THE CONVERSION OF ORNITHINE TO CITRULLINE BY RAT LIVER HOMOGENATES

Urinary metabolites characteristic of urea-cycle amino acid deficiency

Experiments with 45-75-g male rats gave conclusive evidence that the simultaneous absence from the diet of arginine, ornithine, or citrulline caused an immediate and persistent elevation of orotic and citric acids in the urine. The experiments also demonstrated that a deficiency of no other individual amino acid increased urinary citrate and orotate. Elevated urinary excretion of orotic and citric acid occurred independently of the form of nonessential nitrogen. Replacement of arginine isonitrogenously with ornithine or citrulline prevented the rise in urinary orotic acid, but had different effects on growth, urinary citrate, and urinary urea. These differences were probably due to differential uptake of arginine, ornithine, and citrulline by tissues; In the reported experiments employing L-amino acids as sources of dietary nitrogen, a deficiency of any amino acid indispensable for growth and nitrogen balance or a deficiency of arginine, ornithine, or citrulline retarded growth, increased urinary urea, and decreased urinary ammonia. It is concluded that the severe loss of orotic acid during urea-cycle amino acid deficiency arises from a decreased capacity of the urea cycle to detoxify ammonia, thereby causing increased shunting of intramitochondrial carbamyl phosphate into pyrimidine synthesis. The similarities in metabolism during arginine deficiency and ammonia intoxication are discussed. The evidence shows that urinary orotic acid may be a valuable measure of arginine nutrition in mammals.

A STUDY ON THE "IN VITRO" SYNTHESIS OF CITRULLINE: PART II. EFFECT OF GLUTAMIC ACID AND DERIVATIVES IN THE PRESENCE OF FUMARIC ACID

The role of N-carbamyl, N-acetyl, and L-glutamic acids with and without fumaric acid on the "in vitro" synthesis of citrulline was studied by using a particulate fraction obtained from a rat liver homogenate and a partially purified citrulline-synthesizing enzyme system. In the presence of a particulate fraction of rat liver homogenate, N-carbamyl and N-acetyl-L-glutamic acids are unable to replace L-glutamic acid, which is essential for citrulline biosynthesis. However, in the presence of fumaric acid, they both give a better synthesis of citrulline than L-glutamic acid alone. It is postulated that the acyl derivatives serve only in the transport of "activated CO2" whereas fumaric acid enters the citric acid to furnish the essential ATP molecules. Glutamic acid would be able to perform both functions. However, in the presence of a system containing partially purified citrulline-synthesizing enzymes, L-glutamic acid is unable to replace N-carbamyl and N-acetyl-L-glutamic acids with or without fumaric acid. In such a system, L-glutamic acid cannot serve in the transport of "activated CO2". It is postulated that L-glutamic acid must be acetylated prior to its utilization in this respect.With the particulate fraction of rat liver homogenate, N-allyl aspartic acid inhibits totally the synthesis of citrulline both in the presence and absence of fumaric acid with or without glutamic or N-acetyl glutamic acids. It probably interferes with the transport of "activated CO2".

A STUDY ON THE "IN VITRO" SYNTHESIS OF CITRULLINE: PART I. EFFECT OF SOME N-ALKYL DERIVATIVES OF ASPARTIG ACID

The effects of various N-alkyl derivatives of aspartic acid on the synthesis of citrulline in the presence of a particulate fraction obtained from a rat liver homogenate were studied. Even though aspartic acid has no role in this synthesis, both N-methyl and N-isopropyl aspartic acids were found to increase the synthesis of citrulline by 60%, whereas N-cyclohexyl aspartic acid decreased it by 50%. N-Allyl aspartic acid has the strongest effect which is an almost complete inhibition at low concentration of 1.2 × 10−2 M.It seems that N-allyl aspartic acid inhibits directly or indirectly the first step in the synthesis of citrulline leading to the formation of carbamyl phosphate. At various concentrations, none of the intermediates in this synthesis, except magnesium ion, can reverse the inhibition. In order that N-allyl aspartic acid retain its inhibitory action, the ω-carboxyl group has to be free and the double bond in the allyl group must be intact. From these results, it is postulated that N-allyl aspartic acid acts as a chelating agent for magnesium.