METABOLISM OF PROTEINS 573 



active mucosa is so large, that the concentration of peptone or 

 proteose necessary to produce injurious effects could hardly in any 

 case be realized. Again, there is no evidence thai the simpler 

 decomposition products of further hydrolysis are not in equal con- 

 centration as poisonous as proteose and peptone. 



Apart from any influence which it may have in favouring absorp- 

 tion, the complete shattering of the protein molecule has a double 

 significance. In the first place, as already pointed out, the food- 

 proteins cannot be used directly in the upbuilding and repair of the 

 protoplasm (p. 448), since the tissue-proteins differ from them and 

 from each other in the amount and nature of the amino-acids and 

 other groups in their molecule (p. 2). Secondly, under ordinary 

 dietetic conditions a surplus of nitrogen in the protein food has to 

 be got rid of by being converted into urea without being built up 

 into the tissue substance. Here we come upon the fundamental 

 fact that the protein katabolism is not a single uniform process. 

 Two forms may be distinguished which are essentially independent 

 in course and character. One kind varies extremely in its quantita- 

 tive relations, according to the amount of protein in the food. Its 

 chief end-products are urea, representing the nitrogen, and inorganic 

 sulphates, representing the sulphur of the proteins. Since this form 

 of katabolism, as we shall see directly, is not essentially connected 

 with the life and nutrition of the living substance, it is termed 

 exogenous. The other variety is practically constant in amount 

 for one and the same individual, and independent of the quantity 

 of protein in the food. Its characteristic end-products are creatinin 

 and neutral sulphur. This form of protein katabolism is essentially 

 an expression of the waste of the living substance itself, and is 

 therefore spoken of as endogenous. 



Some have supposed that the intestinal mucosa has as one of its 

 special functions the resynthesis of a great part of the digestive 

 decomposition products into the proteins of the blood-plasma. If 

 this is the case, these proteins must be again decomposed in the 

 cells of the various tissues in order that the ' building-stones ' may 

 be recombined to form the tissue-proteins. For the proteins of the 

 organs are not the same as those of the blood, and the proteins of 

 different organs differ characteristically from each other. The 

 significance of the synthetic function of the intestinal wall would 

 then lie in this: that from the varying mixture of amino-acids, etc., 

 derived from the food-proteins an always uniform and suitable' 

 protein mixture (the blood-proteins) is fabricated for the feeding 

 of the tissues. Experiments intended to test this hypothesis have 

 hitherto yielded a negative result. No accumulation of protein in 

 the wall either of the intestine in situ or of the isolated surviving 

 intestine has been detected during absorption of the decomposition 

 products of protein. An alternative assumption, and superficially 

 at least a simpler one, is that no more extensive synthesis of proteins 



