Fig. 2.
Glycerol incorporation to triglycerides. (A) Glycerol in the liver is converted to glycerol 3-phosphate (G3P), a necessary intermediate for fatty acid esterification. [U–13C3]glycerol direct incorporation through this process produces triglycerides containing triple-labeled glycerol backbone (i.e., TG-[13C3]glycerol). (B) A fraction of glycerol enters glycolytic pathways followed by the TCA cycle where carbons from [U–13C3]glycerol are scrambled, which generates 13C-double labeled trioses that can be further converted to G3P for fatty acid esterification. [U–13C3]glycerol indirect contribution to triglycerides through the TCA cycle produces double-labeled glycerol backbone (i.e., TG-[13C2]glycerol). A doublet (D) at TG-glycerol C1 & C3 (62.2 ppm) is the signal from both TG-[13C3]glycerol and TG-[13C2]glycerol. In contrast, a doublet at TG-glycerol C2 (69.1 ppm) is the signal from TG-[13C2]glycerol while a triplet (T) is the signal from TG-[13C3]glycerol only. (C-D) According to 13C NMR of TG-glycerol, empagliflozin treatment reduced plasma triglycerides in a subject with low VAT (1.9 kg) after [U–13C3]glycerol administration while it increased the triglycerides in a subject with high VAT (5.4 kg). A singlet (S) at TG-glycerol C1&C3 reflects the concentration of triglycerides and all spectra were derived from blood drawn at 180 min after oral administration of [U–13C3]glycerol. Signals can be directly compared because they are scaled to an internal standard. GK, glycerol kinase; open circle = 12C; black circle = 13C; blue circle = 13C after metabolism through the TCA cycle. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)