RELATIVE VALUES OF PROTEINS. 105 



Relative nitrogen excretion on different proteins. 





Series I. 



Series II. 



Average. 





101 

 100 

 91 



108 

 100 

 120 



104 

 100 

 104 





Wheat gluten 





Compared in this way, the differences disclosed between the different pro- 

 teins are small in themselves, and, especially in the case of the wheat gluten, 

 are discordant in the two series. Apparently the differences are less than those 

 which may be plausibly ascribed to variations in the conditions of the several 

 experiments. The latter may be to some degree eliminated by comparing the 

 total nitrogen excretion with the fasting nitrogen katabolism of the corre- 

 sponding periods. If the latter be represented by 100, the relative nitrogen 

 excretion on the several proteins was as follows : 





Series I. 



Series II. 



Average. 



Casein 



118.1 

 109.0 

 108.3 



120.0 

 105.4 

 126.5 



119.1 

 107.2 

 117.4 



Muscle protein 



Wheat gluten 





This second method of comparison seems to indicate a distinct, although 

 small, inferiority of the casein as compared with the muscle protein. The 

 same is true of the average result with wheat gluten, but not of the results 

 of the individual series. Entirely similar results are obtained if the calcula- 

 tion is made only upon the protein nitrogen of the feed and excreta instead of 

 the total nitrogen. Zisterer's results are, of course, open to the same criticism 

 made on Michaud's, viz, that the so-called muscle protein was not comparable 

 with the pure proteins used in the other periods. 



RESULTS ABE QUALITATIVE. 



Both Michaud's and Zisterer's results are in a sense qualitative. They show 

 that certain foreign proteins when substituted for tissue caused a relatively 

 greater nitrogen excretion and were therefore less efficient in maintaining the 

 nitrogen balance of the body. For gliadin and edestin, Michaud observed a nota- 

 bly greater difference than did Zisterer for wheat gluten. For casein their results 

 are quite similar. In no case was the amount of foreign protein required to 

 reach nitrogen equilibrium determined, with the exception of one short period 

 upon gliadin in Michaud's experiments. In both cases, the differences appear 

 relatively small. On the basis of average figures for the proportions of four of 

 the principal amino-acids in the different proteins, Zisterer computes much 

 greater possible differences. Representing the amount of muscle protein re- 

 quired to furnish a given amount of each one of the four amino-acids by 100, 

 Zisterer calculates that .the following amounts of casein and of wheat gluten 

 would be required for the same purpose : 





Muscle 

 protein. 



Casein. 



Wheat 

 gluten. 



To furnish equal amounts of : 



Alanin 



100 

 100 

 100 

 100 



444 

 74 



124 

 47 



267 

 154 

 49 

 102 



Leucin 



Glutamic acid 



Tyrosin , 





