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-[13C6]-Homoarginine was used as internal standard. This method provides high sample throughput of 25-μl 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-[13C6]-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 μmol/L and 7.5 ± 2.0% for 2 and 5 μmol/L, respectively. Applying this method, a mean plasma concentration of l-homoarginine of 2.5 ± 1.0 μmol/L was determined in 136 healthy humans.