RELATIVE, VALUES OF PROTEINS. 



109 



seems to render impossible a complete substitution for tissue pro- 

 tein, while differences in the proportions of the different amino 

 acids apparently result in differences in the replacement values of 

 the proteins, although these differences, especially in the experiments 

 of Michaud and Zisterer, are hardly as great as might have been 

 expected. What now can be said regarding the probable significance 

 of these differences for the ordinary problems of nutrition ? 



In the first place, it is to be remarked that both man and animals 

 consume a mixture of proteins. The meat eater gets, along with 

 his gelatin, the various muscle proteins. The animal fed on maize 

 alone receives not only zein but its associated proteins, amounting, 

 according to Osborne, 1 to about 40 per cent of the total protein of 

 the grain, whose chemical constitution has not yet been reported. 

 In the ordinary mixed rations of domestic animals it would appear 

 that there must be a considerable degree of compensation between 

 the different proteins as regards the proportions of the different 

 cleavage products supplied to the organism, although it is difficult 

 to judge to what extent this is the case. In view, however, of the 

 rather small differences observed with pure proteins, it may be 

 questioned whether such differences as exist in mixed rations are of 

 very much significance. 



In the second place, the observed differences in proteins were ob- 

 tained in experiments in which small amounts of protein were con- 

 sumed and in which the animals were on a low level of protein 

 nutrition. As was pointed out in the discussion of those experiments, 

 the consumption of protein in excess of the maintenance requirement, 

 such as usually occurs with domestic animals, tends to obscure the 

 differences between the proteins, owing to the considerable extent to 

 which protein serves for fuel purposes under those conditions. 



Third, almost all writers upon this subject tacitly assume the in- 

 ability of the body to change one amino- acid into another. It does 

 not appear that there is adequate proof of this inability. Most of 

 the amino acids concerned belong to the aliphatic series of com- 

 pounds, characterized by a straight carbon chain, and as between 

 these compounds, at least, mutual changes are not difficult to conceive. 

 As a matter of fact one such change appears to have been dem- 

 onstrated. It is well known that when benzoic acid is consumed it 

 is paired in the body with glycocol, forming hippuric acid which is 

 excreted. It seems to be well established that with large amounts of 

 benzoic acid more combined glycocol may appear in the excreta than 

 can be assumed to have been present as such in the amount of protein 

 katabolized during the same time. In this case, apparently, the body 

 is able to manufacture glycocol from some other substance, pre- 



1 Journal of the American Chemical Society, vol. 19, p. 532. 



