Glucose, labeled with 14c in different carbon atoms, is added to a crude extract of a tissue rich in enzymes of the pentose phosphate pathway. the most rapid production of 14co2 will occur when the glucose is labeled in:
a. c-1
b. c-3
c. c-4
d. c-5
e. c-6

A. C-1

Explanation:

During the pentose phosphate pathway, glucose 6-phosphate is first converted into 6-phosphoglucono-lactone in the reaction catalyzed by glucose 6-phosphate dehydrogenase. The 6-phosphoglucono-lactone is then converted into 6-phospho-gluconate which in turn undergoes decarboxylation to form D-ribulose 5-phosphate and the C-1 of original glucose 6-phosphate is released in form of CO2.  

Therefore, to observe the most rapid production of labeled carbon atom in form of CO2, the C-1 of glucose should be labeled.