Figure 4

Major metabolic pathways are responsible for changes in the natural ¹³C and ¹⁵N abundance in cancerous cultured cells.

Glutamine (Gln) is a major N and C source from which N is removed via hydrolysis (glutaminase ❶) and the urea cycle (❷). These reactions are both fractionating against ¹⁵N thereby yielding ¹⁵N-depleted urea and arginine (Arg). Therefore, build-up and recycling rather than excretion of Arg (dotted arrow) tend to deplete cancer cells of ¹⁵N. Cells are also ¹³C-enriched due to the fixation of bicarbonate by carbamoyl-phosphate (CP) synthesis (to feed the urea cycle ❸) and the anaplerotic pathway (❹), as well as a lower ¹³C content in non-structural lipids (dotted arrow). Lipids that are ¹³C-depleted come from the natural ¹³C-depletion in acetyl-CoA (Ac-CoA) (inherited from naturally ¹³C-depleted C-atom positions in glucose) and the isotope effect of pyruvate dehydrogenase^45,46 (❺). Amine acceptors are denoted as ‘A’ and oxaloacetate (OAA) converted to aspartate (Asp) is provided as an example. 2OG, 2-oxoglutarate; Pyr, pyruvate; Lact, lactate. The potential contribution of respiration (CO₂ loss) and lactate excretion to the natural ¹³C-abundance is described in Supplementary Table S4.