Figure 1.

125-MHz cross-polarization magic-angle spinning ¹³C NMR spectra of intact HeLa cells (left column) and H460 lung-cancer cells (right column). The full spectra obtained from cells labeled for 48 hours with 3 mM 99%-¹³C₃-lactate (red, panels a and b) are compared to the corresponding spectra of unlabeled cells (black, panels a and b). The label-only difference spectra are in blue. The label-only difference spectrum shown in panel c resulted from subtracting the black and red spectra shown in panel A. Similarly, the label-only difference spectrum shown in panel d resulted from subtracting the black and red spectra shown in panel b. Label-only difference spectra were also obtained from cells labeled with 24 mM 50%-¹³C₆-glucose (panels e and f, the latter scaled down by a factor of 2.5). The glucose labeling resulted in secreted lactate with four times the ¹³C concentration of the exogenous lactate labeling of panels c and d (see Supplementary Fig. 1).