citrulline may* be advantageous because it is rate limited so it might supply a steady, longer lasting stream of Arg.*and I'm only guessing here.
Ok bio. You've inspired me to look up a few studies conducted on this issue again. After I took a look, I remembered why I made the decision to find NO products that include a citrulline/arginine blend.
Even though arginine is the direct precursor to NO, it turns out that citrulline also plays an important role in nitric oxide synthase and the administration of both will ensure greater, more effective results.
Studies have recently demonstrated that the citrulline produced by NO synthase can be recycled into arginine in EC (Endothelial Cells), wich may help to maintain a sufficient intracellular concentration of arginine for NO regeneration. Thus with the production of NO from L-arginine by constitutive NO synthase in EC, there is a functional.... (arginine-citrulline cycle) in these cells.
Based on normal intracellular concentrations of L-glutamine and it's metabolites, glutamine markedly decreases NO synthesis from arginine in cultured EC and intact blood vessels, and inhibits cerebral neurogenic vasodilation.
In one study, normal intracellular concentrations of glutamine decreased arginine synthesis.... (from citrulline), whereas glutamine had no effect on arginine, alanine, glutamate, or lysine.
The results from this study suggest that glutamine decreased arginine synthesis from extracellular citrulline, by interfering with it's transport.... (So glutamine disrupts the conversion of citrulline to arginine.)
The study also concluded that inasmuch as NO and citrulline are constantly generated from arginine, with citrulline being actively converted into arginine, that the data had indicated a functioning intracellular (arginine-citrulline cycle). This cycle may function to efficiently scavenge the carbon and alpha-amino group of arginine and to maintain a sufficient cellular concentration of arginine during prolonged synthesis of NO.
http://ajpheart.physiology.org/cgi/content/abstract/265/6/H1965
http://jpet.aspetjournals.org/cgi/content/full/281/1/448