2 THE PROTEIN ELEMENT IN NUTRITION 



connected with protein metabolism, remarkable advances have 

 been recorded, particularly with regard to the nature and 

 chemical constitution of the proteins and their cleavage products. 



By the process of breaking down the large complex protein 

 molecule into smaller, less complex bodies or units, and by the 

 method of building together again such cleavage products into 

 substances in many respects similar to the ordinary nitrogenous 

 compounds, much light has been thrown on the nature of proteins 

 and on the arrangement of their molecules. The main results of 

 these investigations are to show that the protein molecule is built 

 up of a series of units or organic radicles the amino-acids 

 which form the basis of their composition. These amino-acids 

 may be looked upon as* the building-stones or units of the protein 

 molecule, and it is in virtue of their presence that the protein 

 molecule possesses the peculiar property of building itself up 

 into bodies of high molecular weights. The results obtained by 

 means of hydrolytic agents on the different proteins show defi- 

 nitely that they are all composed of the same units ; in some 

 cases certain organic radicles or building-stones are missing, and 

 in some cases certain of these building-stones are greatly in excess. 



The importance of these discoveries can only be grasped when 

 it is borne in mind that the old views regarding the changes in 

 protein material during digestion are rapidly undergoing modifica- 

 tion. The fact that proteins are an essential part of the food of 

 all animals was interpreted to mean that no synthesis of proteins 

 occurred in the animal body, so that all proteins in existence 

 must necessarily be the products of the vegetable cell. In fact, 

 on this basis a fundamental distinction was drawn between the 

 nutrition of plants and animals. The long dominant doctrine 

 of Liebig, that plants in the main live without protein food, and 

 are able to construct their own proteins for themselves, whilst 

 animals have to find their proteins ready-made in their food and 

 are unable to build them up, no longer holds good. The further 

 the study of the nature and constitution of the different proteins 

 advanced, the more clearly evident did it appear that animal and 

 plant proteins were different in type ; the investigations in the 

 science of serology would even make it practically certain that 

 the protein of one species of animal is quite different from that 

 of any other species, and even the proteins of the different organs 

 of animals of the same species differ from one another. 



At present our conception of the effects of the digestive enzymes 



