Arginine is mainly available from protein breakdown in the body and from food intake. The jejunum is the major site for intestinal absorption of dietary arginine. Only approximately 20% of protein synthesis is derived directly from dietary
amino acid intake. This finding implies that approximately 80% of protein synthesis involves recycling of amino acids from protein breakdown. Moreover, arginine is synthesized endogenously or de novo in the proximal renal tubule by conversion of citrulline to arginine through a partial urea cycle by the enzymes argininosuccinate synthase (ASS) and argininosuccinate lyase (ASL) (13, 14, 15, 16). This conversion is part of the intestinal-renal axis, as demonstrated in animal and human studies (6, 17, 18, 19). Under normal conditions, this pathway contributes approximately 10% to 15% to whole body arginine production (20, 21), in which citrulline availability is the limiting factor for renal arginine synthesis (15). In contrast to adults, the conversion to arginine in neonates is limited to intestinal arginine synthesis from dietary proline and the conversion of citrulline to arginine by the enzymes ASS and ASL (22). This first-pass de novo synthesis provides 50% of the arginine needed by neonates (23). In the postabsorptive state, whole body arginine flux in healthy adults is approximately 70 to 90 µmol/kg/hour (24).

Apart from being an essential component of body proteins, arginine plays a key role in several other metabolic pathways that involve various enzyme systems (3, 4, 12, 25, 26, 27), as follows:

In addition to its role as an intermediary in the synthesis of functional products, arginine also acts as a secretagogue because it stimulates release of several hormones such as insulin, glucagon, somatostatin, prolactin, growth hormone, and its peripheral mediator, insulinlike growth factor-I (30, 42). Arginine has the strongest insulinogenic effect of all the amino acids (27).

Citrulline is synthesized by enterocytes in the small intestine that convert glutamine and proline through the glutamate-to-ornithine pathway (43). The final step in this synthesis pathway is the conversion of ornithine to citrulline, catalyzed by the enzyme ornithine transcarbamoylase (OTC) or ornithine carbamoyltransferase. Aside from the liver, where OTC is an enzyme in the urea cycle, OTC is present only in enterocytes (27). Glutamine is considered the main precursor for citrulline synthesis, as demonstrated by the close relationship between glutamine uptake and citrulline release by the intestine (44), which provides 60% to 80% of citrulline (19, 45, 46, 47, 48). Moreover, arginine has been suggested as a source of citrulline through arginase and OTC metabolic pathways (49), and interorgan exchange of ornithine may also contribute to citrulline synthesis in the intestine (50). An additional amount of citrulline comes from nonintestinal sources. The intracellular arginine-citrulline cycle related to NO production in endothelial cells seems a likely candidate, as suggested in mice, humans, and endothelial cell studies (18, 46, 51). In the postabsorptive state, whole body citrulline flux in healthy adults is approximately 10 to 15 µmol/kg/hour (52, 53).

Unlike most amino acids, citrulline is not incorporated into protein, but it can be converted to only arginine.
A large part of circulating citrulline, which is partly derived from the intestinal release of citrulline, is taken up by the kidneys (6, 17), where it is converted to arginine that is released into the circulation. This pathway has been confirmed in humans (18, 19). Although investigators thought that citrulline thus escaped the splanchnic sequestration and bypassed the urea cycle with subsequent nitrogen loss (30), other investigators indicated that the liver does extract substantial amounts of citrulline from the portal vein (18). Moreover, citrulline conversion to arginine is also efficient in other cells such as macrophages, especially under low-arginine conditions (54). This gives citrulline a considerable role in metabolism and regulation of NO (10).

Besides acting as a substrate for arginine, citrulline probably also has a direct anabolic effect on muscle (8, 9). Moreover, citrulline is a major hydroxyl radical scavenger, which, in watermelon, is protective in environments inducing oxidative stress, such as drought (55).

The reason for compartmentalization of metabolism is that the enzymes in arginine-citrulline metabolism are expressed to a different extent in various organs (27, 58), and interorgan exchange occurs (see Fig. 35.4) (44). The direct conversion of citrulline to arginine and subsequently to NO (citrulline-NO cycle) in macrophages (59) or endothelial cells (51) and urea metabolism in the liver or compartmentalized NO production from protein-derived arginine (50) are examples of compartmentalization.

Substrate availability for arginine-requiring catabolic enzymes also depends on arginine transport systems. Several arginine transporters exist, of which system y⁺ is the most important and high-affinity transport mechanism, ascribed on the molecular level to cationic amino acid transporters (CATs). Of these CATs, CAT-1, CAT-2(B), and CAT-3 have been identified, all of which differ in their tissue distribution (27). These transport systems are often colocalized with the catabolic enzymes and as such can modulate cellular arginine metabolism (27). For example, CAT-1 arginine transporter and endothelial NO synthase enzyme are colocalized in plasma membrane caveolae (60), which facilitate specific channelling of arginine to NO production without mixing with the total intracellular pool (58). Lysine, ornithine, and certain endogenous NOS inhibitors use the same transporter as arginine and may thereby compete for transporter capacity in conditions of low arginine (58, 61). For citrulline, no evidence indicates the presence of a specific transporter in any cell type, and transport by the usual generic amino acid transporters is demonstrated (10).