FIG. 1.

Schematic of ¹³C labeling patterns derived from 3-¹³C lactate. 3-¹³C lactate is metabolized to 3-¹³C pyruvate, which then enters the tricarboxylic acid (TCA) cycle via conversion to 2-¹³C acetyl CoA by pyruvate dehydrogenase (PDH) or via pyruvate carboxylase (PC) to 3-¹³C oxaloacetate. 2-¹³C acetyl CoA is metabolized to citrate, isocitrate, and then to 4-¹³C α-ketoglutarate where it can then spin out as 4-¹³C glutamate, which is then converted to 4-¹³C glutamine. Consequently 4-¹³C glutamine is produced on the first turn of the TCA cycle. If 4-¹³C α-ketoglutarate remains within the TCA cycle, it is metabolized to equal proportions of 2-¹³C malate and 3-¹³C malate because of “randomization” of label in the fumarate to malate step, because fumarate is a symmetrical molecule whereas malate is asymmetric. If malate continues within the TCA cycle, equal amounts of 2-¹³C glutamine and 3-¹³C glutamine are produced on the “second turn” of the TCA cycle. When 3-¹³C lactate is metabolized via PC to 3-¹³C oxaloacetate, 2-¹³C α-ketoglutarate is produced, which results in 2-¹³C glutamine produced on the “first turn” of the TCA cycle. For the labeling patterns of the second turn, we have assumed for schematic illustration that the incoming second acetyl-CoA molecule is not enriched with ¹³C. These patterns were consistent with our observations; see Results section and Supplementary Table 2. Colored filled circles indicate position of ¹³C atoms within molecules. Red indicates entry via PDH into the TCA cycle, first turn, and purple for second turn; yellow indicates entry via PC, first turn.