1921] on Quality of Protein in Nutrition 345 



acid would express a difference. We may imagine the proteins of 

 the blood or milk of different species to differ in the arrangements of 

 the units. The one may have the arrangement such as a-b-c-d-e-f-, 

 the other d-a-b-f-e-c- 



Another important difference may be in the so-called tautomerism 

 of the amino acids and polypeptides. With the same arrangement of 

 the amino acids we may have several formulae representing the poly- 

 peptide structure : — 



H 2 N.CH 2 .COOH 

 H.N . GH 2 . CO - HN . CH, . CO - HN . GH 2 . COOH 

 H;N.CH 2 .CO-HN.CH 2 .C(OH):N.CH 2 .COOH 

 H 2 N . CH 2 . C(OH) : N . CH 2 . CO - HN . CH 2 . COOH 

 H 2 N.CH 2 .C(OH):N.CH 2 .C(OH):N.CH 2 .COOH 



H,N.CH„.COO 



H 3 N.CH 2 .CO-HN.CH 2 .CO-HN.CH 2 .COO 

 H 3 N.CH 2 .CO-HN.CH 2 .C(OH):N.CH 2 .COO 

 H 3 N.CH 2 .C(OH):N.CH 2 .CO-HN.CH 2 .COO 

 H 3 N . CH 2 . C(OH) : N . CH 2 . C(OH) : N . CH 2 . COO 



Certain of the properties of the polypeptides can be explained on 

 this basis. 



Fischer's and Kossel's work has revolutionized our conception 

 of protein nutrition. We no longer think, like Liebig and later 

 investigators, that the protein of the food becomes directly the 

 protein of the body, for it has been demonstrated by the physiologists 

 that the protein of the food undergoes hydrolysis during digestion to 

 amino acids, that the amino acids circulate in the blood, and that the 

 tissues receive amino acids from which they build up their protein. 

 Proteins must be regarded as a mixture of amino acids. 



We can look upon a protein as we look upon the contents of a 

 box of assorted biscuits, arranged in rows and in layers of various 

 kinds. Each biscuit should be connected to its neighbour so that we 

 have a continuous chain. The general appearance of the contents of 

 two boxes is different ; in one case we may find sugary biscuits on 

 the top, in another case plain ones. 



In the process of digestion the protein is acted upon by acid in 

 the stomach with the formation of metaprotein. No great chemical 

 change occurs, but we can imagine that the change consists in a 

 tautomeric rearrangement in preparation for the action of the pepsin. 

 Pepsin hydrolyses the protein at certain junctions, forming proteoses 

 and peptones. Their formation can be compared to the separation 

 of the layers of the biscuits. Pancreatic and the further digestion 

 which follow in the intestine separate the individual amino acids or 



