THE METABOLISM OF PROTEIN 633 



nature of the process used for digesting the protein, and partly on the presence or ab- 

 sence of carbohydrate in the digestion mixture. It was found that the products of 

 hydrolysis by acid failed to maintain equilibrium, and it was believed that this was 

 owing to the fact that the acid had more completely disrupted the protein molecule, and 

 had left no polypeptides, which, it was imagined, remained intact during enzyme action 

 and were essential for proper protein metabolism. This view has now been consider- 

 ably altered, since it has been shown that the acid actually destroys certain amino 

 acids which the enzyme leaves! intact. The amino acid particularly concerned is 

 tryptophane. Thus, when different groups of animals were fed with diets, consisting of 

 (1) fully digested casein, (2) fully digested casein from which the tryptophane 

 had been removed, it was found that nitrogen equilibrium could not be maintained on 

 the second diet, whereas it was maintained on the first. When the protein was only 

 partly digested by acid that is, not digested enough so as to break up all the trypto- 

 phane or when tryptophane was added to the second diet, nitrogen equilibrium could 

 be satisfactorily maintained. 



These results obtained in different classes of animals have also been confirmed for 

 the human subject. For example, nitrogen retention has been observed in a case of 

 a boy suffering from a stricture of the esophagus, when he was fed by rectum for 

 fifteen days with digestion products resulting from the action of trypsin and erepsin 

 on flesh. 



Concerning the second type of evidence, many investigators attempted to separate 

 the amino acids themselves from the blood, particularly during the digestion of a large 

 amount of protein, but the results were at first entirely negative because of the lack 

 of methods that were sufficiently delicate to make it possible to detect the slight increase 

 that could be expected even when a maximum absorption of nitrogen had occurred. 

 The very large flow of blood through the portal vein causes such extensive dilution of 

 any substances added to it that the concentration of the substance in an isolated 

 sample of the blood can be only trivial. 



This brief historical survey of the subject brings us to a position where 

 we may proceed to discuss the present-day teaching regarding protein 

 metabolism. Briefly stated, this teaching is to the effect that the protein 

 molecule is broken down into its ultimate building stones, the amino acids, 

 by the digestive enzymes of the gastrointestinal tract. These amino 

 acids are absorbed into the blood, by which they are carried tot the various 

 organs and tissues, which sift out the amino acids and use 'those of themi 

 which they require for the reconstruction of their broken-down protein. 

 The amino acids not required for the process, along with those which may 

 be liberated in the tissues themselves by disintegration of tissue proteins, 

 are then split into two portions, one represented by ammonia and the other 

 by the remainder of the amino acid, molecule. The former is excreted as 

 urea and the latter is oxidized to produce energy. 



CHEMISTRY OF PROTEIN 



Before proceeding to discuss the evidence upon which the above con- 

 clusions depend, it will be necessary to consider some of the most important 

 facts concerning the chemistry of the protein molecule. We shall require 

 this information not only to understand the history of protein in the 



