ARG1 and inflammatory bowel disease: On the other hand, arginine nitric oxide and arginine urea pathways are implicated in the pathogenesis of IBD; in the former case NOS2 (the inducible form of nitric oxide synthase [iNOS]) metabolizes L-arginine to NO and L-citrulline and in the latter arginases (ARG1 and ARG2) catalyse the conversion of arginine to urea and ornithine.