The catalytic rôle of carbamyl glutamate in citrulline biosynthesis.

CC was shown to be necessary for the reaction with a partially purified system (l), confirming previous data obtained with cruder preparations (2, 3).’ However, the mechanism of this reaction was not clear, since CG did not disappear during the biosynthesis of citrulline (1). Some insight into the mechanism of the reaction was obtained from kinetic data (l), indicating an “activation” of CG in proper incubation mixtures. This suggested at least a two-step reaction. It was then postulated that CG was being converted into a highly reactive intermediate, which subsequently would react with ornithine to form citrulline and regenerate CG. It was, therefore, indicated that CC served a catalytic r61e in Reaction 1. This paper presents evidence that Reaction 1 is composed of the two steps, Reactions 1, a and 1, b.