Question

Radioactive labeling experiments can yield estimates of how much glucose 6-phosphate is metabolized by the pentose phosphate pathway and how much is metabolized by the combined action of glycolysis and the TCA cycle. Suppose that you have 2 samples of different tissues as well as 2 radioactively labeled glucose samples, one with glucose labeled with 14C at C-1 and the other with glucose labeled with 14C at C-6. Design an experiment that would enable you to determine the relative activity of the aerobic metabolism of glucose compared with metabolism by the pentose phosphate pathway. What results would you expect?

Answer 1

Ans.

An aliquot of a tissue homogenate is incubated with glucose labeled with ¹⁴C at C-1, and another is incubated with glucose labeled with ¹⁴C at C-6. The radioactivity of the CO₂ produced by the two samples is then compared. The rationale of this experiment is that only C-1 is decarboxylated by the pentose phosphate pathway, whereas C-1 and C-6 are decarboxylated equally when glucose is metabolized by the glycolytic pathway, the pyruvate dehydrogenase complex, and the citric acid cycle. The reason for the equivalence of C-1 and C-6 in the latter set of reactions is that glyceraldehyde 3-phosphate and dihydroxyacetone phosphate are rapidly interconverted by triose phosphate isomerase.