Dual function of pyrimidine metabolism in potato (Solanum tuberosum) plants: pyrimidine salvage and supply of β‐alanine to pantothenic acid synthesis

Uridine and cytidine are major nucleosides and are produced as catabolites of pyrimidine nucleotides. To study the metabolic fates and role of these nucleosides in plants, we have performed pulse (2 h) and chase (12 h) experiments with [2‐14C]uridine and [2‐14C]cytidine and determined the activities of some related enzymes using tubers and fully expanded leaves from 10‐week‐old potato plants (Solanum tuberosum L.). In tubers, more than 94% of exogenously supplied [2‐14C]uridine and [2‐14C]cytidine was converted to pyrimidine nucleotides and RNA during 2‐h pulse, and radioactivity in these salvage products still remained at 12 h after the chase. Little degradation of pyrimidine was found. A similar pyrimidine salvage was operative in leaves, although more than 20% of the radioactivity from [2‐14C]uridine and [2‐14C]cytidine was released as 14CO2 during the chase. Enzyme profile data show that uridine/cytidine kinase (EC 2.7.1.48) activity is higher in tubers than in leaves, but uridine nucleosidase (EC 3.2.2.3) activity was higher in leaves. In leaves, radioactivity from [U‐14C]uracil was incorporated into β‐ureidopropionic acid, CO2, β‐alanine, pantothenic acid and several common amino acids. Our results suggest two functions of uridine and cytidine metabolism in leaves; these nucleosides are not only substrates for the classical pyrimidine salvage pathways but also starting materials for the biosynthesis of β‐alanine. Subsequently, some β‐alanine units are utilized for the synthesis of pantothenic acid in potato leaves.