A sensitive and efficient method for simultaneous profiling of bile acids and fatty acids by UPLC-MS/MS

The carboxyl group is the functional group in both bile acids (BAs) and fatty acids (FAs) (BAFAs). Considering the functional correlation and the structural similarity of these compounds, a sensitive and efficient method was developed here for the first time to simultaneously profile BAFAs based on ultrahigh performance liquid chromatography coupled with tandem mass spectrometry (UPLC-MS/MS). Through optimization of the chromatographic conditions, all BAFAs can be efficiently separated and quantified in 19 min with excellent peak shape. For comprehensive profiling of endogenous FAs without standards, a strategy was established to predict the retention times (RTs) of all theoretically possible FAs on the basis of the good multiple linear regression relationship between RT and FA carbon chain length and double bond number. High-resolution mass spectrometry was employed for the final confirmation of these predicted FAs. Twenty-eight FAs in rat serum were newly identified using this strategy. Though the regulation of collision energies (CEs) for highly abundant compounds, the problems of their poor quantification linearity and accuracy caused by MS signal saturation were solved, facilitating the simultaneous quantification of both high- and low-abundance BAFAs with good linearity and accuracy. The established UPLC-MS/MS method was further used to quantify BAFAs in rat serum and to explore the disturbance of BAFA metabolism in the Tripterygium glycoside-induced liver injury rat model. A total of 25 BAs and 55 FAs in rat serum were identified and quantified. Several BAFAs, including nordeoxycholic acid, taurodeoxycholic acid and some unsaturated FAs, were found to differ significantly in the control and model groups. These BAFAs are very promising biomarkers for the evaluation of Tripterygium glycoside-induced liver injury.