Till. METAB0LI8W OF PRO! 



To account for the indisputable disappears the amini 



the intestine during protein « 1 i -_r < - -« t i • - 1 1 . coupled with the in. 

 detecting any of them in the blood, two many 



years. One of these was thai the amino acids become deaminated Ml. 

 group split up as Ml 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 h.\ pothesis seemed to be obtained by the observation thai there 

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

 systemic circulation. The falsity of this evidenc 

 nitely established by Folin and Denis, >und by mi 



quantitative methods for the estimation of ammonia and urea in the 1.] 

 tli.it the amount of neither of these substances became increased in the 

 portal blood during absorption of amino acids from the int< I 



made the further important discovery that the ammonia in the portal 

 blood is really very Little in amount, and represents that ah* 

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

 of putret';icti\ e bacteria. 



Nor could any evidence 1 btained in favor of the hypotl hat 



the absorbed amino acids become built up in the intestinal epithelium 

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

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

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



amino acid in the blood. 



This brief historical survey of the sub j eel brings us to a position whe 



we may pi-. ed to discuss the present-day teaching regarding protein 



metabolism. Briefl; d, this teaching is to the effect that the \ 



molecule is broken down into Us ultimate building stom vino a< 



by the digestivt ■ f tht gash ttinal tract, and that t) 



acids are absorbed ; nt" tin blood, by which th y an 



organs and tissues, which amino acids and ut ' W 



which they r< for t) 



Tin amino <>> ids not r< qu '■•'• d for il 



Jn lib( rati d in the tissues 



an tht n split info two portions, om \mmon 



by i iaind( r of t) e I form' 



urea and tin hitter is oxidized to p '/. 



CHEMISTRY OF PROTEIN 



Bi ' re pr ding to dif the evidenc upon whi in- 

 clusions den ,nd. it will be n-e, ssarj I insider some of the most importanl 



facts concerning the chemistry of the protein molecule. W< 



