FIGURE 5.

¹³C NMR spectra of plasma glucose reflect hepatic gluconeogenesis and PPP activity. A subject ingested [U-¹³C₃]glycerol under differing nutritional states, and blood was drawn at multiple times. Glucose was derivatized for ¹³C NMR acquisition, and the spectra are from blood drawn at 180 min after [U-¹³C₃]glycerol ingestion. Gluconeogenesis directly from [U-¹³C₃]glycerol produced [1,2,3-¹³C₃]- and [4,5,6-¹³C₃]glucose. [U-¹³C₃]Glycerol metabolism through the TCA cycle prior to gluconeogenesis produced double-labeled glucose ([1,2-¹³C₂], [2,3-¹³C₂], [5,6-¹³C₂], and [4,5-¹³C₂]). Hepatic PPP activity produced additional [1,2-¹³C₂]glucose, which was greatest under a fast. The ratio difference between D12/D23 versus D56/D45 (i.e. [1,2-¹³C₂]/[2,3-¹³C₂] versus [5,6-¹³C₂]/[4,5-¹³C₂] in glucose) reflects hepatic PPP activity, which was sensitive to nutritional states. D12, doublet from coupling of C1 with C2; D23, doublet from coupling of C2 with C3; Q, doublet of doublets, or quartet, arising from coupling of C2 with both C1 and C3 or from coupling of C5 with both C4 and C6; D45, doublet from coupling of C4 with C5; D56, doublet from coupling of C5 with C6; S, singlet. Open circles, ¹²C; black circles, ¹³C; green circles, ¹³C after metabolism through the TCA cycle; red circles, ¹³C after metabolism through the PPP.