Citrulline precursors, synthesis

The synthesis of arginine from citrulline. The latter is produced from other amino acids in the small intestine and then released into the blood. The kidney takes up citrulline and converts it to arginine, which is then released into the blood for use by other tissues (Figure 8.18). Since arginine is a precursor for a number of important compounds, and aids wound healing, this is a significant biochemical role of the kidney. 

It is instructive to note how the main reaction sequences can be divided into two categories those which lead to the liberation of free energy and those which require an outlay of free energy before they can be initiated. Thus the synthesis of citrulline from ornithine - and that of phos-pholipin from various precursors are processes which have to be driven by some concomitant oxidative reaction. The mechanisms by which this linkage of an oxidative and synthetic reaction is accomplished will be considered in a later section. It will be sufficient for present purposes to recognize an important feature of the plan of the C.M. system, viz., the juxtaposition of the oxidative and synthetic enzyme systems and the availability of appropriate devices for linking the two. 

Citrulline more than the precursor for arginine synthesis... 

Precursors for citrulline synthesis enzymes involved in the synthesis of citrulline and the snpply of precursors... 

When discussing which precursors are utilized for the synthesis of citrulline, what we really mean is what sources of ornithine are used for citrulline synthesis. Ornithine can be generated de novo from proline and glutamate by action of ornithine amino transferase (OAT) or it can be preformed , released by hydrolysis of arginine by arginase at the site of citrulline synthesis (Figure 1). In addition, plasma ornithine can be imported by the enterocyte and utilized for the synthesis of citrulline. 

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. 

Marini, J.C., I.C. Didelija, L. Castillo, and B. Lee, 2010b. Glutamine Precursor or nitrogen donor for citrulline synthesis Am. J. Physiol., In press. 

Glutamine precursor or nitrogen donor for the synthesis of citrulline ... 

Precursors for the synthesis of citrulline in mice fed arginine free diets... 

In our previous study, dietary arginine (Arg) provided the precursor for 40% of the circulating citrulline (Cit) in adult mice, while dietary proline (Pro) accounted for only 3.4% and the contrihution of dietary glutamine (Gin) was neghgible (0.4%) (Marini etcJ., 2010). The aim of the present study was to determine which precursors are utilized for the synthesis of Cit when Arg is absent in the diet. 

Marini JC, I. Cajo, L. Castillo and B. Lee, 2010. Glutamine precursor or nitrogen donor for the synthesis of citrulline In Crovetto, G.M. (ed.) Energy and protein metabolism and nutrition. Wageningen Academic Publishers, Wageningen, the Netherlands, p. 123-124. 

The synthesis of citrulline (Cit) relies mainly on the supply of preformed ornithine (Om), rather than on the de novo synthesis of Om from proline (Pro) or glutamine (Gin). Thus utilization of arginine (Arg) for Cit synthesis depends on arginase, which catalyses the hydrolysis of Arg to Om and urea. The aim of the present study was to determine the precursors utilized for the synthesis of Cit when arginase II, the intestinal isoform, is absent. 

Figure 1. Utilization of dietary arginine, glutamine and proline for the synthesis ofcitrulline (pmol/ kg/h). Bars denote the total contribution of the precursor ( V<0.05), shaded portion of the bar the contribution of the precursor through plasma ornithine ( P<0.05), and by difference the contribution of the precursor at the site of citrulline synthesis.

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