Biosynthesis of Ureides from Purines in a Cell-free System from Nodule Extracts of Cowpea [Vigna unguiculata (L) Walp.]

The synthesis of (14)C-labeled xanthine/hypoxanthine, uric acid, allantoin, allantoic acid, and urea from [8-(14)C]guanine or [8-(14)C]hypoxanthine, but not from [8-(14)C]adenine, was demonstrated in a cell-free extract from N(2)-fixing nodules of cowpea (Walp.). The (14)C recovered in the acid/neutral fraction was present predominantly in uric acid and allantoin (88-97%), with less than 10% of the (14)C in allantoic acid and urea. Time courses of labeling in the cell-free system suggested the sequence of synthesis from guanine to be uric acid, allantoin, and allantoic acid. Ureide synthesis was confined to soluble extracts from the bacteroid-containing tissue, was stimulated by pyridine nucleotides and intermediates of the pathways of aerobic oxidation of ureides, but was completely inhibited by allopurinol, a potent inhibitor of xanthine dehydrogenase (EC 1.2.1.37). The data indicated a purine-based pathway for ureide synthesis by cowpea nodules, and this suggestion is discussed.