Measuring Complete Isotopomer Distribution of Aspartate Using Gas Chromatography/Tandem Mass Spectrometry

We have developed a simple and accurate method for determining the complete positional isotopomer distribution of aspartate carbon atoms by gas chromatography/tandem mass spectrometry for C-13-metabolic flux analysis. First, we screened tandem mass spectrometry (MS) spectra of the tert-butyldimethylsilyl (TBDMS) derivative of aspartate for daughter fragments with the necessary carbon atom fragmentations to fully resolve all 16 isotopomers of aspartate. Tandem MS scanning parameters were optimized for each daughter fragment, and the accuracy of tandem MS measurements were evaluated. We selected five accurate fragments that provided a redundant set of 47 labeling measurements to quantify the complete isotopomer distribution of aspartate by least-squares regression. The validity of the approach was demonstrated using six C-13-labeled aspartate standards and natural aspartate.