820 PHYSIOLOGY 



fatty acid) necessary to the building up of the tissue proteins. The nucleins, 

 on the other hand, can .certainly be synthetised by the animal. This is 

 shown by the fact that the hens' egg before incubation contains practically 

 no nuclein or purine bases. During incubation tissues are formed, and 

 there is a- rapid increase in the number of nuclei, so that the chick just 

 before it is hatched contains a considerable amount of nuclein from which 

 purine bases can be extracted. This nuclein must have been formed by, a 

 synthesis from the phospho-proteins and phosphatides (phosphorised fats) 

 which form so important a constituent of the egg-yolk, and in the same way 

 the purines must have been formed by a process of synthesis. This syn- 

 thesis may occur by a conjugation of two urea molecules with the 3-carbon 

 chain which is so prominent a feature in the proximate principles of the 

 body (e.g. in lactic acid, alanine, and all the compound amino-acids of 

 which alanine is a constituent). Methyliminazol, representing one-half of 

 the purine ring, can be formed simply by allowing ammonia and glucose to 

 stand in contact with zinc hydroxide. The power of synthesis of purines 

 possessed by the body must complicate the question of their fate after 

 ingestion, since it is evident either that they can be destroyed and excreted 

 in some other form or that the products of their destruction may be built 

 up into fresh purine or nuclein molecules. In the same way, in the growing 

 child there is a rapid increase in the nuclein of the body, although the only 

 food ingested is milk, which contains but an insignificant amount of nuclein. 



FATE OF NUCLEINS IN THE BODY 



Nucleins and nucleic acids are dissolved by the pancreatic juice, but no 

 digestion of the nucleic acid occurs in the alimentary tract other than by 

 the action of micro-organisms. We must assume therefore that the* nucleic 

 acid is taken up by the cells of the intestinal wall unchanged. 



Ingestion of nucleic acid is, in man, followed by an increased excretion of 

 uric acid in the urine, so that we regard this substance- as the end-product 

 of nuclein metabolism in the body. It is evident that the uric acid of the 

 urine may be derived either from the nucleins of the food or from the 

 nucleins of the tissues of the body, the uric acid in these two cases being 

 spoken of as exogenous and endogenous respectively. By digestion of 

 nucleic acids with animal tissues or extracts of animal tissues under varying 

 conditions, it is possible to bring about all the changes involved in the 

 conversion of the purine bases contained in them into uric acid. In the 

 intestinal wall, or after absorption into other tissues of the body, the nucleic 

 acid is subjected to hydrolytic changes by the agency of ferments which 

 may be classed as nucleases. These are however of different kinds, the 

 phosphonuclease splitting off the phosphoric acid and leaving the nucleo- 

 sides, while the purine nucleases, which are more effective in a slightly 

 alkaline medium, split off the purines, leaving the phosphoric acid com- 

 bined with the carbohydrate. The purines set free in this way undergo 

 further changes. The hypoxanthin derived from inosinic acid is con von <1 

 under the action of an oxidase first into xan thine and then into uric acid. 



