Appearance of Purine-Catabolizing Enzymes in Fix+ and Fix− Root Nodules on Soybean and Effect of Oxygen on the Expression of the Enzymes in Callus Tissue

The appearance of enzymes involved in the formation of ureides, allantoin, and allantoic acid, from inosine 5'-monophosphate was analyzed in developing root nodules of soybean (Glycine max). Concomitant with development of effective nodules, a substantial increase in specific activities of the enzymes 5'-nucleotidase (35-fold), purine nucleosidase (10-fold), xanthine dehydrogenase (25-fold), and uricase (200-fold), over root levels was observed. The specific activity of allantoinase remained constant during nodule development. With ineffective nodules the activities were generally lower than in effective nodules; however, the activities of 5'-nucleotidase and allantoinase were 2-fold higher in ineffective nodules unable to synthesize leghemoglobin than in effective nodules. Since the expression of uricase has been shown to be regulated by oxygen (K Larsen, BU Jochimsen 1986 EMBO J 5: 15-19), the expression of the remaining enzymes in the purine catabolic pathway were tested in response to variations in O(2) concentration in sterile soybean callus tissue. Purine nucleosidase responded to this treatment, exhibiting a 4-fold increase in activity around 2% O(2). 5'-Nucleotidase, xanthine dehydrogenase, and allantoinase remained unaffected by variations in the O(2) concentration. Hence, the expression of two enzymes involved in ureide formation, purine nucleosidase and uricase, has been demonstrated to be influenced by O(2) concentration.