THE PROTEINS 45 



As a result of this hydrolytic method of procedure, we now know 

 that the proteins differ greatly in composition; for example, the 

 protein of the spleen is different from that of the thymus or of the 

 pancreas. Further, the protein of the same tissue differs in animals 

 of different species e.g., the serum albumin of the blood of one animal 

 has a different constitution to the serum albumin of an animal of 

 another species; the same is true of the chief protein (caseinogen) of 

 milk. We can understand, therefore, why it is that the proteins of 

 the food have to be broken down into such numerous end products 

 in the digestive tract; of these end products those are selected 

 which are of value in building up the animal's own particular forms 

 of protein, forms which differ in various parts of the body and 

 differ from the protein ingested. It is only by a very complete hydro- 

 lysis that the particular valuable end products can be obtained 

 free from products of lesser value (see also under Digestion). 



The results of the second procedure, the synthetic, have been 

 highly interesting. Starting with a simple end product such as 

 glycin, two of its molecules have been combined together, forming a 

 dipeptide, glycyl-glycin, with the elimination of water, thus : 



OH H 

 NHjCItjCO NHCHjCOOH = NHaCH^O.NHCH.^COOH + HjO 



Glycin 4- Glycin Glycyl-glycin + Water 



The addition of another molecule forms a tripeptide, and so on until 

 polypeptides are formed. Penta-glycl-glycin, for example, is 



NH 2 CH 2 CO(NHCH 2 CO) 4 NHCH 2 COOH 



Not only has glycin been combined with glycin ; other end 

 products, such as alanin, leucin, tyrosin, have been combined together. 

 An example of such is the polypeptide (do-dekapeptide) leucyl-deka- 



G 4 H 9 CH(NH 2 )CO 

 Leucyl 



(NHCH 2 CO) 10 

 Glycyl 



NHCH 2 COOH 



Glycin 



By many such operations polypeptides have been obtained, which, 

 if not actually having the same composition, have many resemblances 

 to peptones. 



SECTION II. 

 The Physical and Chemical Properties of Proteins. 



The proteins possess certain well-marked physical and chemical 

 properties. 



1. All proteins, with the exception of a few vegetable proteins, 

 are insoluble in alcohol and ether. They vary as to their solubility 

 in water, the more common proteins (albumins and globulins) being 

 either soluble in water (albumins) or soluble in weak saline solutions 

 (globulins). 



