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 manner 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 produced by acids. We may conjecture that while in acid hydrolysis the products are all simple amino-acids, in enzymolysis combinations of the amino-acids, i. 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 synthesis 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 digestion 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 certain products that are not essential or not immediately necessary. On the other hand, it is equally apparent that in the more profound breaking 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:
- "Problems in Animal Metabolism," 1906, p. 132.