CHAPTER LIII 



THE METABOLISM OF NUCLEIN 



NUCLEIN linked on to protein forms the compound body nucleo- 

 protein, the chief protein constituent of the cell nucleus. Little or 

 nothing is known as to the building up of nuclein or of nucleo -protein 

 within the body. Such a synthesis is undoubtedly always taking 

 place, but the exact nature of the precursors employed in the process 

 is not known. In the adult body there is no evidence to show that 

 the purin bodies of the food are used in the process. In the young, 

 in the early months of growth, there takes place an abundant forma- 

 tion of nuclein, and upon a food (milk) which contains practically 

 no nuclein. During the incubation of the hen's egg, nucleo-protein 

 is formed within the cells of the embryo at the expense of the food 

 yolk, which contains almost no nuclein or its derivatives, 



Nuclein breaks down as follows : 



Nuclease 

 of pancreas 



V 



= Protein 



Purin bodies 

 adenin 

 guanin 



Nuclein 



Nucleic acid 



Enzymes of 

 tissues 



Phosphoric acid 



Carbohydrate 



(hexose or 



pentose) 



Pyrimidin V 

 bases = 

 (chiefly 

 cytosin) 



Adenin and guanin are amino-purins, being respectively amino- 

 purin and amino-oxypurin (cf. p. 50). In some cases, guanin alone 

 is formed. Probably the ingested nuclein is broken down by the 

 nuc lease of the pancreatic juice to nucleic acid and protein. The 

 nucleic acid thus formed is absorbed into the blood, perhaps by the 

 action of the pale corpuscles, and taken to tissues, where it is further 

 acted upon, particularly in tissues such as the spleen, liver, and 

 thymus, which contain enzymes capable of breaking it down into the 

 purin bodies adenin and guanin, phosphoric acid, carbohydrate and 

 pyrimidin bases. The adenin and guanin thus formed are, by the 



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