Study of carbon dioxide fixation in the synthesis of citrulline.

The ornithine + citrulline -+ arginine cycle of urea synthesis originally proposed by Krebs and Henseleit (1) has had increasing experimental support (2-10). The position of citrulline in the cycle, however, has been questioned (11). With the advent of the successful separation of the two over-all enzymatic steps, ornithine -+ citrulline and citrulline -+ arginine (lo), from the intact cellular system the position of citrulline appeared to be well established. However, it seemed desirable to determine the intermediary role of citrulline by the use of carbon dioxide containing C14. In the present paper it is demonstrated that the incorporation of Cl4 into the carbonyl group of citrulline and urea is of such a magnitude that citrulline must be considered as an obligatory intermediate in the urea synthesis cycle.