UPLC–MS/MS measurement of S-nitrosoglutathione (GSNO) in human plasma solves the S-nitrosothiol concentration enigma

We developed and validated a fast UPLC–MS/MS method with positive electrospray ionization (ESI+) for the quantitative determination of S-nitrosoglutathione (GSNO) in human plasma. We used a published protocol for the inactivation of plasma γ-glutamyltransferase (γGT) activity by using the γGT transition inhibitor serine/borate and the chelator EDTA for the stabilization of GSNO, and N-ethylmaleimide (NEM) to block SH groups and to avoid S-transnitrosylation reactions which may diminish GSNO concentration. S-[15N]Nitrosoglutathione (GS15NO) served as internal standard. Fresh blood was treated with NEM/serine/borate/EDTA, plasma spiked with GS15NO (50 nM) was ultrafiltered (cut-off 10 kDa) and 10 μL aliquots of the ultrafiltrate were analyzed by UPLC–MS/MS. Five HILIC columns and an Acquity UPLC BH amide column were tested. The mobile phase was acetonitrile–water (70:30, v/v), contained 20 mM ammonium formate, had a pH value of 7, and was pumped isocratically (0.5 mL/min). The Nucleoshell column allowed better LC performance and higher MS sensitivity. The retention time of GSNO was about 1.1 min. Quantification was performed by selected-reaction monitoring the mass transition m/z 337 ([M+H]+) → m/z 307 ([M+H14NO]+) for GSNO (i.e., GS14NO) and m/z 338 ([M+H]+) → m/z 307 ([M+H15NO]+) for GS15NO. NEM/serine/borate/EDTA was found to stabilize GSNO in human plasma. The method was validated in human plasma (range, 0–300 nM) using 50 nM GS15NO. Accuracy and precision were in generally acceptable ranges. A considerable matrix effect was observed, which was however outweighed by the internal standard GS15NO. In freshly prepared plasma from heparinized blood donated by 10 healthy subjects, no endogenous GSNO was determined above 2.8 nM, the limit of quantitation (LOQ) of the method. This study challenges previously reported GSNO plasma concentrations being far above the present method LOQ value and predicts that the concentration of low-molecular-mass and high-molecular-mass S-nitrosothiols are in the upper pM- and lower nM-range, respectively.