Bioanalytical method for the simultaneous determination of D- and L-serine in human plasma by LC/MS/MS.

D-Serine is an endogenous modulator of N-methyl-D-aspartate (NMDA) receptors. Plasma concentrations of D-serine and the ratio of D-serine to total serine may be used as clinically-translatable biomarkers in NMDA receptor-related disease. We developed a highly sensitive and specific method using high performance liquid chromatography tandem mass spectrometry (LC/MS/MS) for the simultaneous determination of the D- and L-isomers of serine in human plasma. Since D- and L-serine are endogenous components, phosphate buffered saline was used as the surrogate matrix. D- and L-serine in human plasma and PBS were treated by cationic exchange solid phase extraction. D-Serine (m/z 106.1 > 60.0), L-serine (m/z 106.1 > 60.1) and DL-serine-d3 (m/z 109.1 > 63.0) were detected using a multiple reaction monitoring. The enantiomer separation of D- and L-serine was successfully achieved without any derivatization step using tandemly-arranged and ice-cold CROWNPAK CR-I(+) columns with an isocratic mobile phase comprised of 0.3% trifluoroacetic acid in 10% acetonitrile. The standard curves were linear throughout the calibration range with 0.01-10 μg/mL (D-serine) and 0.1-100 μg/mL (L-serine), respectively. Intra-day and inter-day precision and accuracy of the quality control samples were within relative standard deviations of less than 15%. The endogenous concentrations of D- and L-serine in human plasma were 0.124-0.199 and 7.97-13.1 μg/mL, respectively.