Estimations of Pathway Contributions to Glucose Metabolism and the Transaldolase Reactions

A model for the estimation of pathway contributions has been extended to include the effect of the transaldolase reactions on the randomization of 14 C of specifically labeled glucoses. With glucose-2- 14 C as substrate, the contribution of the pentose cycle to glucose metabolism and the extent of isomerization of fructose 6-phosphate to glucose 6-phosphate in a tissue represented by the model can be estimated solely from the distribution of 14 C in the 6 carbon atoms of glucose 6-phosphate or a derivative. Data for adipose, adrenal, and thyroid tissue support the adequacy of this approach. In these tissues, the transaldolase exchange reaction, as well as the reaction of the pentose cycle catalyzed by transaldolase, appears to introduce 14 C from glucose-2- 14 C into carbon atoms 4, 5, and 6 of hexose-6-P. The maximum rate of the transaldolase exchange reaction can be estimated. The transaldolase reactions do not affect estimates of pentose cycle contributions with glucose-1- 14 C and -6- 14 C when triose-P derivatives are employed, but do alter the relative specific activities in carbon atoms 1 to 6 of hexose-6-P derivatives.