Simultaneous determination of 18 D-amino acids in rat plasma by an ultrahigh-performance liquid chromatography-tandem mass spectrometry method: application to explore the potential relationship between Alzheimer's disease and D-amino acid level alterations

d-Amino acids are increasingly being recognized as important signaling molecules, and abnormal levels of d-amino acids have been implicated in the pathogenesis of Alzheimer’s disease. To evaluate the potential relationship between Alzheimer’s disease and d-amino acids, a simple, sensitive, and reliable UPLC-MS/MS method with pre-column derivatization was developed and validated for simultaneous determination of 18 d-amino acids in rat plasma. The analytes were extracted from plasma samples by a protein precipitation procedure, and then derivatized with (S)-N-(4-nitrophenoxycarbonyl) phenylalanine methoxyethyl ester [(S)-NIFE]. Chromatographic separation was achieved using an ACQUITY UPLC BEH C18 column (2.1 mm × 50 mm, 1.7 μm) with a mobile phase composed of acetonitrile containing 8 mM ammonium hydrogen carbonate at a flow rate of 0.6 mL min−1. The analytes were detected by electrospray ionization in positive ion multiple reaction monitoring modes. Under the optimum experimental conditions, all the linear regressions were acquired with r > 0.9932. The limits of quantitation of all derivatized d-amino acids were within 0.05–40.0 ng mL−1 in rat plasma. The intra- and inter-day precisions, expressed as percentage relative standard deviations (%RSD), were within the range of 12.3 and 10.1 %, respectively. The recoveries for all the analytes were observed over the range of 82.8–100.5 % with RSD values less than 12.5 %. Finally, the proposed method was successfully applied to simultaneous determination of the 18 d-amino acids in plasma from Alzheimer’s disease rats and age-matched normal controls. Results showed that the concentrations of d-serine, d-aspartate, d-alanine, d-leucine, and d-proline in Alzheimer’s disease rat plasma were significantly decreased compared with those in normal controls, while d-phenylalanine levels increased. It was revealed that some of these d-amino acids would be potential diagnostic biomarkers for Alzheimer’s disease.