FIGURE 1.

Possible metabolic routes for unequal ¹³C enrichment of carbons 1–3 versus 4–6 of glucosyl units of glycogen in heart supplied with [U-¹³C₃]glycerol. A, during glucose production through standard gluconeogenic pathways, carbons 1–3 of glucose originate from DHAP whereas carbons 4–6 originate from GA3P. B, in hearts exposed to [U-¹³C₃]glycerol, one would anticipate that incomplete equilibration at the level of TPI would yield more [1,2,3-¹³C₃]glucosyl units than [4,5,6-¹³C₃]glucosyl units because [U-¹³C₃]glycerol is converted first to [U-¹³C₃]DHAP before [U-¹³C₃]GA3P is produced at the level of TPI. C, in comparison, rapid equilibration of [U-¹³C₃]GA3P with S7P at the level of transaldolase would yield only [4,5,6-¹³C₃]F6P and E4P. D, transaldolase exchange between carbons 4–6 of F6P and [U-¹³C₃]GA3P would also yield only [4,5,6-¹³C₃]F6P. OAA, oxaloacetate; E4P, erythrose 4-phosphate; CAC, citric acid cycle; open circles, ¹²C; filled circles, ¹³C. The x indicates that flow of ¹³C from the citric acid cycle into glycogen production is not considered in these examples. The asterisks in C and D indicate the carbons that are replaced by [U-¹³C₃]GA3P through transaldolase activity to form [4,5,6-¹³C₃]F6P.