Arginosuccinate

The addition of the second ammonia to the backbone involves the joining of the carbamoyl group of citrulline to the x-amino group of aspartate, leading to a complex compound, arginosuccinate. ATP is hydrolyzed in this step. 

The next step is cleavage of arginosuccinate to arginine and fumarate by the enzyme arginosuccinate lyase. Lyases cleave bonds with the creation of a double bond in one of the products. In this case, the double bond is the carbon-carbon double bond of fumarate. 

Figure 20.9 The urea cycle. The bicarbonate is marked by an asterisk to facilitate its tracing through the pathway a, b, c, d, and e indicate urea cycle lesions, hyperammonemia I and II, citrullinemia, arginosuccinic aciduria, and argininemia, respectively.

A number of inherited disorders of urea cycle metabolism are known. Hy per-ammonemia I and II are associated with CPS I and ornithine transcarbamylase deficiencies, respectively. Citrullinemia, arginosuccinic aciduria, and argininemia are associated with low levels of arginosuccinic acid synthetase, arginosuccinase, and arginase, respectively. All such disorders are associated with mental retardation, convulsions, and failure to thrive if not treated. Treatment involves the feeding of low-protein diets or, experimentally, the administration of a-keto analogs of essential amino acids instead of protein. 

Citrullinemia Unclear Urea cycle Arginosuccinate synthetase... 

Arginosuccinic Argininosuccinate Episodic symptoms similar to classic... 

Citrullinema Arginosuccinate lyase Lethergy, siezures, reduced muscle tension... 

A significant increase in the measured COj concentration was observed after the addition of 10 mmol/1 of a-keto-isovaleric acid (148% increase from baseline), a-keto-isocaproic acid (120%), phenylacetic acid (100%), DL-a-keto-P-methyl-A-valeric acid (100%), homogentisinic acid (50%), P-phenylpyruvic acid (45%), hydroxyphenyl acetic acid (32%), propionic acid (25%), ace-toacetic acid (25%), hydroxyphenyl pyruvate acid (23%), and 20 mmol/1 of hydroxyphenyl lactid acid (65%) or salicylic acid (76%). A significant decrease was observed after the addition of 20 mmol/1 of ascorbic acid (33%), DL- -hydroxybutyric acid (25%) or imidazole lactic acid (25%). No effect after the addition of 10 mmol/1 of imidazole-4-acetic acid, methylmalonic acid, 5-hy-droxyindole acetic acid or 20 mmol/1 arginosuccinic acid, Z.-(+)-hydroxybu-tyric acid (E525). 

B2. Baumgartner, R., Scheidegger, S., Stalder, G., and Hettinger, A., Argininbern-stein.saure-Krankheit des neugeborenen mit latelem Verlauf. (Neonatal death due to arginosuccinic aciduria.) Helv. Paediat. Acta 23, 77-106 (1968). 

The term Krebs bicycle refers to two interlocking cyclic reaction pathways. The aspartate-arginosuccinate shunt of the citric acid cycle is responsible for regenerating the aspartate needed for the urea cycle from fumarate. The molecule that the two cycles have in common is arginosuccinate. 

Nussler, A. K., Billiar, T. R., Liu, Z. Z., and Morris, S. M. (1994). Coinduction of nitric oxide synthase and arginosuccinate synthetase in a murine macrophage cell line. /. Biol. Chem. 269, 1257-1261. 

Five oc-amino acids are involved directly in the urea cycle (ornithine, citrul-line, aspartate, arginosuccinate, and arginine). Of those, only aspartate and arginine are also found in proteins. 

Aspartate and arginosuccinate are the amino acids that link the two pathways. Aspartate is made by transamination of OAA. The aspartate then combines with citrulline to form arginosuccinate, which then releases a fumarate to go back to the TCA cycle. 

Each round of the urea cycle costs 4 ATP, two to make carbamoyl-phosphate and effectively two (ATP AMP) to make arginosuccinate. 

The limits of detection were less then 200 fmol and the concentrations of nitrite and nitrate were typically 2 and 12 mM, respectively. A more recent report involved CE separations of arginine, citmllene, nitrate, and nitrite with LIE and conductivity detection Contrary to previous results, it was determined that nitrates are not always reliable indicators of NO synthase activity. Similarly, other metabolites associated with NO—arginine, citmllene, arginosuccinate, ornithine, and arginine phosphate—were quantified in Pleurobranchaea and Aplysia with limits of detection ranging from 5 nM to 17... 

To return to the behavior of sulfur amino acids on automatic ammo acid analyzers, as we have already said, cystathionine can coelute on an amino add analyzer with arginosuccinic acid (and also with alloisoleucine). Treatment of the urine with peroxide followed by two-dimensional paper or thin-Iayer chromatography will confirm that the compound is indeed cystathionine... 

The primary step in the urea cycle is the synthesis of carbamyl phosphate from ammonia and carbon dioxide (11.76). This first stage, and the later stage of synthesis of arginosuccinic acid from citrulline and aspartic acid, both require the transfer of energy from ATP hydrolysis. The pyrophosphate formed in the latter reaction is itself hydrolysed which, together with the former reaction. 

Urea cycle Ornithine carbamoyl transferase Arginosuccinate synthetase Arginosuccinase Arginase Carbamoyl phosphate synthase... 

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