Enzymes of ammonia assimilation and ureide biogenesis in developing pigeonpea (Cajanus cajan L.) nodules.

Ammonia assimilatory and ureide biogenic enzymes were measured in the cytosol fraction of pigeonpea nodules during the period 15-120 days after sowing. The activity of enzymes involved in the initial assimilation of ammonia, i.e. glutamine synthetase, glutamate synthase, asparagine synthetase and aspartate aminotransferase, substantially increased activities during the period of plant growth and reached a maximum value around 105 days after sowing. These increases paralleled the increase in nodule mass, nitrogenase activity and ureide content in nodules. Though no regular pattern was obtained for their specific activities, yet these activities when expressed relative to the specific activity of nitrogenase were many fold higher at each stage of development. Similar increases were observed in the activities of enzymes associated with the formation of ureides from purines. In almost all cases, the activities were again maximum around 90-105 days after sowing. The specific activities of nucleotidase, nucleosidase, xanthine dehydrogenase, uricase and allantoinase, when expressed relative to the specific activity of nitrogenase at vegetative, flowering and podsetting stages were again many fold higher indicating the sufficiency of the levels of these enzymes for the biosynthesis of ureides. The data presented are consistent with the proposal that in ureide producing legumes, ammonia is initially assimilated into glutamine, aspartate, etc., which are metabolised for the de novo synthesis of purines. The purines are then utilised for the production of ureides by a group of enzymes investigated here.

Ureide biogenesis and the enzymes of ammonia assimilation and ureide biosynthesis in nitrogen fixing pigeonpea (Cajanus cajan) nodules.

Allantoic acid production from IMP, XMP, inosine, xanthosine, hypoxanthine, xanthine, uric acid and allantoin was investigated by incubating each of these substrates with Cajanus cajan cytosol and bacteroid fractions separately in the presence and absence of NAD+ and allopurinol. Allantoic acid synthesis by bacteroid fraction could only be observed with uric acid and allantoin as substrates. Addition of NAD+ or allopurinol to the reaction mixtures had no effect. However, with cytosol fraction, allantoic acid was produced by each of these substrates, with maximum rate with allantoin. With NAD+ or with allopurinol, allantoic acid was produced only with uric acid and allantoin as substrates. NADH production with cytosol fraction could again be observed with all the substrates. Except with uric acid and allantoin, allopurinol completely inhibited NADH formation. Regardless of the presence or absence of allopurinol, none of the substrates exhibited significant activity with bacteroid fraction. Based on the activities of glutamine synthetase, glutamate synthase, glutamate dehydrogenase, aspartate aminotransferase, asparagine synthetase, nucleotidase, nucleosidase, xanthine dehydrogenase, uricase and allantoinase and their intracellular localisation in various nodule fractions, a probable pathway for the biogenesis of ureides in pigeonpea nodules has been proposed.