METABOLISM 171 



" building stones " it would appear that we have here, as in the 

 case of the simple proteins, a complex of reversible enzym 

 reactions. 



Phosphoric acid 



Nucleic acid 



Pentose 



Purin or pyrimidin bases 



241. Deaminization. The phosphoric acid which is split 

 off from the nucleic acids is, of course, added to the general 

 stock of this substance in the body. The pentose may be pre- 

 sumed to be katabolized or possibly built up into a hexose. 



The bases, on the other hand, like the amino acids derived 

 from the proteins, undergo, as the second general stage of their 

 katabolism, an enzymatic deaminization and oxidation. The 

 NH 2 groups are split off as ammonia and converted into urea, 

 while the ring formations are largely unbroken, the principal 

 end products of purin katabolism being uric acid in man and 

 allantoin in most other mammals. Of the katabolism of the 

 pyrimidin bases little is known. The deaminization is never 

 complete, however, purin and pyrimidin bases appearing in 

 the urine along with the end products of katabolism. 



242. Synthesis of uric acid. In birds and reptiles, uric 

 acid is the principal nitrogenous constituent of the semi-solid 

 urine. Since no considerable portion of its nitrogen can have 

 existed as preformed purins in the feed, it is evident that these 

 animals must synthesize uric acid. This synthesis appears 

 to take place in the liver, the antecedents probably being lactic 

 acid and urea. 



6. THE METABOLISM OF 'THE FATS 

 Anabolism 



243. Re synthesis of feed fat. In considering the resorp- 

 tion of the fats (152) it was shown that, while the products of 

 their digestion are glycerol and fatty acids (or their salts), 

 after resorption only neutral fats have been recognized in the 

 epithelial cells and in the lymph of the intestinal lacteals. 

 The cleavage of the fats in digestion is reversed in the epithelial 

 cells. It seems altogether plausible to ascribe this resyn thesis 

 to the action of an intracellular lipase, the more since the action 



