fully detected. In this work, a highly efficient isotope labeling strategy was developed to detect acylcarnitines in biological samples using liquid chromatography-mass spectrometry (LC-MS). A pair of reagents with or without deuterium labeling tags containing both a positive charge and a primary amine group were synthesized, which were used to label the carboxyl group of acylcarnitines. The reaction was performed under mild conditions to avoid hydrolysis of acylcarnitines. The reaction products had two positive charges, forming a double charge peak in MS spectra and with mass-to-charge ratio difference (Δ m/ z) of 4.0251 Da between the light- and heavy-labeled products. The feature made it possible to distinguish signals of acylcarnitines from other carboxyl metabolites with Δ m/ z of 8.0502 Da between their light- and heavy-labeled products. On the basis of the characteristics and the tandem mass spectrometry (MS/MS) spectra, a total of 108 acylcarnitines were discovered and identified in urine samples. Our established approach will be very helpful for the studies of diseases associated with acylcarnitines metabolism.