METABOLISM, NUTRITION AND DIETETICS 497 



Apart from any influence which it may have in favouring 

 absorption, 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. 419), 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 lias 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 quantitative 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 kreatinin 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. An alternative 

 assumption, and superficially at least a simpler one, is that no 

 more extensive synthesis of proteins occurs in the wall of the 

 alimentary canal than is necessary for the needs of the tissues 

 composing it, and that the decomposition products of the 

 proteins are mainly absorbed as such, and pass in the blood to 

 the tissues for which they are destined. If this is the case, the 

 blood-proteins can no longer be looked upon as representing 

 the main current of protein supply for the organs, but rather the 



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