Figure 2. Glu isotopomers from Krebs cycling.

Isotopomers created via Krebs cycle activity by: (A) ¹³C₆ glucose. Black dots are ¹²C. ¹³C₆ glucose (red dots) produces ¹³C₃ pyruvate which can enter the Krebs cycle either via PDH (2 ¹³C) or PCB (3 ¹³C, shown as green dots), which give rise to different isotopomers of Krebs cycle intermediates and anabolic products such as uracil and its precursor Asp, distinguishable by NMR as ¹³C_1,2 + ¹³C_3,4 via PDH in the forward direction, and ¹³C_1,2,3 Asp via PCB (B) ¹³C₅,¹⁵N₂ Gln plus unlabeled glucose. Red dots are ¹³C atoms from Gln, blue dots are the two nitrogen atoms. Open circles are ¹²C. The anaplerotic input of fully labeled Gln produces the fully labeled Krebs cycle intermediates. Fully labeled OAA from Gln condenses with glucose-derived AcCoA to produce quadruple labeled citrate, which becomes via the Krebs cycle triply labeled αKG, and doubly labeled succinate. AST will transaminate OAA with Glu, transferring the amino nitrogen to Asp. The isotopomers produced evolve on further cycles, and differ with other inputs to the cycle. Isotopomer analysis is needed to sort out the resulting complex patterns.