SOME NEW VIEW POINTS IN NUTRITION 395 



enzymes is, however, in some measure peculiar, in that the fragments 

 are not wholly akin to those formed by hydrolysis with acids. When 

 a given protein is boiled with a dilute acid it is speedily broken down 

 and many of the fragments are identical with those produced by the 

 action of trypsin and erepsin. Thus tyrosine, leucine, arginine, lysine, 

 histidine, etc., appear in both cases, but the enzymes presumably leave 

 intact certain groups or combinations which acids break up or in some 

 way modify. As a result, it is found that the cleavage products formed 

 from casein, for example, by acids, will not take the place of casein, 

 or the products formed by trypsin proteolysis, in meeting the needs of 

 the body. On a diet of protein cleavage products formed in this man- 

 ner the animal steadily loses nitrogen; it is impossible to maintain a 

 condition of nitrogenous equilibrium, since for some reason the tissue 

 cells can not synthesize protein from the mixture of fragments pro- 

 duced by acids. We may conjecture that while in acid hydrolysis the 

 products are all simple amino-acids, in enzymolysis combinations of the 

 amino-acids, *. e., polypeptides, remain intact. There is experimental 

 evidence that such is actually the case, but equally good evidence seems 

 to show that the presence of polypeptides is not essential for the syn- 

 thesis of protein by the animal body. Thus, Henriques and Hansen 

 found that by treating the mixed products of pancreatic digestion with 

 phosphotungstic acid, which precipitates, so far as at present known, 

 all the basic bodies including polypeptides, the mixture of monoamino- 

 acids and possibly other nitrogenous substances contained in the filtrate 

 is apparently able when fed to keep animals in a condition of nitrogen 

 equilibrium. Further, the same investigators found, on treating the 

 nitrogenous products (free from biuret reaction) of pancreatic diges- 

 tion with strong alcohol, thereby separating the substances into two 

 portions, the alcohol-soluble part was quite able to maintain animals 

 in nitrogen equilibrium, i. e., it was equivalent in action to the original 

 protein, while the portion insoluble in alcohol was wholly ineffective. 

 It is thus apparent that all of the fragments resulting from proteolysis 

 are not needed for the synthesis of protein; there are apparently cer- 

 tain products that are not essential or not immediately necessary. On 

 the other hand, it is equally apparent that in the more profound break- 

 ing down of proteins by acids, something is done which constitutes a 

 physiological obstacle to utilization of the products in the synthesis of 

 protein by the animal body. 

 As stated by Leathes: 6 



The great bulk of the substances set free in the hydrolysis of proteids by 

 enzymes and by acids are the same, and these substances enter into the com- 

 position of the proteids synthesized in the body in similar proportions to those 

 in which they occur in the proteids of the food. For the present, all we can 



Problems in Animal Metabolism," 1906, p. 132. 



