Free fatty acid (FFA) and acylcarnitine (AcCar) are key elements of energy metabolism. Dysregulated levels of FFA and AcCar are associated with genetic defects and other metabolic disorders. Due to differences in the physicochemical properties of these two classes of compounds, it is challenging to quantify FFA and AcCar in human plasma using a single method. In this work, we developed a chemical isotope labeling (CIL)-based liquid chromatography-multiple reaction monitoring (LC-MRM) method to simultaneously quantify FFA and AcCar. Dansylhydrazine (DnsHz) was used to label the carboxylic acid moiety on FFA and AcCar. This resulted in the formation of a permanently charged ammonium ion for facile ionization in positive ionization mode and higher hydrophobicity for enhanced retention of short-chain analogs on reversed-phase LC columns and enabled absolute quantification by using heavy labeled DnsHz analogs as internal standards. Labeling conditions including the concentration and freshness of cross-linker, reaction time, and temperature were optimized. This method can successfully quantify all short-, medium- and long-chain FFAs and AcCars with greatly enhanced sensitivity. Using this method, 25 FFAs and 13 AcCars can be absolutely quantified and validated in human plasma samples within 12 min. Simultaneous quantification of FFA and AcCar enabled by this CIL-based LC-MRM method facilitates the investigation of fatty acid metabolism and has potential in clinical applications.
Keywords: Acylcarnitine; Chemical isotope labeling; Free fatty acid; Human plasma; LC-MRM-MS; Targeted metabolomics.