METABOLISM 169 



passing into the circulation being essentially phosphoric acid, 

 pentoses, and purin and pyrimidin bases. By analogy, with 

 the simple proteins, one might expect, therefore, to find that 

 these fragments of the nucleic acid molecule are rebuilt into 

 nucleoproteins in the body cells, of which they constitute such 

 an indispensable ingredient (75). 



The occurrence of such a synthesis, however, has been seriously 

 questioned. One argument against it is the fact that the in- 

 gestion of nucleoproteins, or more specifically purin bases, re- 

 sults in a prompt excretion in the urine of end products of their 

 katabolism which, although it has not been proved to be quan- 

 titative, is certainly large, while the amounts excreted on a 

 purin free diet are small and notably uniform. It has been 

 argued, therefore, that the so-called " exogenous " purins, i.e., 

 the nucleic acid constituents derived from the feed, are simply 

 katabolized and excreted without serving to rebuild nucleic 

 acids in the cells. Precisely the same argument might be made, 

 however, against the synthesis of the simple proteins from their 

 cleavage products, since in this case also an increase in the supply 

 causes a prompt and almost quantitative increase in the ex- 

 cretion of the end product, urea (402). 



238. Autogenesis. It is true that the formation of nucleo- 

 proteins differs from that of the simple proteins in that the 

 latter is a reconstruction of the molecule from its fragments 1 

 rather than a synthesis in the stricter sense, while it has been 

 demonstrated that the body can build up nucleic acids out 

 of a feed supply containing neither purins, pyrimidins nor 

 pentoses. One of the most striking instances of this is 

 seen in the development of the embryo of birds and insects. 

 The eggs contain practically none of the substances just men- 

 tioned, yet the bodies of the young animals contain normal 

 amounts of nucleic acid. Equally significant is the case of the 

 'suckling mammal, which receives in the milk a food very poor 

 in purins, pyrimidins and pentoses, yet which maintains a rapid 

 growth and cell multiplication with its accompanying formation 

 of nucleoproteins. So, too, in Osborne and Mendel's extensive 

 investigations 2 upon the nutritive values of the proteins, normal 



1 The possibility of the formation of proteins from ammonia (234) is of little sig- 

 nificance under ordinary conditions of nutrition. 



2 Carnegie Institution of Washington, Publication No. 156, p. 85. 



