An improved mass spectrometry-based measurement of NO metabolites in biological fluids

Assessment of NO metabolism in vivo relies on the accurate measurement of its metabolites nitrite (NO2−), nitrate (NO3−), and nitrosothiols (RSNOs) in biological fluids. We report a sensitive method to simultaneously determine NO2− and NO3− in biological matrixes. Tetraoctylammonium was used to catalyze the complete conversion of NO2− and NO3− to stable pentafluorobenzyl (PFB) derivatives directly from aqueous acetone medium before gas chromatography and negative-ion chemical ionization mass spectrometry (GC/NICI/MS). This catalyst dramatically improved the yield of PFB derivatives for NO2− (4.5 times) and NO3− (55 times) compared to noncatalyzed derivatization methods. Analysis was performed using 15N-labeled internal standards by selected-ion monitoring at m/z 46 for fragment NO2− and m/z 47 for its isotope analogue, 15NO2−, and m/z 62 for NO3− and m/z 63 for 15NO3−. This method allowed specific detection of both PFB derivatives over a wide dynamic range with a limit of detection below 4.5 pg for NO2− and 2.5 pg for NO3−. After the specific conversion of RSNOs by HgCl2 to NO2−, this GC/NICI/MS analysis was used to measure RSNOs in plasma. A further comparison with the widely used tri-iodide chemiluminescence (I3−-CL) assay indicated that the GC/MS assay validated the lower physiological RSNO and nitrite levels reported using I3−-CL detection compared with values obtained using UV–photolysis methods. Plasma levels of RSNOs determined by GC/MS and I3−-CL were well correlated (r = 0.8). The improved GC/MS method was successfully used to determine the changes in plasma, urinary, and salivary NO2− and NO3− as well as plasma RSNOs in humans after either a low-NO3− or a high-NO3− meal.

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► The study of NO biology requires accurate assessment of its metabolites in vivo.
► We developed an improved GC/MS method for measurements of NO2−, NO3−, and RSNOs.
► The results are in close agreement with those obtained using the tri-iodide assay.
► The methods were tested on NO changes in humans after a low-NO3− or high-NO3− meal.
► A technical advance was achieved in a challenging area of NO metabolism.