De Novo Purine Synthesis in Nitrogen-Fixing Nodules of Cowpea (Vigna unguiculata [L.] Walp.) and Soybean (Glycine max [L.] Merr.)

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Abstract
Partially purified, cell-free extracts from nodules of cowpea (V. unguiculata L. Walp. cv. Caloona) and soybean (G. max L. Merr. cv. Bragg) [inoculated with Rhizobium] had high rates of de novo purine nucleotide and purine base synthesis. Activity increased with rates of N2 fixation and ureide export during development of cowpea plants; maximum rates (equivalent to 1.2 .mu.mol N2/h per g fresh nodule) being similar to those of maximum N2 fixation (1-2 .mu.mol N2/h per g fresh nodule). Extracts from actively fixing nodules of a symbiosis not producing ureides, Lupinus albus L. cv. Ultra, showed rates of de novo purine synthesis 0.1-0.5% those of cowpea and soybean. Most (70-90%) of the activity was associated with the particulate components of the nodule, but up to 50% was released from this fraction by osmotic shock. The accumulated end products with particulate fractions were inosine monophosphate and aminoimidazole carboxamide ribonucleotide. Further metabolism to purine bases and ureides was restricted to the soluble fraction of the nodule extract. High rates of inosine monophosphate synthesis were supported by glutamine as amide donor, lower rates (10-20%) by ammonia and negligible rates with asparagine as substrate.