Nitrogen Eoccretion 189 



but apparently hypoxanthine is an intermediate product in the process. 



Trimeth)damine Oxide. This constitutes a high proportion of excreted 

 nitrogen in certain marine fishes. Possibly ammonia is methylated, and the 

 resulting trimethylamine oxidized. 



Guanine (2-amino-6-oxr purine). Spiders excrete most of their nitrogen as 

 the purine, guanine, a substance less soluble than uric acid in water. Its 

 method of synthesis is unknown. ^ 



Purine Excretion Products. Roughly 5 per cent of excretory nitrogen comes 

 from nucleic acid metabolism, which yields principally the purines adenine 

 and guanine. 



Purines. The purines are degraded to different degrees by animals of various 

 groups. Some animals excrete a high proportion as purines without further 

 degradation. In some animals adenine may be oxidized, but not other purines; 

 guanine is accumulated or excreted as such in the pig, but in most mammals 

 it is deaminated. Different enzymes act on the purine adenine and the nucleo- 

 side adenosine (adenine-ribose). •'•^ 



Uric Acid. Uric acid is formed as an end-product by oxidative deamination. 



adenase 



adenine *■ hypoxanthine 



I , xanthine .xanthine 



guanase \ oxidase oxidase 

 guanine •• xanthine *■ uric acid 



N=C-NH2 H-N-C=0 H-N-C=0 



I I II 11 

 H-C C-NH OzrC C-NH 0=C C-NH\ 



II It ^CH I II 7CH— > 1 II C-0 

 N-C-N^ H-N-C-N^ H-N-C-NH/ 



Adenine Xanthine Uric acid 



Allantoin. The pyrimidine ring of uric acid may be opened by uricase and 

 the substance allantoin formed (Fig. 33). 



Allantoic Acid. Further oxidation bv allantoinase forms allantoic acid 

 (Fig. 33). 



Urea. The molecule of allantoic acid is broken by allantoicase to glyoxylic 

 acid and urea. 



Ammonia. A few animals and many plants possess urease, which converts 

 urea to ammonia and carbon dioxide. 



The complete sequence of purine breakdown is shown in Figure 33, modi- 

 fied from Florkin. '^'-^ 



Miscellaneous Nitrogenous Compounds. Other nitrogenous excretion 

 products include: 



Hippiiric Acid and Ornithuric Acid. Benzoic acid entering in the diet of 

 mammals is removed by combination with glycine to form hippuric acid 

 (benzoylglycine). In birds benzoic acid combines with ornithine to form 

 ornithuric acid. 



Creatine and Creatinine. Creatine is important in muscle development 

 (Ch. 16), and its metabolic roles are numerous. Some creatine is apparently 



