alsberg: biochemical analysis of nutrition 271 



was so complete that no trace of any reaction for protein could 

 be obtained. This predigested material as the sole nitrogenous 

 element of the diet was then fed to dogs and it was found that 

 upon such a diet the animals maintained their weight. These 

 results were remarkable because they tended to show that it 

 was not absolutely necessary to life that protein be an element 

 of the diet. For a time these findings were ignored, but it has 

 since been shown that life can be supported for a time at least 

 upon a diet containing no complexes known as protein, but 

 instead a suitable mixture of amino-acids. 



For the information of those of you who have not followed 

 the chemistry of the proteins during recent years I may say 

 that the proteins are combinations of the amino-acids, by which 

 we mean ordinary organic acids in which one or two hydrogen 

 atoms have been replaced by the amino group, NH 2 . The 

 simplest amino-acid found in protein is glycine which, as you 

 see, is derived from acetic acid. 



NH 2 



I II 

 H— C— C— OH 



I 

 H 



There are seventeen of these amino-acids commonly found 

 in proteins. Several others have been reported more or less 

 definitely. Perhaps the commonest one is leucine, which has 

 six carbon atoms. 



O NH 2 H H H 



II I I I I 

 HO— C— C— C— C— C— H 



I I I I 

 H H H 



H— C— H 



I 



. H 



As you see, in all these substances the amino group is associated 

 with the carbon atom adjacent to a carboxyl, COOH, group, the 

 alpha position, as it is known technically. When two amino 



