Abstract
Both arginine (Arg) and its precursor citrulline (Cit) have received much interest during the past two decades because of their potential effects on whole-body nitric oxide (NO) production and augmentation of NO-dependent signaling pathways. However, the usefulness of Arg supplementation for NO production is questionable due to its high splanchnic first-pass metabolism (FPM), which limits its systemic availability. Both hepatic- and extra-hepatic arginases critically limit the availability of Arg for the NO synthase enzymes (NOSs) and therefore, a limited amount of oral Arg can reach the systemic circulation for NO synthesis. Arg has also some undesired effects including induction of arginase activity, an increase of urea levels, a decrease of cellular uptake of Cit, and decrease of recycling of Arg from Cit. In contrast, Cit has more availability as an NO precursor because of its high intestinal absorption, low FPM, and high renal reabsorption. At the cellular level, co-localization of Cit transport systems and the enzymes involved in the Cit-Arg-NO pathway facilitates channeling of Cit into NO. Furthermore, cells preferably use Cit rather than either intra- or extra-cellular Arg to improve NO output, especially in high-demand situations. In conclusion, available evidence strongly supports the concept that Cit leads to higher NO production and suggests that Cit may have a better therapeutic effect than Arg for NO-disrupted conditions.
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