Enantioseparation of N-acetyl-glutamine enantiomers by LC-MS/MS and its application to a plasma protein binding study

A novel chiral method was developed and validated to determine N-acetyl-glutamine (NAG) enantiomers by liquid chromatography-tandem mass spectrometry (LC-MS/MS). Enantioseparation was achieved on a Chiralpak QD-AX column (150 x 4.6 mm i.d., 5 mu m) using methanol-water (50 mm ammonium formate, pH 4.3; 70:30, v/v) at a flow rate of 500 mu L/min. The detection was operated with an electrospray ionization source interface in positive mode. The ion transition for NAG enantiomers was m/z 189.0 -> 130.0. The retention time of N-acetyl-l-glutamine and N-acetyl-d-glutamine were 15.2 and 17.0 min, respectively. Calibration curves were linear over the range of 0.02-20 mu g/mL with r > 0.99. The deviation of accuracy and the coefficient of variation of within-run and between-run precision were within 10% for both enantiomers, except for the lower limit of quantification (20 ng/mL), where they deviated 88% and no obvious matrix effect was observed. This method was successfully applied to investigate the plasma protein binding of NAG enantiomers in rats. The results showed that the plasma protein binding of NAG enantiomers was stereoselective. The assay method also exhibited good application prospects for the clinical monitoring of free drugs in plasma.