THE PROTEINS AND THEIR METABOLISM 125 



DOG WAS FED 25 GRAMS OF THE AMINO ACID MIXTURES AS THE SOLE SOURCE OF NITROGEN 

 SUPPLY. EXPERIMENT PROVES THAT NITROGENOUS EQUILIBRIUM AND BODY WEIGHT 

 CAN BE MAINTAINED ON IT 



These experiments are of the utmost importance because they show 

 the value of tryptophan in the physiological economy. They prove defi- 

 nitely that if an animal is kept on a diet free from tryptophan, the body 

 has to burn its own protein to supply tryptophan to the cells that require 

 it. (See the relationship between tryptophan and thyroxin, the active 

 principle of the thyroid secretion, page 115.) 



The proteins that do not contain all the indispensable amino acids are 

 designated incomplete proteins, and the above experiment shows that a 

 complete protein like casein can be made incomplete and cause it to be a, 

 non-sustainer of nitrogenous equilibrium by merely removing the trypto- 

 phan. 



The study of the physiological values of the incomplete proteins and 

 the influence of the individual amino acids have been carried on in- 

 tensively for the past fifteen years. 



In 1907 Hopkins and Willcock published a series of experiments on 

 mice. They fed mice on a diet in which all the protein was supplied in 

 the form of zein, a protein derived from maize, containing neither lysin 

 nor tryptophan. The zein was mixed with carbohydrates, fats, lecithin 

 and salts. In the first series of experiments five young mice were kept on 

 this diet for seven days. On the seventh day they all showed the follow- 

 ing losses in weight in per cent: 11.8, 17.6, 13.1, 23.2, 27.1. 



As a control, four mice were kept on a similar diet, but the zein was 

 replaced by a similar quantity of casein. On the seventh day the following 

 increases in weight in per cent were recorded: 20.2, 21.8, 9.1, 21.0. 



One of the mice of the first series was then given half of its protein 

 in the form of zein and the other half in the form of casein, and it promptly 

 began to gain in weight. After fifteen days it gained in weight to the 

 extent of 46 per cent. 



