This pathway (also known as Ornithine and Proline Metabolism) describes the co-metabolism of arginine, ornithine, proline, citrulline and glutamate in humans. Arginine is synthesized from citrulline by the sequential action of the cytosolic enzymes argininosuccinate synthetase (ASS) and argininosuccinate lyase (ASL). Citrulline can be derived from ornithine via the catabolism of proline or glutamine/glutamate. Many of the reactions required to generate proline and glutamate from ornithine are located in the mitochondria. Proline is biosynthetically derived from glutamate and its immediate precursor, 1-pyrroline-5-carboxylate. The pathways linking arginine, glutamine, and proline are bidirectional. Thus, the net utilization or production of these amino acids is highly dependent on cell type and developmental stage. On a whole-body basis, synthesis of arginine occurs principally via the intestinal–renal axis, wherein epithelial cells of the small intestine, which produce citrulline primarily from glutamine and glutamate, collaborate with the proximal tubule cells of the kidney, which extract citrulline from the circulation and convert it to arginine, which is returned to the circulation. Consequently, impairment of small bowel or renal function can reduce endogenous arginine synthesis, thereby increasing the dietary requirement. Both proline and arginine are protegenic amino acids and are incorporated into proteins by prolyl-tRNA and arginyl-tRNA, which are synthesized by their respective tRNA synthetases. Arginine can also serve as a precursor for the synthesis of creatine and phopshocreatine through the intermediate guanidoacetic acid. A key component of the arginine/proline metabolic pathway is ornithine. In epithelial cells of the small intestine, ornithine is used primarily to synthesize citrulline and arginine, in liver cells surrounding the portal vein, ornithine functions primarily as an intermediate of the urea cycle, in liver cells surrounding the central vein, ornithine is used to synthesize glutamate and glutamine while in many peripheral tissues, ornithine is used for the synthesis of glutamate and proline.