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

Purpose: L-Arginine (ARC) 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 ARC, labeled ARC (N-15(4)-ARG). CIT, ADMA, and its inactive isomer, symmetric dimethylarginine (SDMA) in biological samples. Methods: Concentrations of unlabeled ARC. N-15(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. C-13(6)-ARC, D-4-CIT and D-7-ADMA were used as internal standards for ARC and N-15(4)-ARC, CIT, and dimethylarginines, respectively. Results: The calibration curves of ARC, N-15(4)-ARC, 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 N-15(4)-ARG, which allowed the observation of generation of N-15(3)-CIT and N-15(3) -ARG in the cell lyate. Use of these isotopes adds insights into the cellular handling of endogenous vs. exogenous ARC. Application of this method for rat plasma and rat urine assays was demonstrated after ARC oral supplementation in rats. Conclusion: An LC-MS/MS method was developed to quantify 6 ARC-related compounds simultaneously, utilizing 3 separate internal standards. This assay allows concurrent monitoring of uptake, efflux and metabolic processes when isotope-labeled ARC and CIT are measured, and can be applied for determination of these compounds in rat plasma and rat urine. (C) 2011 Elsevier B.V. All rights reserved.