a–c, Regions of 600 MHz two-dimensional heteronuclear multiple-bond correlation spectra showing cross-peaks between the amine nitrogen and the β carbon protons. Only those amino acids that incorporated a significant amount of [15N]amine show cross-peak signals. d–f, Regions of 600 MHz one-dimensional 1H spectra. Each proton peak is assigned as indicated. DSS, 2,2-dimethyl-2-silapentane-5-sulfonate. a, d, Mixture of reference standards of the indicated amino acids. b, e, K562 cell sample cultured in the medium containing (amine-15N)-glutamine. c, f, K562 cell sample cultured in the non-labelled standard medium. g, Percentage of newly synthesized 15N-labelled BCAAs within total intracellular pool at 72 h after post-labelling for each amino acid indicated. ‘Total BCAAs’ shows the percentage including all three BCAA species.