 Citrulline is also produced as a byproduct of the enzymatic production of nitric oxide from the amino acid arginine, catalyzed by nitric oxide synthase.Citrulline is made from ornithine and carbamoyl phosphate in one of the central reactions in the urea cycle.It is also produced from arginine as a byproduct of the reaction catalyzed by NOS family (NOS; EC 1.14.13.39).