Figure 3.

Atom transition networks and positional fractional labeling for selected glucose and glutamine tracers. Fractional labeling is indicated by a colormap, where dark red indicates all atoms at that position are ¹³C and dark blue that all atoms are ¹²C. No natural labeling was assumed in the creation of these maps. Atom transitions are indicated for all positions where fractional ¹³C labeling exceeds 10%. (A) [2]Gluc effectively characterizes glycolytic and PPP fluxes because DHAP (and by extension GLP) is labeled in multiple positions by different combinations of fluxes, leading to greater measurement sensitivity. (B) [4]Gluc poorly identifies fluxes in glycolysis and the PPP because its sole labeled carbon is caught in a cycle and can only reach the 1st carbon of DHAP. (C and D) [3,4]Gln and [4]Gln are both able to label a majority of the carbon atoms in the TCA cycle; however, the two labeled atoms in the former lead to larger, clearer measurements and therefore more precises fluxes.