Figure 2.

¹³C distribution in first turn of the Krebs cycle with [¹³C₃]- or [lsqb]¹³C₁-1]pyruvate. A, [¹³C₃]pyruvate enters the Krebs cycle either through pyruvate carboxylase (PC, blue circles) or pyruvate dehydrogenase (PDH, red circles). Colored circles indicate ¹³C label for selected metabolites. PDH-derived and PC-derived metabolites are represented in the inner and outer circle, respectively. PC notably produces M + 3 citrate, malate, and fumarate during the first turn of the Krebs cycle, whereas PDH notably produces M + 2 citrate. [¹³C₂-2,3]Glu and [¹³C₂-3,4]Glu isotopomers are associated with PC and PDH activity, respectively. B, labels from [¹³C-1]pyruvate (blue circle, derived from [¹³C₂-3,4]glucose) enter the Krebs cycle exclusively through PC as the labeled carbon is lost in the PDH reaction. Other isotopomers are possible but not shown. IDH, isocitrate dehydrogenase; GDH, glutamate dehydrogenase; GS, glutamine synthetase; GLS, glutaminase; ME, malic enzyme; GOT, glutamic-oxaloacetic transaminase.