Validation of a fast and sensitive UPLC-MS/MS quantitative method for N-acyl taurine analysis in biological samples

The recent discovery of N-acyl taurines (NATs) as a class of endogenous bioactive lipids and the perspective of their possible pharmacological applications stimulated the development of mass spectrometry-based methods for their quantitative measurements in biological tissues and fluids. We report here for the first time a procedure validated both in liver surrogate matrix and neat solvent (MeOH) based on UPLC-ESI-QqQ analysis for the identification and quantification of NATs in biological tissue extracts. The LC-MS method was based on five representative lipid analogues, including saturated, monounsaturated and polyunsaturated species, namely N-palmitoyl taurine (C16:0 NAT), N-oleoyl taurine (C18:1 NAT), N-arachidonoyl taurine (C20:4 NAT), N-docosanoyl taurine (C22:0 NAT) and N-nervonoyl taurine (C24:1 NAT), and evaluated for specificity, linearity, matrix effect, recovery, repeatability and intermediate precision and accuracy. The method validated in MeOH by internal standard approach (d₄-C20:4 NAT) showed excellent linearity in the range 1-300 ng/ml with R always ≥ 0.9996 for all NATs; intra-day and inter-day precision and accuracy were always within the acceptable range. Specificity was assessed on NAT standards in MeOH, applying the confirmation ratio of two diagnostic MRM ion transitions for product ions at m/z 80 and m/z 107 to true samples in the adopted BEH C18 UPLC conditions. Limit of detection (LOD) and limit of quantification (LOQ) were 0.3-0.4 and 1 ng/ml, respectively, for all compounds. The method was successfully applied to assess the levels of NATs in the mouse liver and, for the first time, in varying sections of the intestine (duodenum, jejunum, ileum and colon). NAT levels increased from duodenum to colon, evidencing a remarkable prevalence in the large intestine of C22:0 NAT, typically occurring mainly in the central nervous system. These findings prompt further studies to disclose the biological function of the various members of this class in different peripheral tissues.