54 THE AMINO ACIDS 



That all of the older investigators did not regard the 

 direct absorption of protein or even of such large 

 molecules as peptone as essential for nutrition may be 

 seen from the view formulated by Salkowski and 

 Leube. According to this suggestion leucine may be 

 regarded as a substance capable after absorption of 

 being built up into protein and therefore leucine might 

 be looked upon as a stage in protein regeneration. 

 Against such a view, however, stood the fact that the 

 amino acids were not at that time demonstrable in the 

 blood or lymph. 



From the numerous researches carried through con- 

 cerning the absorption of proteoses and peptones from 

 the intestines, the conclusion may be drawn that 

 although these proteins disappear when placed in con- 

 tact with the intestinal mucosa, there is no evidence of 

 their absorption as such for they can be found neither 

 in the blood nor in the lymph. On the other hand, 

 inasmuch as their disappearance from the intestine is 

 coincident with the appearance of amino acids, an 

 enzyme being furnished which specifically effects such 

 a transformation, it is probable that these proteins are 

 absorbed only in the form of amino acids. 



The Absorption of Amino Acids 



As has been stated previously the presence of amino 

 acids in the small intestine has long been known. Their 

 absorption therefrom, however, has been a matter of 

 conjecture. Inasmuch as their presence in the blood 



