Ornithine, biosynthesis citrulline from

FIGURE 3-8 Uncommon amino acids, (a) Some uncommon amino acids found in proteins. All are derived from common amino acids. Extra functional groups added by modification reactions are shown in red. Desmosine is formed from four Lys residues (the four carbon backbones are shaded in yellow). Note the use of either numbers or Creek letters to identify the carbon atoms in these structures, (b) Ornithine and citrulline, which are not found in proteins, are intermediates in the biosynthesis of arginine and in the urea cycle. 

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... 

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