First detection and quantification of Nδ-monomethylarginine, a structural isomer of NG-monomethylarginine, in humans using MS3

The l-arginine metabolites methylated at the guanidino moiety, such as NG-monomethyl-l-arginine (LNMMA), asymmetric NG,NG-dimethyl-l-arginine (ADMA), and symmetric NG,NG'-dimethyl-l-arginine (SDMA), are long known to be present in human plasma. Far less is known about the structural isomer of LNMMA, Nδ-monomethyl-l-arginine (δ-MMA). In prior work, it has been detected in yeast proteins, but it has not been investigated in mammalian plasma or cells. In this work, we present a method for the simultaneous and unambiguous quantification of LNMMA and δ-MMA in human plasma that is capable of detecting δ-MMA separately from LNMMA. The method comprises a simple protein precipitation sample preparation, hydrophilic interaction liquid chromatography (HILIC) gradient elution on an unmodified silica column, and triple stage mass spectrometric detection. Stable isotope-labeled D6-SDMA was used as internal standard. The calibration ranges were 25-1000 nmol/L for LNMMA and 5-350 nmol/L for δ-MMA. The intra- and inter-batch precision determinations resulted in relative standard deviations of less than 12% for both compounds with accuracies of less than 6% deviation from the expected values. In a pilot study enrolling 10 healthy volunteers, mean concentrations of 48.0 ± 7.4 nmol/L for LNMMA and 27.4 ± 7.7 nmol/L for δ-MMA were found.