Figure 3. Tracing TCA metabolism using ¹³C glucose and glutamine.

In this example, labeling on citrate and other intermediates from fully labeled [U-¹³C₆]glucose changes depending on routes used for anaplerosis and AcCoA generation. Oxidation of glucose-derived pyruvate by PDH results in M+2 citrate. Carboxylation through PC results in M+3 or M+5 citrate. [U-¹³C₅]glutamine oxidation or reduction results in M+4 and M+5 citrate, respectively. Taken together, relative flux changes in well-connected nodes (e.g. TCA cycles) result in measureable differences in labeling. Open circles depict ¹²C carbon atoms, filled circles depict ¹³C carbon atoms.