Carbon dioxide and ammonia fixation in the biosynthesis of citrulline.

In the preceding paper (1) an enzyme preparation was described which catalyzed the synthesis of citrulline from ornithine in the presence of adenosinetriphosphate (ATP), ammonia, magnesium and bicarbonate ions, and carbamylglutamate. Glutamic acid could not replace the latter compound. Balance studies have failed to provide satisfactory evidence for the r61e of carbamylglutamate in this system. Therefore we have attempted to obtain more certain information as to the source of the carbamyl group of citrulline by the use of isotopically labeled compounds. Experiments were carried out with C14-labeled carbon dioxide, carbamylglutamic acid labeled with Cl4 in the carbamyl group, and with N16-labeled ammonia. The experiments reported in this paper show clearly that carbamylglutamate does not participate in the synthesis of citrulline by the transfer of its carbamyl group to ornithine as previously postulated (2). It will be shown in the present paper that the carbamyl group of the citrulline synthesized can be accounted for in terms of the free ammonia and bicarbonate ions added to the system.