Addition of which of the following exhaustively 14C labeled substrates would lead to evolution of 14CO2 from a cell-free suspension containing all the enzymes and substrates required for the synthesis of uridylic acid?

A. Aspartate
B. Carbamoyl phosphate
C. Glutamine
D. Glycine
E. N10-Formyltetrahydrofolate

Is it A?

The correct answer is A. In the first step of pyrimidine synthesis, carbamoyl phosphate condenses with aspartate to form carbamoyl aspartate, in a reaction catalyzed by aspartate transcarbamoylase. In subsequent steps, ring closure occurs with the loss of water, followed by oxidation to yield orotic acid. Addition of ribose-5-phosphate produces orotidylic acid, which is decarboxylated by orotidylate decarboxylase to yield uridylic acid. The carbon dioxide that is evolved is derived from the alpha carboxyl group of aspartate.
Carbamoyl phosphate (choice B) condenses with aspartate with the loss of inorganic phosphate to produce carbamoyl aspartate. The carbamoyl moiety of carbamoyl phosphate is retained.

Glutamine (choice C), glycine (choice D) and N10-formyltetrahydrofolate (choice E) are all used in purine synthesis. Glutamine also donates an amino group to UTP to form CTP, but this step occurs after the synthesis of uridylic acid is complete.