104 PHYSIOLOGY 



THYMINE is 5-methyl uracil, NH CO 



CO C.CH 3 



I II 

 NH CH 



while CYTOSINE is 6-amino-2-oxypyrimidine, 



N =C.NH 2 



CO CH 



I II 

 NH CH 



Besides these two groups of nitrogenous compounds derived from the 

 purine and pyrimidine rings, many nucleic acids yield on hydrolysis a carbo- 

 hydrate. Thus, Hammarsten has isolated a pentose, from the nucleo- 

 proteins of the pancreas. It is supposed that the nucleic acid of the thymus 

 gland contains a hexose, since it is possible to split off from it laevulinic acid, 

 which is one of the first products of the decomposition of a hexose. The com- 

 plex constitution of the nucleic acids and nucleoproteins may be rendered 

 clearer from the following schema : 



Nucleo -protein 

 on digestion yields ^ ^^^ 



nuclein proteoses and peptones 

 dissolved in alkali and precipitated 

 with hydrochloric acid yields 



nucleic acid acid derivatives of protein, histones 



or protamines 

 hydrolysed yields 



cytosine 



It must not be imagined, however, that all these disintegration products 

 are present in all nucleic acids. Thus the nucleic acid derived from the 

 pancreas, the so-called guanylic acid, yields of the purine bases only guanine, 

 and of the pyrimidine bases only thymine and uracil, and every variety is 

 met with as we analyse the nucleic acids of different origin. The fact that 

 nucleic acid is a characteristic and necessary constituent of all nuclei adds 

 interest to the divergence of its constituent radicals from those which dis- 

 tinguish the proteins of the cell protoplasm. Further importance is lent to 

 this section of the chemistry of the body by the close relationship which we 

 shall have to study later between the nuclein metabolism of the body and 

 the production and excretion of uric acid. 



(c) THE GLYCOPROTEINS. In the glycoproteins the prosthetic group is 

 represented by a carbohydrate radical, generally containing nitrogen, such 

 as glucosamine or galactosamine. They are split into their two constituents, 



