Stable isotope dilution assay for liquid chromatography-tandem mass spectrometric determination of L-homoarginine in human plasma

Nitric oxide (NO), the endogenous modulator of vascular tone and structure, originates from oxidation of L-arginine catalysed by NO synthase (NOS). The L-arginine derivative L-homoarginine serves as an alternative NOS substrate releasing NO, competing with L-arginine for NOS, arginase, and arginine transport. In the present article we report a liquid chromatography-tandem mass spectrometric (LC-tandem MS) method for the determination of L-homoarginine in human plasma by stable-isotope dilution. L[(13)C(6)]-Homoarginine was used as internal standard. This method provides high sample throughput of 25-mu l aliquots of plasma with an analysis time of 4 min using LC-tandem MS electrospray ionisation in the positive mode (ESI+). Specific transitions for L-homoarginine and L-[(13)C(6)]-homoarginine were m/z 245 -> m/z 211 and m/z 251 -> m/z 217, respectively. The mean intra- and interassay CVs were 7.4 +/- 4.5% (+/- SD) for 0.1-50 mu mol/L and 7.5 +/- 2.0% for 2 and 5 mu mol/L, respectively. Applying this method, a mean plasma concentration of L-homoarginine of 2.5 +/- 1.0 mu mol/L was determined in 136 healthy humans. (C) 2011 Elsevier B.V. All rights reserved.