Radioactive Carbon?

Question

If pyruvate molecules containing radioactive carbon are incubated with all the components of the citric acid cycle long enough for one turn of the cycle, carbon dioxide is produced. If only 1/3 of it is radioactive, what would explain this?

Answer 1

Based on what you have given in the question, this tells me that pyruvate is only directly responsible for the creation of 1/3rd of all of the CO2 produced in TCA. Other sources of CO2 are the non-radioactive carbons from citrate, oxaloacetate and other TCA intermediates.

Answer 2

That would mean that 2/3 of the carbon in the CO2 came from carbon already present in the other components of the cycle.