METABOLISM 203 



sized from the food protein is always the same. The selective 

 action of the digestive enzymes in not hydrolyzing certain poly- 

 peptides would be in accord with such results. The animal 

 body in its digestive action upon proteins would retain certain 

 polypeptides in an unhydrolyzed form, while all others would 

 be completely hydrolyzed to amino-acids. These unhydrolyzed 

 polypeptides would then act as a nucleus with which the various 

 amino-acids could combine to synthesize a protein of uniform 

 character, viz. the serum albumin. 



Body Protein. — What now is the fate of the synthesized 

 serum albumin of the blood? We know that the hlood protein 

 synthesized from the absorbed products oi food protein is built 

 up in the muscle cells into body protein, but we do not know the 

 different steps in this metabolic change. It is important here 

 to emphasize the fact that under normal conditions body pro- 

 tein can be formed, either directly or indirectly, only from food 

 protein. Unlike plants, animals are unable to use other nitrogen 

 compounds together with carbohydrates to synthesize body pro- 

 tein. It is true that for a short time the animal body can live 

 with a protein free diet, but this is always at the expense of 

 body protein, and any formation of body protein from carbohy- 

 drates can only take place when protein residues, probably 

 polypeptides and amino-acids obtained from the katabolism of 

 body protein, are present to combine with carbohydrate to form 

 new body protein. Under such conditions body protein is 

 gradually lost and protein katabolism grows gradually less and 

 less until life ceases. Food proteins in some form, or com- 

 pounds analogous to them, such as certain combinations of 

 amino-acids or polypeptides or both, and these only under 

 limited conditions, are absolutely essential to the formation of 

 body protein and the existence of animal life. 



Oxidation of Protein. — We do not know certainly whether 

 all food protein first converted into blood protein is built up 

 into body protein and then this body protein torn down and 

 oxidized, or whether only part of it forms body protein while 

 another part is oxidized without ever having been converted 



