Biosynthesis of urea; arginine synthesis from citrulline in liver homogenates.

A preceding paper described the characteristics of the isolated enzyme system, prepared from ox liver acetone powder, which catalyzes the conversion of citrulline and aspartic acid to arginine and malic acid. It was shown that the fundamental requirements for arginine synthesis from citrulline are aspartic acid, Mg++, and adenosine triphosphate (ATP), the latter as a reactant in substrate concentrations (1). At the optimum enzymatic conditions established, a large proportion of the arginine-synthesizing activity of the tissue was found in the acetone powder extract. Aspartic acid was shown to be the specific --NH2 donor. Glutamic acid was unreactive but the combination of glutamic and oxalacetic acids could replace aspartic acid in proportion to the glutamic-aspartic transaminase activity of the enzyme preparation. The relative activity of these two amino acids is reversed in liver homogenates. Cohen and Hayano (2), corroborated by Krebs and Eggleston (3), have found that glutamic acid is about 4 times as effective as aspartic acid. Since these observations raise the question as to whether the enzyme system studied by us is the same as the one concerned with arginine synthesis in homogenates and slices, studies of liver homogenates bearing on this point were carried out. The experimental observations presented here show that the same enzymatic system is involved in all cases. It has been possible to explain conflicting observations by a uniform mechanism and to indicate the physiological pathway of amino nitrogen from amino acids to urea. In order to relate the behavior of liver homogenates in oxygen to the behavior and requirements of the isolated system, certain general properties of homogenates, affecting arginine synthesis, should be mentioned. The adenosinetriphosphatase (ATPase) activity is much greater than that of acetone powder extracts; hence rapid generation of mph is necessary. In addition to the enzyme system catalyzing the citrulline to arginine re-