Question

Describe how the results of carbon-14 labeling experiments demonstrated that the citrate cycle is a cycle and not a linear pathway with regard to the appearance of radioactivity in the carbon dioxide product.

Answer 1

In experiments carried out in 1941 to investigate the citric acid cycle, oxaloacetate labeled with 14C in the carboxyl carbon atom furthest from the keto group was introduced to an active preparation of mitochondria

Analysis of the α-ketoglutarate formed showed that none of the radioactive label had been lost.

Decarboxylation of the α-ketoglutarate then yielded succinate devoid of radioactivity. All the label was in the released CO2.This demonstrates that the citrate cycle is a cyclic not a linear pathway.