THE METABOLISM OF PROTEIN 597 



To account for the indisputable disappearance of the amino acids from 

 the intestine during protein digestion, coupled with the impossibility of 

 detecting any of them in the blood, two views were current for many 

 years. One of these was that the amino acids become deaminated (NH 2 

 group split up as NH 3 ) by the intestinal epithelium, and the other, that 

 these cells are endowed with the power of reconstructing the amino acids 

 into protein, which then passes into the blood. Justification for the de- 

 amination hypothesis seemed to be obtained by the observation that there 

 is more free ammonia in the blood of the portal vein than in that of the 

 systemic circulation. The falsity of this evidence was, however, defi- 

 nitely established by Folin and Denis, 32 who found by means of delicate 

 quantitative methods for the estimation of ammonia and urea in the blood 

 that the amount of neither of these substances became increased in the 

 portal blood during absorption of amino acids from the intestine. They 

 made the further important discovery that the ammonia in the portal 

 blood is really very little in amount, and represents that absorbed as 

 such from the intestinal lumen, where it is produced chiefly by the action 

 of putrefactive bacteria. 



Nor could any evidence be obtained in favor of the hypothesis that 

 the absorbed amino acids become built up in the intestinal epithelium 

 into proteins, which are then transformed or carried away by the blood. 

 This hypothesis was based entirely on negative findings, and had there- 

 fore to be dropped when discovery was made of the actual presence of 

 amino acid in the blood. 



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, and that these amino 

 acids are absorbed into the blood, by which they are carried to 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 wliich 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 



