Abstract
A model for the estimation of pathway contributions has been extended to include the effect of the transaldolase reactions on the randomization of 14C of specifically labeled glucoses. With glucose-2-14C 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 14C 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 14C from glucose-2-14C 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-1-14C and -6-14C when triose-P derivatives are employed, but do alter the relative specific activities in carbon atoms 1 to 6 of hexose-6-P derivatives.
Highlights
B-phosphate and fructose 6-phosphate in a tissue presented with glucose-2-14C, and containing the pathways as assumed in the model, the relative rates of the reactions and the relative contributions of the pathways could be estimated
Estimation of Pathway Contributions with Use of Glucose-l-W and -6J4C-In the absence of a non-triose-P pathway contribution, the introduction of the transaldolase reactions, with glucose-l-% and -6-14C as substrates, will not alter the specific activities of dihydroxyacetone-P and glyceraldehyde-P
It was recognized that the reactions would not change l4C ratios of carbon atoms I, 2, and 3 of glucose-6-P and fructose-6-P which were the determinants employed in the estimation of the pentose cycle pathway contribution and extent of recycling (1, 10)
Summary
A model for the estimation of pathway contributions has been extended to include the effect of the transaldolase reactions on the randomization of 14C of labeled glucoses. With glucose-Z- 14C 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 14C 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 14C from glucose-2-14C into carbon atoms [4,5], and 6 of hexose-6-P. B-phosphate and fructose 6-phosphate in a tissue presented with glucose-2-14C, and containing the pathways as assumed in the model, the relative rates of the reactions and the relative contributions of the pathways could be estimated. The first of these occurs in the pentose cycle (Reaction 3)
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