Highly sensitive determination of fatty acid esters of hydroxyl fatty acids by liquid chromatography-mass spectrometry

Recently, a new class of endogenous lipids, branched fatty acid esters of hydroxy fatty acids (FAHFAs), was discovered with anti-diabetic and anti-inflammatory effects in mammals. FAHFAs attracted increasing attention because of their critical physiological function. However, accurate quantitation of FAHFAs is still a challenge due to their high structure similarity and low abundance in biological samples. Herein, we developed a highly sensitive method for the determination of 16 FAHFAs (PAHSAs, OAHSAs, SAHSAs and POHSAs) in biological samples by coupling strong anion exchange solid phase extraction (SAX-SPE) with chemical labeling assisted ultra-high performance liquid chromatography/mass spectrometry (SAX-SPE-CL-UHPLC/MS). In the developed method, SAX-SPE was employed to selectively enrich and purify FAHFAs from biological samples. And then a pair of isotope labeling reagents, 2-dimethylaminoethylamine (DMED) and d(4)-DMED were used to label the purified samples and standard FAHFAs, respectively. The labeled samples were mixed and further subjected to UHPLC/MS analysis. Our results demonstrated that the detection sensitivities of FAHFAs increased by 7-72 folds upon DMED labeling and the limits of detections (LODs) of labeled FAHFAs ranged from 0.01 to 0.14 pg. Moreover, a good separation of FAHFAs isomers was achieved on C18 column in a UHPLC system and all FAHFAs could be analyzed in 20 min with sharp peak shape. The established method provided substantial sensitivity, high specificity, and broad linear dynamic range (3 orders of magnitude). Using this method, we successfully measured the contents and distribution of FAHFAs in rat white adipose, lung, kidney, thymus, liver and heart tissues. The results showed that 7 FAHFAs (13-, 12-, 9-, 5-PAHSA, 13-, 12- and 9-SAHSA) were observed in different tissues of rat. In addition, we successfully detected the above 7 FAHFAs in human serum samples; and among the 7 FAHFAs, 13-, 9-PAHSA, 13- and 12-SAHSA were found remarkably decreased in human breast cancer serum. The proposed method could be successfully applied for the detection of FAHFAs in various biological samples, which will facilitate the understanding of the physiological functions of FAHFAs.