L-arginine is an essential amino acid. In the mitochondrion, and probably in the cytosol, L-arginine is converted by Arg to L-ornithine, a precursor for the biosynthesis of polyamine, L-proline and L-glutamate. L-arginine is also a precursor for the biosynthesis of the signalling molecule nitric oxide (NO). L-citrulline, a by-product of NO synthesis, can be converted back to L-arginine by the sequential action of Ass and Argl. In D. melanogaster, L-arginine is used to produce phospho-L-arginine which serves as the main high-energy phosphate reservoir instead of phosphocreatine. (Adapted from PMID:40081835.)