Stable isotope dilution HILIC-MS/MS method for accurate quantification of glutamic acid, glutamine, pyroglutamic acid, GABA and theanine in mouse brain tissues.

In this study, we developed the stable isotope dilution hydrophilic interaction liquid chromatography with tandem mass spectrometry (HILIC-MS/MS) technique for the accurate, reasonable and simultaneous quantification of glutamic acid (Glu), glutamine (Gln), pyroglutamic acid (pGlu), γ-aminobutyric acid (GABA) and theanine in mouse brain tissues. The quantification of these analytes was accomplished using stable isotope internal standards and the HILIC separating mode to fully correct the intramolecular cyclization during the electrospray ionization. It was shown that linear calibrations were available with high coefficients of correlation (r(2)  > 0.999, range from 10 pmol/mL to 50 mol/mL). For application of the theanine intake, the determination of Glu, Gln, pGlu, GABA and theanine in the hippocampus and central cortex tissues was performed based on our developed method. In the region of the hippocampus, the concentration levels of Glu and pGlu were significantly reduced during reality-based theanine intake. Conversely, the concentration level of GABA increased. This result showed that transited theanine has an effect on the metabolic balance of Glu analogs in the hippocampus.