The enzymatic steps in urea synthesis.

lnvestigations on the synthesis of arginine from citrulline (Step II of the Krebs-Henseleit urea cycle) and citrulline from ornithine (Step I) by rat liver homogenates have been reported previously (l-3). The study of the latter reaction was made possible by the apparently critical magnesium ion concentration required for the function of the liver transimination (citrulline to arginine) system. An attempt to isolate these reactions further has led to the use of differential centrifugation methods. By this means a separation of the two reactions has now been effected. The system for the synthesis of citrulline was found to be associated 16th the insoluble residues of the rat liver cell, while the system involved in the synthesis of arginine required two fractions, a soluble protein component plus the insoluble residue. The use of these tissue fractions has led to more detailed information regarding the optimum requirements for Steps I and II. It has now been shown that the citrulline to arginine reaction involves the transfer of hydrogen through the cytochrome system. Furthermore, the step ornithine to citrulline is shown to require the presence of magnesium ions and high concentrations of either ATP (adenosine triphosphate) or AMP (adenylic acid), as well as ornithine, glutamic acid, ammonium, potassium, phosphate, and bicarbonate ions, carbon dioxide, and oxygen at the concentrations used previously with whole homogenate. It is the purpose of the present paper to describe in detail the experiments leading to the above findings.