GENERAL METHODS HISTORY OF PROTEIN FOOD. 903 



use of rats, which were fed with a suitable mixture of inorganic 

 salts, carbohydrate, fats, and some single protein representing the 

 sole form in which nitrogenous material was supplied. They 

 found that in addition to gelatin the protein zein obtained from 

 maize, which is deficient in the amino-acids, tryptophan, lysin, and 

 glycin, is also an inadequate or incomplete protein. Gliadin and 

 hordein which are alcohol soluble proteins obtained from wheat, 

 rye, and barley when fed alone sufficed for maintenance, but not for 

 growth. A young rat fed upon gliadin alone ceased to grow, but 

 did not lose in weight, although the power of growth was not lost, 

 since at any time satisfactory growth could be re-established by 

 substituting a suitable dietary for the gliadin mixture. By the 

 same method it was shown that the leguminous proteins when 

 fed alone are inadequate for growth, although in this case appa- 

 rently the defect is not due to lack of necessary amino-acids, but 

 to some other undetermined cause. There are, however, many 

 single proteins which, when fed alone, seem to be entirely sufficient 

 to provide all the necessary nitrogenous material for maintenance 

 and growth. Such proteins are, for example, casein from milk, 

 edestin from hemp seed, glutein from wheat, lactalbumin from 

 milk, vitellin, etc. Experiments of this kind bring out in a decisive 

 way the important fundamental fact that proteins differ among 

 themselves in regard to their utility in furnishing material for cell 

 repair and growth, and the evidence at hand goes to show that the 

 qualitative differences are dependent upon variations in the amino- 

 acids of which they are composed. A protein-like zein is inade- 

 quate because it is lacking in certain essential amino-acids, namely, 

 tryptophan and lysin. Gliadin and hordein are also inadequate, 

 so far as growth is concerned, because they lack lysin. If the miss- 

 ing amino-acids are supplied, either directly or by using other pro- 

 teins, then the functions of maintenance and growth are adequately 

 supported. In an ordinary diet we do not, of course, use single 

 proteins. Our foods contain a variety of proteins, and it is not at all 

 probable that a natural diet would be lacking entirely in any of the 

 essential building-stones. Work of the kind described above helps 

 to bring out important data in regard to the particular role played 

 by the different amino-acids. In time, no doubt, it will be possible 

 to describe more or less completely the transformations that each 

 of these substances undergoes in the body and the special reactions 

 that may depend upon certain ones of them. At present the most 

 evident outcome of the work reported seems to be that some of 

 these amino-acids, glycin, for example, may be formed de novo in the 

 animal body by synthetic processes or by transmutation of other 

 amino-acids, while others, such as tryptophan, lysin, and probably 

 tyrosin, cannot be so manufactured, but have to be supplied from 



