text: Stable isotope labeling involves the use of non-radioactive isotopes that can act as a tracers used to model several chemical and biochemical systems. The chosen isotope can act as a label on that compound that can be identified through nuclear magnetic resonance  (NMR) and mass spectrometry (MS). Some of the most common stable isotopes are H, C, and N, which can further be produced into NMR solvents, amino acids, nucleic acids, lipids, common metabolites and cell growth media. The compounds produced using stable isotopes are either specified by the percentage of labeled isotopes (i.e. 30% uniformly labeled C glucose contains a mixture that is 30% labeled with  carbon isotope and 70% naturally labeled carbon) or by the specifically labeled carbon positions on the compound (i.e. 1-C glucose which is labeled at the first carbon position of glucose).
  A network of reactions adopted from the glycolysis pathway and the pentose phosphate pathway is shown in which the labeled carbon isotope rearranges to different carbon positions throughout the network of reactions. The network starts with fructose 6-phosphate (F6P), which has 6 carbon atoms with a label C at carbon position 1 and 2. 1,2-C F6P becomes two glyceraldehyde 3-phosphate (G3P), one 2,3-C T3P and one unlabeled T3P. The 2,3-C T3P can now be reacted with sedoheptulose 7-phosphate (S7P) to form an unlabeled erythrose 4-phosphate(E4P) and a 5,6-C F6P. The unlabeled T3P will react with the S7P to synthesize unlabeled products. The figure demonstrates the use of stable isotope labeling to discover the carbon atom rearrangement through reactions using position specific labeled compounds.