4 THE PROTEIN ELEMENT IN NUTRITION 



by hydrolysis, which in different combinations and in different 

 proportions are found in the proteins of the different tissues, 

 these different combinations and proportions conferring on the 

 nitrogenous material of the several organs and tissues a specific 

 character. 



But while the cleavage products of the intestinal digestion of 

 proteins are able to maintain an animal in nitrogenous equilibrium, 

 those formed by the hydrolytic action of mineral acids are quite 

 unable to do so ; animals fed on these substances live no longer 

 than other animals whose food contained no nitrogenous materials 

 at all. There is therefore some difference in the hydrolytic action 

 of the digestive enzymes as contrasted with that of mineral acids, 

 although the products obtained by the action of these two hydro- 

 lyzing agents are to a large extent identical. However, it is 

 evident that the acid breaks up some building-stone or stones 

 which the digestive ferments retain, and which are absolutely 

 essential in the synthesis and nutrition of the bioplasm. What 

 the nature of those organic radicles may be whose presence are of 

 such importance in the assimilation of the products of tryptic 

 digestion, at present it is impossible to say. It may be, as 

 Leathes suggests, that there is some kind of linkage between 

 certain groups in the protein molecules which is not broken down 

 by the digestive enzymes, but is in acid hydrolysis. When this 

 coupling is destroyed, it is impossible for the cells of the body 

 to make use of these groups, and successfully resynthesize them 

 into materials necessary for the nutrition of their bioplasm. 



The study, therefore, of the intermediate stages in the processes 

 of protein metabolism has openedup a field for research, which has 

 been and is being successfully cultivated. As pointed out above, 

 the researches of Fischer, Kossel, Schutzenberg, and a host of 

 others have thrown a flood of light on the composition of the 

 protein molecules. Protein has been shown to consist of a com- 

 plicated chain of units capable of being broken down into bodies 

 of gradually decreasing complexity. Thus, the first real decom- 

 position products of acid or enzyme proteolysis are the albumoses 

 which exhibit distinct characteristics, and are easily isolated by 

 precipitation with ammonium sulphate. The next products the 

 peptones are less definitely characterized, and are at present 

 identified by one positive reaction the biuret test. Fischer has 

 applied the term " peptides " to a number of bodies which are 

 derived from peptones, but which no longer give the biuret test. 



