Citrulline is created in the body by one of two pathways, either recycled from arginine (the conversion of arginine into nitric oxide leaves citrulline as a byproduct)[1][2] or produced from the nitrogen (and some carbon) contained in L-glutamine,[3] a process in which the enzyme ornithine transcarbamylase uses both ornithine and carbamoyl phosphate (which requires glutamine) to produce citrulline in enterocytes.[4][5]
It appears that the arginine pathway accounts for about 10% of circulating citrulline and the glutamine pathway accounts for 90%;[1] reducing plasma glutamine levels can reduce plasma citrulline.[6]