Figure 1. Maps of glutamine metabolism.

(A) Anaplerotic conversion of glutamate to α-ketoglutarate (αKG) is highlighted by green arrows (alternate enzymatic reactions for this step are contained within the brackets); cataplerotic reactions are shown by red arrows; TCA cycle is within the shaded area. Dashed lines indicate secretion of metabolites. Genes for key steps are shown: gls1, glutaminase-1; got, glutamate-oxaloacetate transaminase (aspartate aminotransferase); gpt, glutamate-pyruvate transaminase (alanine aminotransferase); glud, glutamate dehydrogenase. Other abbreviations: Pyr, pyruvate; OAA, oxaloacetate; AcCoA, acetyl-coenzyme-A. (B) Using ¹³C-labeling to quantify glutaminolysis. Metabolic trafficking from universally ¹³C-glutamine is indicated by brown arrows, with ¹³C atoms, including ¹³C lost as CO₂, indicated by brown circles, and recombination with unlabeled (open circles) acetyl-CoA in the TCA cycle. Routes to glutamate, aspartate and lactate are shown, as well as example mass profiles of cellular glutamate and aspartate. The x axis designations of “m+1”, “m+2”, etc., indicate metabolite mass greater than the unlabeled (no ¹³C) mass of “m+0”, and together indicate the distribution of ¹³C label. ¹³C-Glutamine is deamidated to (m+5) glutamate. This is not glutaminolysis, as no energy is produced. But glutamate can then be converted to α-ketoglutarate and circuit the TCA cycle. After one circuit, two carbons are lost as CO₂ and replaced by unlabeled acetyl-CoA. This results in m+3 glutamate. Further circuits of the TCA cycle result in exchange of more ¹³C carbon. Similarly for aspartate, the initial pass through the TCA cycle yields m+4 aspartate (one carbon lost as CO₂), and another circuit through the cycle yields m+2 aspartate (three carbons lost as CO₂). Glutaminolysis can be quantified in terms of CO₂ production by comparing the ¹³C content and quantities of metabolites after ¹³C-glutamine labeling with the amount of ¹³C-glutamine taken up by cells.