Simultaneous bioanalysis of l-arginine, l-citrulline, and dimethylarginines by LC-MS/MS

Purpose: l-Arginine (ARG) is converted to nitric oxide (NO) and l-citrulline (CIT) by endothelial nitric oxide synthase which is competitively inhibited by asymmetric dimethylarginine (ADMA). We have developed a liquid chromatography-mass spectrometric method for the simultaneous determination of endogenous ARG, labeled ARG ( ¹⁵N ₄-ARG), CIT, ADMA, and its inactive isomer, symmetric dimethylarginine (SDMA) in biological samples. Methods: Concentrations of unlabeled ARG, ¹⁵N ₄-ARG, CIT, ADMA, and SDMA in EA.hy926 human endothelial cell lysate, cell incubation media, rat plasma or rat urine were measured by hydrophilic-interaction liquid chromatography electrospray tandem mass spectrometry. ¹³C ₆-ARG, D ₄-CIT and D ₇-ADMA were used as internal standards for ARG and ¹⁵N ₄-ARG, CIT, and dimethylarginines, respectively. Results: The calibration curves of ARG, ¹⁵N ₄-ARG, CIT, ADMA, and SDMA were linear and independent of several sample matrices. Intra- and inter-day variabilities for the quantification of all the compounds were below 15% in quality control samples. Application of this method to determine the uptake as well as efflux of these compounds was illustrated through in vitro cell study by exposing human endothelial cells to ¹⁵N ₄-ARG, which allowed the observation of generation of ¹⁵N ₃-CIT and ¹⁵N ₃-ARG in the cell lyate. Use of these isotopes adds insights into the cellular handling of endogenous vs. exogenous ARG. Application of this method for rat plasma and rat urine assays was demonstrated after ARG oral supplementation in rats. Conclusion: An LC-MS/MS method was developed to quantify 6 ARG-related compounds simultaneously, utilizing 3 separate internal standards. This assay allows concurrent monitoring of uptake, efflux and metabolic processes when isotope-labeled ARG and CIT are measured, and can be applied for determination of these compounds in rat plasma and rat urine.