Citrulline and arginine

The concept of the ornithine cycle arose from the observation that ornithine, citrulline and arginine stimulated urea production in the presence of ammonia without themselves being consumed in the process. 

Particular interest attaches to these amino acids which are intermediary metabolites of the urea cycle, and which are normally present in the plasma, ornithine, citrulline, and arginine. Earlier reports do not mention the levels of these amino acids. Moser et al. (M13) found a normal plasma ornithine level in their two patients, but the citrulline was, as would be expected, several times the normal level, while the plasma arginine was at the lower limit of normal. In the cerebrospinal fluid also the citrulline level was greatly increased, while the ornithine and arginine levels were normal. Similar results were obtained also by Carton et al. (C3) in their neonate who died at 6 days old, and by Levin et al. (L5, Lll), also in a neonate who died in the first week of life. In the last two cases, however, the levels of citrulline were considerably higher than in the older patient of Moser et al. (M13) at 2.2 mg and 3.0 mg/100 ml, compared with a level of only 0.77 mg/100 ml for Moser s... 

There are small but significant alterations in the plasma levels of two of the amino acids involved in the urea cycle. Both citrulline and arginine levels are low, about one-third to one-half the normal levels (Lll) sometimes even lower (Table 6). These changes would be expected since these acids are in the metabolic pathway beyond the block. What appears surprising is that the level of ornithine, the amino acid whose further metabolism is blocked, which would be expected to rise, is actually usually within normal limits. There are two possible explanations for this. The biosynthesis of urea forms a cycle in which each intermediate is re-formed in the process. The block in the synthesis of citrulline from ornithine results in a decreased formation of arginine which in turn will... 

NO is formed enzymatically from arginine with the help of NO synthase, producing citrulline (Fig. 6.18). Citrulline and arginine are intermediates of the urea cycle, and arginine can be regenerated from citrulline by urea cycle enzymes. 

Figure 9.2. (A) Secondary structure proposed previously for the citrulline- and arginine-specific aptamers, based on co-variations of selected sequences, on the chemical footprinting pattern obtained in the presence of the cognate amino acid, as well as in damage selection experiments. The bases which were conserved among different isolates arc shown in upper case variant bases are in lower case. The three nucleotides critical for arginine specificity (13,29 and 31) arc indicated by circles (for citrulline) and boxes (for arginine). (B) Tertiary structure of the L-arginine aptamer complex resolved by NMR spectroscopy. Yellow L-arginine red the three mutations. (Illustration adapted from [9].)...

In their in vitro studies using liver slices, Krebs and Hense-leit observed that urea formation was stimulated by the addition of ornithine, citrulline, and arginine. Other amino acids had no effect. Explain these observations. 

The sequential labeling of first citrulline and then arginine are consistent with the idea that citrulline is a substrate for the synthesis of arginine. In cyanobacteria, citrulline appears to be formed by the condensation of carbamyl phosphate with ornithine (27). The inhibition of formation of citrulline (and arginine) by methionine sulfoximine, azaserine, and aminooxy acetate could be due to the reduced formations of ornithine from glutamate and carbamyl phosphate from glutamine. In... 

In trifunctional amino acids, the R radical is connected to an acid or basic radical. Aspartic acid and glutamic acid are acid amino acids, while lysine, histidine, ornithine, citrulline and arginine are basic amino acids. Other trifunctional amino acids have no marked acid or basic character. They have hydroxylated (serine, threonine and tyrosine), thiol or sulfide radicals (cysteine and methionine). 

This same full-scan and SRM approach used for ACs is also done for amino acids. Basic amino acids such as citrulline and arginine are characterized by loss of 102 Da and ammonia or other basic amino side chain. Selective reaction monitoring (SRM) is used rather than full scan and is based on NL of 119 for citrulline (102 -I-17, where 17 is ammonia) and NL of 161 for arginine (102 -i- 59, where 59 is the amino side chain). These SRMs are often grouped together in the visual spectrum as shown in Figure 13.12. Note that citrulline can also be acquired in an NL 102 scan since source-induced dissociate may cause the ammonia to be lost and hence detection of m/z 215 (MH+ minus 17) versus m/z 232. Details of this fragmentation are described elsewhere. 

Citrulline Synthesis. The first syntheses of citrulline and arginine by the addition of NHa and CO2 to ornithine were achieved in the laboratory of Cohen, where soluble enzymes of liver supplemented with cell particles were found to carry out these syntheses. The soluble extract was found to contain the several activities involved in the reactions of the urea cycle, while the particles were found to function solely as a source of ATP. The synthesis of citrulline was found to require ornithine, CO2, NH3, and... 

Soluble Enzymes. Very many enzymes appear to be present in solution in cytoplasm. The glycolytic enzymes are all found in the soluble fraction of homogenates. The report that glycolysis is stimulated by the addition of other components cannot be evaluated now it may be that glycolysis does involve additional elements of the cell, or it may be that some enzymes are adsorbed to particles. The enzymes of many ATP-requiring reactions are found in the soluble fraction. Early work on many of these systems, such as those that form citrulline and arginine, showed a requirement for mitochondria, but this requirement was eventually found to be related only to the ability of mitochondria to form ATP. 

Gross elevations of many amino acids, particularly glutamine and alanine in blood, have been reported in moribund children, however, elevations of the branched-chain amino acids, citrulline and arginine are probably secondary to hypoxia and liver failure. Post-mortem blood shows similar but more pronounced amino acid changes. 

Here, we have a biosynthetic pathway which, starting from a-ketoglutaric acid in the Krebs cycle, divides at the level of glutamic acid into three branches, one leading to glutamine, a second to proline and hydroxyproline, and the third to ornithine, citrulline and arginine. 

Glutamine is an important precursor for de novo synthesis of arginine in humans. Am. J. Clin. Nutr. 87, 1282-1289. Ligthart-Melis, G.C., M.C.G. van de Poll, C.H.C. Dejong, P.G. Boelens, N.E.P. Deutz, and P.A.M. van Leeuwen, 2007. The route of administration (enteral or parenteral) affects the conversion of isotopically labelled l-[2-N-15] glutamine into citrulline and arginine in humans. J. Parenter. Enteral Nutr. 31, 343-348. 

Wu, G.Y, 1997. Synthesis of citrulline and arginine from proline in enterocytes of postnatal pigs. Am. J. Physiol.-Gastrointestinal and Liver Physiology 35, G1382-G1390. 

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