GC-MS measurement of biological NG-hydroxy-L-arginine, a stepmotherly investigated endogenous nitric oxide synthase substrate and arginase inhibitor

Abstract

L-Arginine is converted by nitric oxide synthase (NOS) to L-citrulline and nitric oxide (NO). N^G-Hydroxy-L-arginine (NOHA) is the isolable intermediate of this reaction. NOHA has been identified in biological samples by gas chromatography-mass spectrometry (GC-MS) and quantified by high-performance liquid chromatography (HPLC). Reportedly, NOHA concentrations in human plasma and serum range over four orders of magnitude (e.g., 2 nM-34 µM). The natural occurrence of NOHA in urine has not been reported thus far. Here, we report a validated stable-isotope dilution GC-MS method for the quantitative determination of NOHA in 10-µL aliquots of human serum and urine samples. The method is based on a two-step derivatization of NOHA to the methyl ester pentafluoropropionyl (PFP) derivatives using newly synthesized trideuteromethyl ester NOHA (d₃Me-NOHA) as the internal standard and GC-MS quantification. NOHA was found to form a methyl ester-N^G,N^δ,N^α-pentafluoropropionyl derivative, i.e., Me-(PFP)₃ (M, 642) with the N^G-hydroxy group remaining non-derivatized. Selected-ion monitoring of mass-to-charge (m/z) ratio of 458 for endogenous NOHA and m/z 461 for d₃Me-NOHA in the negative-ion chemical ionization mode revealed NOHA concentrations of the order of 0.2 µM in human serum and 3 µM in urine samples. Accuracy (recovery, %) was 91.6 ± 1.6% in serum and 39.9 ± 4.5% in urine. Inorganic nitrate was found to decrease NOHA recovery from urine presumably through the reaction of the OH group of NOHA with nitric acid. Imprecision (RSD, %) ranged between 1.4 and 14.8% in serum, and between 5.3 and 18.4% in urine in the investigated concentration range (0-15 µM NOHA). Ten healthy kidney donors excreted in the urine (mean ± SEM) 13.9 ± 1.81 µmol NOHA per day before and 10.9 ± 1.4 µmol NOHA per day after kidney donation (P = 0.24). Similar results were observed for dimethylamine (DMA), the major urinary metabolite of asymmetric dimethylarginine (ADMA). Changes in NOHA and DMA correlated positively (r = 0.718, P = 0.019). This is the first report on the occurrence and measurement of NOHA in human urine and on the effect of human unilateral nephrectomy on urinary NOHA and DMA. Healthy kidney donation may be useful as a model for kidney disease.

Keywords: Arginase; Asymmetric dimethylarginine; Dimethylamine; L-Arginine; Mass spectrometry; N G-Hydroxy-L-arginine; Nitrate; Nitric oxide; Nitric oxide synthase.