METABOLISM 181 



of proteins in the tissues is not general, but for the purpose of 

 supplying some amino acid required for the manufacture of a 

 necessary substance. Otherwise, it would be difficult to under- 

 stand why the body can build enough protein to repair that 

 broken down, but no more for growth unless lysine also is fed. 



The problem of protein requirement thus is resolved into a 

 problem of amino acid requirement, limited on the one hand by 

 the body's needs, and on the other hand by its ability to con- 

 struct amino acids itself. 



Since amino acids are the real requirement of the body, one 

 would expect it to be possible to supply an animal's protein re- 

 quirement solely by a mixture of amino acids. Such experiments 

 have been attempted and their results point in general to this 

 conclusion. It may be noted, however, that the difficulty of 

 experiments of this sort is much increased by the unpalatable 

 nature of such a mixture, so that positive results have not been 

 always the rule. Dogs fed on such mixtures often refuse to eat, 

 and if fed by a stomach tube, frequently vomit. In a German 

 laboratory, an attempt by one of the assistants to live for a 

 period upon a mixture of amino acids and the building stones 

 of the fats and carbohydrates as well, was abandoned after the 

 first attempted meal as a result of the unappetizing nature of 

 the mixture. By injection of protein decomposition products 

 into the blood, it has been possible to maintain an animal on 

 nitrogen equilibrium. 



The role played by the individual amino acids in the organism 

 is still obscure. Probably certain of them are used as repair 

 material for the tissue proteins. Others may be employed for the 

 manufacture of important products of internal secretion, whereas 

 excess of these, and such acids as are not required, undoubtedly 

 are deaminized and the residues either used for constructing 

 other compounds, or burned as fuel, as are the fats and car- 

 bohydrates. 



The composition of the proteins of the tissues is remarkably 

 constant, and quite independent of the nature of the food pro- 

 tein. A horse was bled to remove much blood protein. This 



