Ornithine: at the crossroads of multiple paths to amino acids and polyamines.

After the 20 amino acids that make up the proteins in all living organisms, ornithine perhaps occupies the most critical position among the non-protein amino acids. It sits at the crossroads of interconversions of glutamate and arginine on the one hand and the production of proline, polyamines and several alkaloids on the other: all products of tremendous importance to living cells. While ornithine is typically not an abundant amino acid, the metabolic flux of nitrogen through this amino acid is presumably quite rapid because of the cellular contents of the products for which it serves as the substrate. Our current knowledge of the regulation of ornithine biosynthesis is rather limited and mostly dependent upon the research targeted at understanding arginine and proline metabolism, and to some extent polyamine and alkaloid biosynthesis. Whereas most of the ornithine biosynthesis in animals occurs on the way to the production of glutamate, proline and putrescine from dietary arginine, in plants the pathway works in the reverse order, i.e. glutamate, the first product of nitrogen assimilation is the primary source of arginine, proline and putrescine. An understanding of the regulation of ornithine metabolism could help us in the genetic manipulation of plants for stress tolerance (via manipulation of proline and γ-aminobutyric acid, and the polyamine pathway), as well as in nutritional improvement (the cellular contents of important amino acids-arginine and citrulline among others). Based upon the recent observations of a strong connection between polyamine biosynthesis, ornithine metabolism and glutamic acid metabolism, it can be proposed that ornithine may act as a controlling metabolite to enhance nitrogen assimilation and, consequently, increased carbon assimilation.