Citrulline, argininosuccinate from

Figure 24-6 Elution profile of 13N-containing metabolites extracted from liver 15 s after injection of 13NH3 into the portal vein of an anesthetized adult male rat. CIT, citrulline ARG SUC, argininosuccinate. From Cooper et al.89...

L-argininosuccinate from citrulline and L-aspartate in the arginine biosynthesis pathway potential Cdc28p substrate... 

Urea is synthesized in the liver in the urea cycle. The first step is formation of carbamoyl phosphate from ammonia, C02, and ATP. This is followed by a number of other steps, including formation of citrulline, argininosuccinate, and arginine, which is split to urea plus ornithine. The second nitrogen of urea is donated by aspartate in the formation of argininosuccinate. 

In a 2-step reaction, catalyzed by cytosolic argininosuccinate synthetase, citrulline is converted to argininosuccinate. The reaction involves the addition of AMP (from ATP) to the amido carbonyl of citrulline, forming an activated intermediate on the enzyme surface (AMP-citrulline), and the subsequent addition of aspartate to form argininosuccinate. 

Cedrangolo et al (C5) and De Lorenzo (Dl) also postulated an alternative pathway on the basis of their experimental results on rats. The animals were injected with a-methyl aspartate, a specific inhibitor for argininosuccinate synthetase. No effect on urea excretion was observed, but there was complete inhibition of urea synthesis from citrulline in liver homogenates prepared from injected animals, as well as increased susceptibility to ammonia intoxication. However, this inhibition was not confirmed by Crokaert and Baroen (C15, C16, C17), although they did confirm the lack of any effect on urea excretion. The experimental basis for this suggestion is therefore in doubt. 

T3. Tedesco, T. A., and Mellman, W. J., Argininosuccinate synthetase activities and citrulline metabolism in cells cultured from a citrullinaemic subject. Proc. Nat. Acad. Sci. U.8. 67, 829-834 (1967). 

The intestine contains the enzymes for the urea cycle, but the for argininosuccinate synthetase and argininosuccinate lyase are very low, suggesting that the primary role of the urea cycle enzymes in the gut is to produce citrulline from the carbons of glutamine (glutamine glutamate glutamate semialdehyde ornithine citrulline). The citrulline is released in the circulation for use by the liver. 

Adenylosuccinate formed by adenylosuccinate synthetase is cleaved by adenylosuccinate lyase to form AMP. The reaction steps are illustrated in Fig. 1. Included in the sequence is the additional reaction catalyzed by AMP deaminase. These three enzymes have been suggested to function in a cyclic process termed the purine nucleotide cycle 7,8). The two-step conversion of IMP to AMP is very similar to both the conversion of citrulline to arginine, which involves formation of argininosuccinate as an intermediate, and formation of 5-amino-imidazole 4-carboxamide ribonucleotide from 5-aminoimidazole 4-carboxylate ribonucleotide as part of IMP biosynthesis. Adenylosuccinate lyase is a dual function enzyme catalyzing the cleavage of both adenylosuccinate and 5-aminoimidazole 4-N-succinocarboxamide ribonucleotide. 

The six reactions listed in Table 5-IV are similar in that (1) protonated ammonia or a carrier of protonated ammonia is involved (2) all are reversible (3) no isotope exchange or reaction occurs in either direction unless all three reactants are present (4) all require ATP or GTP (5) 0 from phosphate is transferred to the amino acceptor. (The case of argininosuccinate synthesis is different in that pyrophosphate is formed, and from citrulline is found in adenylate rather than in pyrophosphate.)... 

The transfer of nitrogen from aspartate to citrulline occurs via a condensation to form an intermediate, argininosuccinate, ASA. The condensation requires the participation of ATP, which is split to AMP and inorganic pyrophosphate during the reaction. The condensation is See general references by S. Grisolia. 

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