FIGURE 6.

Double-labeled trioses produced through the metabolism of [U-¹³C₃]glycerol in the TCA cycle. After phosphorylation, [U-¹³C₃]glycerol may be further metabolized to [U-¹³C₃]pyruvate entering the TCA cycle. a, the entry of [U-¹³C₃]pyruvate through pyruvate carboxylase produces [1,2,3-¹³C₃]oxaloacetate, which equilibrates with the symmetric fumarate pool, producing both [1,2,3-¹³C₃]- and [2,3,4-¹³C₃]oxaloacetate. These oxaloacetate isotopomers may exit the TCA cycle through PEP carboxykinase generating [U-¹³C₃]PEP and [2,3-¹³C₂]PEP. b, the entry of [U-¹³C₃]pyruvate through pyruvate dehydrogenase generates [U-¹³C₂]acetyl-CoA. The condensation of [U-¹³C₂]acetyl-CoA and oxaloacetate produces [4,5-¹³C₂]citrate and [4,5-¹³C₂]α-ketoglutarate. After decarboxylation and experiencing a symmetric fumarate pool, [1,2,-¹³C₂]- and [3,4-¹³C₂]oxaloacetate are produced. These isotopomers become [1,2-¹³C₂]PEP and [3-¹³C₁]PEP through PEP carboxykinase. Open circle, ¹²C; black circle, ¹³C; green circle, ¹³C after metabolism through the TCA cycle. kG, ketoglutarate; OAA, oxaloacetate; PC, pyruvate carboxylase; PDH, pyruvate dehydrogenase; PEPCK, phosphoenolpyruvate carboxykinase.