298 PRACTICAL PHYSIOLOGY 



Recent research has shown that nitrogen may be detected in the protein 

 molecule after complete hydrolysis with 30 % sulphuric acid as : 



(1) Amide or Ammonia Nitrogen. This is the loosely combined nitrogen 

 mentioned above. 



(2) Diamino Nitrogen, precipitated by phosphotungstic acid after the amide N 

 has been removed. 



(3) Monamino Nitrogen estimated in the residuum when the above two have 

 been removed. 



By this means the difference in composition of proteins is further brought out : 



Total N. Amide N. Diamino N. Monamino N. 



Caseinogen, - - - 15*62 1-61 3'49 10'31 



Edestin (from hemp), - IS'64 1'88 5 '91 10'78 



Gliadin (from wheat), - 17 '66 4-20 '98 12'41 



Glutenin (from wheat), 17 '49 3'30 2'05 11 '95 



Zein (from maize), - 16 '13 2 -97 '49 12-51 



(T. B. Osborne.) 



The complex constitution has been studied recently in two ways 

 (I) by working out the products of hydrolysis (breaking down) of the 

 different proteins; (2) by endeavouring to link together simple cleavage 

 products, and thereby produce some form of protein. As the result of 

 the first method of procedure, we now know that the proteins of the 

 various parts of the body 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; likewise, 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. From these end products chiefly those are required which are 

 of value in building up the animal's own particular forms of protein, 

 forms, as we have seen, differing in various parts of the body, and 

 wholly different from the protein ingested. It is only, therefore, by 

 very complete hydrolysis, that these valuable end products can be 

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



In the following table will be seen the varying yields of the different 

 amino acids obtained from 100 parts of various proteins, after complete 

 hydrolysis with hydrochloric or sulphuric acid. Tyrosin and cystin 

 are separated by crystallisation, after neutralising and concentrating 

 the liquid. The diamino acids arginin, histidin, lysin are separated 

 from the rest of the products by being precipitated by phosphotungstic 

 acid in acid solution. Tryptophan is separated by precipitation with 

 mercuric sulphate in the presence of 5 % sulphuric acid after tryptic 

 digestion. The other amino acids are separated (after hydrolysis of 



