FIGURE 3.

¹³C NMR spectrum of a heart tissue extract after perfusion with [U-¹³C₃]glycerol. Only natural-abundance singlets were detected in glutamate, glutamine, and taurine, with the exception that [4,5-¹³C₂]glutamate was observed at a very low level (0.022 ± 0.006%, n = 5). Resonance of glutamate C4 is expanded. The multiplets in lactate resonances are from [U-¹³C₃]lactate, which is in exchange with [U-¹³C₃]pyruvate in tissues. Because glutamate is in rapid exchange with α-ketoglutarate (α-kG), the presence of the small doublets due to ¹³C-¹³C spin-spin coupling between C4 and C5 (D45) indicates that a small portion of [U-¹³C₃]pyruvate entered the citric acid cycle through acetyl-CoA. D45, doublet arising from coupling of C4 with C5; S, singlet.