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Argininosuccinate synthetase (ASS) catalyses the condensation of citrulline and aspartate to form argininosuccinate, the immediate precursor of arginine. First identified in the liver as the limiting enzyme of the urea cycle, ASS is now recognized as a ubiquitous enzyme in mammalian tissues.<ref> http://www.ncbi.nlm.nih.gov/pubmed/12709047</ref>Since its discovery, the function of argininosuccinate synthase has been linked almost exclusively to hepatic urea production despite the fact that alternative pathways involving argininosuccinate synthase were defined, such as its role in providing arginine for creatine and for polyamine biosynthesis <ref>http://www.ijbmb.org/files/IJBMB1009003.pdf</ref>Argininosuccinate synthase plays an important role as the rate-limiting step in providing arginine for an assortment of metabolic processes, both catabolic and anabolic. Thus, the metabolic pathways in which argininosuccinate synthase participates are linked to the varied uses of the amino acid arginine. There are five major pathways in which argininosuccinate synthase plays a key role. These are (a) urea synthesis,(b) nitric oxide synthesis, (c) polyamine synthesis, (d) creatine synthesis, and (e) the de novosynthesis of arginine to maintain serum levels.<ref>http://www.ijbmb.org/files/IJBMB1009003.pdf</ref>