584 METABOLISM, NUTRITION AND DIETETICS 



The resultant products, probably along with unchanged nucleic 

 acid, are absorbed, mainly at least, by way of the bloodvessels. 



It will be well, however, to remember that our knowledge of 

 the digestion of the nuclein bodies is still incomplete, and the 

 natural tendency qf the mind to think in diagrams is apt to give it 

 greater precision than is justified by the facts; for example, it is 

 known that even gastric juice is capable of liberating some of the 

 phosphoric acid from nucleo-proteins. 



In the tissues the absorbed products of the digestion of nucleic 

 acids may be partially utilized without further decomposition for 

 the synthesis of nucleo-proteins, to take the place of those which are 

 destroyed in the metabolism of the cells; or they may be split com- 

 pletely into their components, and these resynthesized. Finally, 

 and this fate is probably not long delayed in the case of the surplus 

 of purin compounds contained in ordinary dietaries, both the purins 

 of the food and the purins arising from the waste of the tissues are 

 for the most part converted into uric acid and excreted in the urine. 

 Small quantities of purins leave the body in the faeces (p. 419). 

 The phosphoric acid can be utilized not only for the building of 

 nucleo-proteins, but for the synthesis of phosphatides. Eventually 

 it is eliminated as phosphates in the urine. The carbo-hydrate 

 groups, so far as they are not utilized in the synthesis of nucleic 

 acids, may be supposed to undergo metabolism like other carbo- 

 hydrates. The metabolic history of the pyrimidin bases has not 

 been made clear. 



Steps in Formation of Uric Acid. As to the manner in which 

 uric acid arises from the nuclein substances, we may picture the 

 process as taking place by the following steps : Certain organs have 

 been shown to contain ferments which split up nucleo-proteins into 

 protein and nucleic acid. This nucleic acid, or nucleic acid arising 

 in other ways in the metabolism of nuclein, and also any nucleic 

 acid absorbed as such from the alimentary canal in the digestion of 

 nuclein-containing substances, are then decomposed by another 

 ferment, similar to or identical with the nuclease or nucleic-acidase 

 previously encountered in the intestine. The resulting nucleotids are 

 split up by a special ferment (nucleotidase) so as to yield nucleosides. 

 These are in turn decomposed by appropriate enzymes (nucleo- 

 sidases), so that we finally arrive at the individual ' building-stones,' 

 the nucleic acid molecule, phosphoric acid, the carbo-hydrate group, 

 pyrimidin and purin bases, especially adenin and guanin. Then 

 follows the action of ferments (adenase and guanase), which remove 

 the amino-group from these purin bases, transforming adenin into 

 hypoxanthin, and guanin into xanthin (Jones). The deaminiza- 

 tion is associated with hydrolysis. Thus : 



C 5 H 5 N 5 + H 2 =C 5 H 4 N 4 + NH 3 ; C 5 H 5 N 5 O + H 2 O=C 5 H 4 N 4 



Adenin. Hypoxanthin. Guanin. Xanthin. 



