ALBUMINS OR PROTEINS. 213 



These experiments show clearly that the composition of the serum- 

 albuminous bodies remains unchanged and is independent of the nature 

 of the albumin administered. The amounts of glutamic acid remained 

 very constant, in spite of the fact that the horse had to renew a supply of 

 serum-albumin, due to a large loss of blood. The albumin in the food 

 must certainly have undergone a complete change before it entered into 

 the general circulation. The blood was taken from the Vena jugularis in 

 these experiments. As the albuminous substances do not seem to enter 

 the general circulation through the lymph-stream, but only through the 

 blood, it was conceivable that the transformation of the nutrient albumin, 

 that is, the synthesis of the cleavage-products, was carried out in the 

 liver. We have, as yet, no exact proof of this. Our present knowledge 

 indicates that a synthesis of the albuminous cleavage-products takes 

 place in the walls of the intestine. From the various different albumins 

 in the food, the serum-albumins are formed first. From the latter, each 

 cell constructs the protein that it requires. The cell, therefore, obtains 

 the same nourishment entirely independent of external conditions. The 

 functions of the intestine and of the digestive ferments are, according to 

 this conception, to be regarded in a quite particular light. First of all, 

 they guarantee collectively the correct course of the general metabolism. 

 The digestive ferments act before the intestine does. They furnish the 

 intestine with the building materials from which it forms homogeneous 

 products for the cell-metabolism. It now becomes apparent why any 

 derangement of the alimentary tract should have such a far-reaching effect 

 upon all processes of metabolism. It is not the deranged absorption which 

 is so important. It is the deranged assimilation. The intestine itself is 

 one of the most important organs. Many important syntheses and changes 

 are carried on within its walls. 



That syntheses play necessarily in albumin-metabolism a part as impor- 

 tant as in the case of fats and carbohydrates, is evident from the fact that 

 animals which are supplied with greatly disintegrated albuminous material, 

 can be easily maintained in nitrogen equilibrium, as the experiment on 

 the next page shows. 1 



The casein decomposed by tryptic digestion consisted of 80-85 per cent 

 of simple cleavage-products, the amino acids constituting, by far, the 

 larger portion, while the smaller part was composed of substances akin to 

 polypeptides. At most, 15-20 per cent of the total material administered 

 consisted of complicated polypeptides, which, however, no longer gave the 

 biuret reaction. Whether the organism is capable of producing albumins 

 from the amino acids alone remains undecided, and at present is not sus- 



1 E. Abderhalden and P. Rona: Z. physiol. Chem. 42, 528 (1904); 44, 108 (1905). 

 Cf. O. Loewi: Arc. exp. Path. Pharm. 48, 303 (1902). 



