594 



METABOLISM, NUTRITION AND DIETETICS 



origin has been previously alluded to (p. 481) namely, the methyl 

 purins forming the active "principles of tea, coffee, and cocoa, caffein, 

 or i, 3, 7-trimethylxanthin (C 8 H 10 N 4 O 2 ), theobromin, or 3, y-dimethyl- 

 xanthin (C 7 H 8 N 4 O 2 ), and theophyllin, or i, 3 - dimethylxanthin 

 (C 6 H 8 N 4 2 ). 



CH 3 .N CO 



CO C N.CH 3 



> H 



CH,.N C N 



Caffein. 



NH CO 



:o 



CH 3 .N C 



Theobromin. 



CH 3 .N CO 



<u 



CO C NH 



CH 3 .N C N 



Theophyllin. 



Nucleic acid, as stated, can be partially decomposed by the 

 succus entericus, by means of a ferment called nuclease or, more 

 accurately, nucleic-acidase. The groups into which it is split 

 are nucleotids (see above). By another ferment, nucleotidase, a 

 portion, at any rate, of the nucleotids is further decomposed to 

 yield nucleosides, bodies of the glucoside class containing a com- 

 pound of a purin base with the carbo-hydrate group of the nucleic 

 acid, to which phosphoric acid is also coupled. Beyond this stage 

 the hydrolysis of nucleic acid does not proceed in the intestine. 

 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 of 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. 425). 

 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 



