FIGURE 1.

¹³C symmetry between the top and bottom half of glucose from liver supplied with [U-¹³C₃]glycerol. DHAP and GA3P condense during gluconeogenesis. Carbons 1–3 of glucose originate from DHAP, whereas carbons 4–6 originate from GA3P (A). The liver would produce equal amounts of [1,2,3-¹³C₃]glucose and [4,5,6-¹³C₃]glucose from [U-¹³C₃]glycerol if there is complete equilibrium at the level of TPI, no PPP, and no transaldolase activity (B). If a small portion of [U-¹³C₃]glycerol enters the CAC prior to gluconeogenesis, a set of GA3P isotopomers ([2,3-¹³C₂]-, [1,2-¹³C₂]-, [U-¹³C₃]GA3P) are produced, which are common intermediates for both top and bottom half carbons of glucose. Consequently, there would be equal in the ratios of [1,2-¹³C₂]/[2,3-¹³C₃] and [5,6-¹³C₂]/[4,5-¹³C₂] if the PPP is not active (C). Transaldolase activity or disequilibrium at the level of TPI does not influence the relative amount of [1,2-¹³C₂]- and [2,3-¹³C₂]glucose (or the relative amount of [5,6-¹³C₂]- and [4,5-¹³C₂]glucose) because neither pathway disrupts the information encoded by the ¹³C distribution within the three-carbon unit. OAA, oxaloacetate; open circle, ¹²C; filled circle, ¹³C.