METABOLISM l6l 



of these substances the body builds up the great variety of 

 specific proteins which are peculiar to itself and which differ in 

 properties and chemical structure from the proteins of the feed, 

 especially from those of the vegetable kingdom (147). This 

 process of building animal proteins from the fragments of 

 vegetable proteins is the most conspicuous example at once of 

 the synthetic powers of the animal organism and of the object 

 of the digestive cleavage. 



227. Seat of protein synthesis. As regards the place where 

 this synthesis of proteins occurs, opinions are divided. Until 

 recently, most experimenters have not been able to detect the 

 products of digestive cleavage with certainty in the blood, either 

 in the general circulation or in the portal vein, and the current 

 view has been, therefore, that of Abderhalden, viz., that the 

 " building stones " of the proteins are synthesized in the 

 epithelial cells of the intestine and that the resulting proteins 

 in particular serum albumin are passed on to the blood 

 to serve as nourishment to the protein tissues of the body. 



Various investigators, however, have reported the presence in 

 the blood of greater or less amounts of non-protein nitrogen 

 and with the aid of more refined chemical methods Folin and 

 Denis 1 and Van Slyke and Meyer 2 seem to have shown beyond 

 question that amino acids may pass through the resorbing 

 epithelium unchanged and be found in the blood and tissues 

 in amounts sufficient to account for practically all that was 

 administered (152). The latter experimenters have likewise 

 shown that after meat feeding the proportion of amino acids in 

 the blood may be doubled and that the increase affects the blood 

 of the entire circulatory system and not that of the portal vein 

 only, while Abel, 3 by a diffusion method, has been able to 

 secure considerable amounts of amino acids from the circulat- 

 ing blood of living animals. The evidence at the present time, 

 therefore, seems decisively in favor of the view that the frag- 

 ments into which the protein molecule is split during digestion 

 pass without material change into the blood current and serve 

 as a common source from which the proteins, both of the blood 

 and the various tissues, are built up and that every living cell, 

 each in its own measure, has this anabolic power. 



1 Jour. Biol. Chem., 11 (1912), 87. 2 Jour. Biol. Chem., 12 (1912), 399. 



3 Jour. Pharmacol. and Expt'l Therap., 5 (1914), 275. 

 M 



