Enzymes of Purine Biosynthesis and Catabolism in Glycine max

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Abstract
During the period examined from 12-63 days after planting, the ureides, allantoin and allantoic acid, were the predominant nitrogenous solutes in the xylem exudate of soybeans (G. max [L.]) growing solely on symbiotically fixed N, accounting for .apprx. 60% and > 95% of the total N in the xylem exudate before and after the onset of active N fixation, respectively. For plants from 18-49 days of age, the apparent rate of ureide export estimated from concentrations of ureides in xylem exudate collected over a period of 1 h was closely related to the rate of N fixation estimated from measurements of C2H2 reduction by nodulated root systems. After this time, the apparent rate of ureide export per plant continued to increase, reaching a maximum value at day 63 of 12 .mu.mol/plant per h, even though the rate of C2H2 reduction per plant declined .apprx. 4-fold. The most probable pathway for the biosynthesis of ureides involves the catabolism of purines. The levels of phosphoribosylpyrophosphate (PRPP) synthetase, which catalyzes the formation of the PRPP required for purine synthesis, increased in parallel with the rates of N fixation (C2H2) from day 18 reaching a maximum value of 13.9 .mu.mol/plant per h at day 49, and then both activities declined rapidly. During the period of active N fixation the ratio of PRPP synthesis estimated from measurements of PRPP synthetase activity in cell-free extracts to the apparent rate of ureide export was 1-2. The activities of the enzymes of purine catabolism, xanthine dehydrogenase uricase and allantoinase, increased in parallel with the increases in nodule mass and the export of ureides with maximum activities of 13, 119 and 79 .mu.mol/plant per h, corresponding with apparent rates of ureide export in the range of 9.5-119 .mu.mol/plant per h. There is a close association between N fixation, PRPP synthetase activity and ureide export in soybeans. The proposal that recently-fixed N is utilized in the de novo synthesis of purines which are subsequently catabolized to produce the ureides is supported.
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