Fig. 5.

¹³C-Metabolic labeling of lactate and N-lac-Phe occurs at a similar speed in HEK 293 cells. HEK 293 cells were grown to confluence in six-well plates, at which point the medium was replaced with medium containing ¹³C₆-glucose. At several time points, the amount of lactate and N-lac-Phe containing only ¹²C (U-¹²C; unlabeled) or one to four ¹³C atoms per molecule (¹³C_1–4) was determined in lysate by accurate mass LC/MS. Fully ¹³C-labeled lactate contains three ¹³C-atoms, which is reflected in the ¹³C₃-labeling of N-lac-Phe. The minor presence of ¹³C₁, ¹³C₂, and ¹³C₄ isotopologs can be explained by the natural occurrence of the ¹³C isotope (∼1% of all carbon atoms). Levels are expressed as absolute (arbitrary units) and relative values (percentage of isotope total). Data are presented as mean (n = 3) and SD (only for absolute values). Isotope data of corresponding culture medium samples are presented in Fig. S6.