Ornithine arginine synthesis

An anomaly associated with citrulline that became evident when detailed kinetic studies were made in the 1950s (R.B. Fisher and J.R. Bronk) was the irreproducibility of its catalytic activity in liver slices on the formation of urea, despite the clear evidence from Ratner and Petrack of its importance in arginine synthesis. Initially the discrepancy in catalytic activity between ornithine and citrulline was ascribed to the possible impermeability of the liver cell plasma membrane to the latter intermediate, a hypothesis which was rapidly disproved experimentally. Only recently has it been shown that ornithine transcarbamylase is clearly associated with the ornithine/... 

C. Synthesis and Catabolism of Proline, Ornithine, Arginine, and Polyamines... 

Proline is synthesized from glutamate in three steps. The second step is a spontaneous cyclization reaction. In arginine synthesis the acetylation of glutamate prevents the cyclization reaction. In mammals the reactions that convert ornithine to arginine are part of the urea cycle. 

Amino sugars Asparagine Other compounds Glutamate donor for synthesis of Glutathione GABA Ornithine Arginine Proline... 

Fig. 3. Proline and arginine synthesis and degradation to show interrelationships between the pathways. The structures are glutamic acid (GLU), ornithine (ORN), citrulline (CIT), arginine (ARG), urea. 2-oxo-5-amino valeric acid (OAV), A -pyrroline-2-carboxylic acid (P2C), proline (PRO), A -pyrroline-5-carboxylic acid (P5C), glutamic semialdehyde (GSA).

The enzymes of the urea cycle, however, are not only expressed in the liver, but also in other tissues and cell types. In fact, it is believed that the urea cycle evolved from the arginine metabolic pathway present in lower organisms (Takiguchi et al, 1989). This difference in function between urea and arginine synthesis is reflected by the different tissue localization, function and regulation of the enzymes of the urea cycle and other enzymes involved in the metabolism of the urea cycle intermediates, arginine, citrulline and ornithine. 

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