Direct 13C observation of ALT activity using hyperpolarized sodium pyruvate. (a) A stacked plot of individual spectra following conversion of hyperpolarized 13C2-pyruvate into 13Cα-L-alanine. The resonances corresponding to these nuclei are indicated below the horizontal axis. (b) A plot showing observed intensities extracted from the spectra shown in panel (a). (c) A stacked plot of ALT-catalyzed conversion of hyperpolarized 13C3D3-pyruvate into 13Cβ-L-alanine. The resonances corresponding to these nuclei are indicated below the horizontal axis; note the resolution of peaks corresponding to the 13CD3 (tallest peak), 13CD2H, 13CDH2 and 13CH3 isotopologues, from right to left, respectively, for both nuclei. (d) A plot showing observed intensities extracted from the spectra shown in panel (c).