Nitrogen ureide

Release of Ammonia. Of the five enzymes, three lead to release of amide/ami no nitrogen as ammonia during ureide metabolism ... 

Figure 25-18 Pathways of catabolism of purine nucleotides, nucleosides, and free bases. Spiders excrete xanthine while mammals and birds excrete uric acid. Spiders and birds convert all of their excess nitrogen via the de novo pathway of Fig. 25-15 into purines. Many animals excrete allantoin, urea, or NH4+. Some legumes utilize the pathway marked by green arrows in their nitrogen transport via ureides.

Plants also form the ureides allantoin and allantoic acid, and in some legumes, such as soy beans, these compounds account for 70-80% of the organic nitrogen in the xylem. They appear to function in nitrogen transport.337 As indicated in Fig. 25-18, the hydrolysis to glyoxylate, NH4+, and C02 follows a different pathway than in animals. See also Chapter 24, Section C. 

Ammonium, the primary product of nitrogen fixation, is transported to the host cell cytoplasm where it is assimilated into amides and, in some cases, further converted into ureides before being transported to the shoot. Since the physiological environment within the nodule is apparently different from the other parts of the plant, nodule-specific or nodule-abundant forms of several enzymes of the nitrogen and carbon assimilation pathways have evolved, and are induced to improve the efficiency of nitrogen and carbon metabolism in nodules. 

Formulae (1.6) and (1.7) show other functions which present problems of classification. The former has a group which may be regarded as either a ureide or a carboxamide, and is classified as a carboxamide in this book. Since it is the ester group of (1.7) that reacts in the second example, the conversion is classified under carbamate and ring-nitrogen (Chapter 61). 

Carbamate or Ureide and Ring-carbon or Ring-nitrogen... 

A similar classification may be appropriate for N2-fixing actinorhizal species. Asparagine is the major product of N2 fixation in Myiica species (32), w hereas the ureide, citrulline, predominates in Alnus species (20, 33). The structures of the difiFerent nitrogenous components transported from N2-fixing plants are given in Figure 3. 

The hydantoins have a 5-membered ring structure containing two nitrogens in an ureide configuration (Fig. 20.4) and were tested as antiepileptics by Merritt and Putnam (23,24). These drugs suppressed electrically induced convulsions in animals but were ineffective against convulsions induced by pentylenetetrazole, picrotoxin, or bicuculline. The structures for the clinically available hydantoins are listed in Figure 20.5. 

Ureide planis plant families that accumulate al-lantoin and/or all toic add, and use these compounds as nitrogen reserves, U.p. are members of the Aceraceae, Boraginaceae, Hippocastanaceae and Pla-... 

The so-called ureide plants use allantoin and allantoic acid as a nitrogen store from which ammonia may be liberated by further degradation (E 2.2), In the liver of lungfish a glycine-allantoin cycle (Fig. 182) causes the formation of urea (cf. the formation of urea via L-ornithine derivatives, D 19). 

Other derivatives that are not ureides may, however, also be subject to this reaction. This is the case, for example, of theophylline, which is endowed with a weak acidic character due to its mobile hydrogen brought by the nitrogen in position 7. Theophylline s pK value is 8.6. To perform the titration, an excess of silver nitrate is added and the titration is carried out by sodium hydroxide. The indicator is bromothymol blue. 

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