202 ORGANIC AGRICULTURAL CHEMISTRY 



acids. It was formerly believed that all absorption of digested 

 protein was in the form of amino-acids. The presence of the 

 enzyme erepsin in the intestinal juice seemed to uphold this 

 view for, as this enzyme completes all partially digested protein, 

 carrying the hydrolysis through to the amino-acids, its presence 

 would indicate that the complete hydrolysis is necessary for 

 absorption. It is now pretty generally accepted, however, 

 that all protein food is not converted into amino-acids. The 

 experimental evidence for this is that amino-acids themselves, 

 at least singly, are not used as nutritive material when fed to 

 animals. It seems possible, however, that certain combinations 

 of amino-acids, including some of the more complex ones like 

 tr3T)tophane, can be utilized directly as food. 



Polypeptide Nucleus. — On the other hand, it has been found 

 that tryptic digestion is inactive toward certain polypeptides, 

 and it is believed that this indicates a sparing or protective 

 action toward the further hydrolysis of these compounds in 

 order that they may be absorbed as such. Being absorbed 

 through the intestinal wall together with the amino-acids, these 

 polypeptides act as a nucleus with which the amino-acids com- 

 bine to form new protein. It has been established that neither 

 polypeptides nor amino-acids are present in the blood except 

 perhaps small amounts of the latter. This would indicate that, 

 as in the case of fats, the resynthesis of the protein from the 

 polypeptide and amino-acid products of digestion occurs during 

 the absorption of these materials through the intestinal wall. 



Serum Albumin. — As stated, the absorbed products of pro- 

 tein digestion are found in the blood capillaries leading to the 

 portal vein. In the blood of this part of the circulatory system 

 there is always present in the serum a protein known as 

 serum albumin. It is interesting that while absorbed and 

 resynthesized fat is found in the animal body of exactly the 

 same nature as the food fat, even though this is different in 

 character from the normal body fat, yet, in the case of pro- 

 teins, no matter how different the protein food may be from 

 the serum albumin of the animal, the serum albumin synthe- 



