Triple chemical derivatization strategy assisted liquid chromatography-mass spectrometry for determination of retinoic acids in human serum.

Retinoic acids (RAs), an important class of fatty acid derived from vitamin A, are closely associated with various human diseases including cancer. Hence, determination of endogenous RAs would help to uncover the mechanisms underlying RAs-related diseases. However, accurate quantification of RAs is still a challenge due to their high structure similarity, low abundance in biological samples and the lack of isotope internal standards (ISs). In this study, a liquid chromatography-mass spectrometry (LC-MS) method with high-sensitivity and high-throughput was developed to simultaneously determine 5 RAs (all-trans-retinoic acid, 13-cis-retinoic acid, 9,13-cis-retinoic acid, all-trans-4-oxoretinoic acid and 4-hydroxy-retinoic acid) in human serum. In the method, three derivatization reagents, N, N-dimethylethylenediamine (DMED), d4-N,N-dimethylethylenediamine (d4-DMED) and 13C2-N,N-dimethylethylenediamine (13C2-DMED), were used for triple chemical derivatization of RAs, thus the detection sensitivity and analysis throughput of endogenous RAs could be achieved. Benefiting from the developed strategy, the analysis throughput was enhanced and the detection sensitivity of RAs was increased by 14-398 folds. The limits of quantification (LOQs) of RAs were found to be between 8.4 and 130 pg/mL, which were better than previously reported methods. Good linearities of RAs were obtained with determination coefficient (R2) ranging from 0.9774 to 0.9999. The intra- and inter-day relative standard deviations (RSDs) were below 11.7% and 12.7%, respectively, indicating the acceptable reproducibility of the method. Using the developed method, we successfully quantified 4 RAs (all-trans-retinoic acid, 13-cis-retinoic acid, 9,13-cis-retinoic acid and all-trans-4-oxoretinoic acid) in health controls and cancer patient serum samples. Furthermore, t-test analysis showed that the concentration of three RAs (all-trans-retinoic acid, 13-cis-retinoic acid and all-trans-4-oxoretinoic acid) in cancer patient serum samples were significantly decreased compared with health controls, which indicated that RAs might play an important role in the formation and development of cancer.

High Coverage Profiling of Carboxylated Metabolites in HepG2 Cells Using Secondary Amine-Assisted Ultrahigh-Performance Liquid Chromatography Coupled to High-Resolution Mass Spectrometry.

Carboxylic metabolites are an important class of metabolites, which widely exist in mammals with various types. Chemical isotope labeling liquid chromatography-mass spectrometry (CIL-LC-MS) has been widely used for the detection of carboxylated metabolites. However, high coverage analysis of carboxylated metabolites in biological samples is still challenging due to improper reactivity and selectivity of labeling reagents to carboxylated metabolites. In this study, we used N-methylphenylethylamine (MPEA) to label various types of carboxylated metabolites including short-chain fatty acids (SCFAs), medium-chain fatty acids (MCFAs), long-chain fatty acids (LCFAs), polycarboxylic acids (polyCAs), amino acids (AAs), and aromatic acids. Additionally, metabolites containing other functional groups, such as phenol, sulfhydryl, and phosphate groups, could not be labeled under the conditions of MPEA labeling. After MPEA labeling, the detection sensitivity of carboxylic acids was increased by 1-2 orders of magnitude, and their chromatographic retention on a reversed-phase (RP) column was enhanced (RT > 3 min). Under optimized labeling conditions, we used MPEA and d3-N-methylphenylethylamine (d3-MPEA) for high coverage screening of carboxylated metabolites in HepG2 cells by ultrahigh-performance liquid chromatography coupled to high-resolution mass spectrometry (UHPLC-HRMS). As a result, a total of 403 potential carboxylated metabolites were obtained of which 68 were confirmed based on our established in-house chemically labeled metabolite database (CLMD). SCFAs, MCFAs, LCFAs, polyCAs, AAs, and aromatic acids were all detected in HepG2 cell extracts. Due to the successful identification of AAs, the current method increased the coverage of carboxylated metabolites compared with our previous work. Moreover, 133 and 109 carboxylated metabolites with changed contents were obtained in HepG2 cells incubated with curcumin and R-3-hydroxybutyric acid, respectively. In general, our established method realized high coverage analysis of carboxylated metabolites in HepG2 cells.