THE PEOTEINS 



47 



NUCLEO-PROTEIN 



subjected to gastric digestion yields 



I 



i 



Protein converted into 

 peptone, which goes 

 into solution. 



Nuclein, which remains as an insoluble 

 residue. If this is dissolved in alkali 

 and then hydrochloric acid added it 

 yields 



i 



Protein converted into 

 acid albumin in solu- 

 tion. 



i 



A precipitate consisting of nucleic acid. 

 If this is heated in a sealed tube with 

 hydrochloric acid, it yields a number 

 of substances. But the best known 

 and constant products of its decom- 

 position are 



Phosphoric acid. 



i 



Purine bases, viz. 

 Adenine 

 Hypoxanthine 

 Guanine 

 Xanthine. 



Protein-hydrolysis 



Pyrimidine 

 Uracil 



Cytosine 

 Thymine. 



, viz. 



When protein material is subjected to hydrolysis, as it is when 

 heated with mineral acid, or superheated steam, or to the action of 

 such ferments as pepsin or trypsin in the alimentary canal, it is 

 finally resolved into the numerous amino-acids of which it is built. 

 But before this ultimate stage is reached, it is split into substances of 

 progressively diminishing molecular size, which still retain many of 

 the protein characters. These may be classified in order of formation 

 as follows : 



1. Infra-proteins. 



2. Proteoses. 



3. Peptones. 



4. Polypeptides. 



The polypeptides are linkages of two or more amino-acids as already 

 explained. They do not give the biuret reaction. Although most 

 of the polypeptides at present known are products of laboratory 

 synthesis, some have been definitely separated from the digestion of 



