Ammonium fluoride-induced stabilization for anion attachment mass spectrometry: Facilitating the pseudotargeted profiling of bile acids submetabolome

Mass spectrometry-based approaches enable us to capture changes in the metabolome in biological systems with high sensitivity and resolution. But global MS-based profiling of the bile acids (BAs) submetabolome is still a challenging task. Particularly for unconjugated BAs, the collision-induced dissociation (CID) fragment ions showed low ion intensities which were insufficient for analysis. This study is aimed at the development of an anion attachment MS-based approach for pseudotargeted profiling of the BAs submetabolome. We demonstrated that anion attachment MS with the combination use of ammonia fluoride (NH4F) and formate could provide stable anionic adduct ([M + HCOO]−) with good MS responses for unconjugated BAs. A mechanistic study revealed that the underlying rationale is due to the NH4F-induced approximate matching of attractions between BAs and anion for the 24-carboxyl hydrogen. This 24-carboxyl hydrogen regioselectivity is useful to screen for potential unconjugated BAs from the biological matrix. The stability and regioselectivity of anion attachment allowed the establishment of SRM transitions for unconjugated BAs for the first time. To profile conjugated BAs that come from the conjugation of glycine or taurine at 24-carboxyl hydrogen, specific precursor/fragment ion transitions were used for the detection. Finally, SRM-based UPLC-MS/MS method was developed for the pseudotargeted profiling of the BAs submetabolome with good linearity (r2 > 0.995) and high sensitivity (0.20–1.37 ng mL−1 for LLOQ). With this method, a total of 83 BAs, covering 45 unconjugated BAs and 38 conjugated BAs, were successfully determined in different biosamples from experimental colitis mice. The BAs metabolism homeostasis was disrupted by colitis, characterized by the decreased BAs levels in serum and excessive BAs accumuation in the gall bladder and colon. Overall, the present anion attachment MS-based approach is sufficiently sensitive and robust to comprehensively measure various BAs.