Nucleotides are derived from biosynthetic precursors of carbohydrate amino acid metabolism, and from and ia and carbon dioxide. The liver is the major organ of de novo synthesis of all four nucleotides. gradation in humans, however, is only complete for pyrimidines(C, T, U), but not purines(G, A), which are creted from the body in form of uric acid(coo366). De novo synthesis of midines and purines follows two ferent pathways. Pyrimidines are synthesized first from aspartate and ca phosphate in the cytoplasm to e common precursor ring structure orotic acid(C00295), onto which a phosphorylated ribosyl unit is covalently ed. Purines, however, are first synthesized from the sugar template onto which the ring synthesis occurs.