Fig. 1 ∣. Isotopomer depiction of ¹³C-glutamine incorporation into the TCA cycle.

Schematic example of uniformly ¹³C-labeled glutamine ([U]¹³C-Gln) metabolism in the TCA cycle. Labeled carbons are depicted in red; unlabeled carbons are white. Gln carbon incorporation into alpha-ketoglutarate (αKG) during the first turn of the TCA cycle results in fully labeled αKG (M+5). Successive turns contribute other carbon sources to the TCA cycle, resulting in partial labelling. The full spectrum of labeled positions is denoted in the box at the top right. Gln-labeled malate can leave the mitochondria, where it is converted into pyruvate. This can equilibrate with glucose-derived pyruvate and the resulting mixed label is depicted as red hash marks. The mixed labelling can exist in several forms. Mixed labelling within a pyruvate isotopolog (e.g., M+2) can exist as three possible isotopomers. Using the SRM protocol, each isotopomer is identified individually and the ion currents are summed to obtain the labelling data for the isotopolog. M+, mass plus.