582 METABOLISM, NUTRITION AND DIETETICS 



increased excretion of homogentisinic acid, but even in starvation 

 this substance still continues to appear in the urine. Since homo- 

 gentisinic acid is undoubtedly formed from tyrosin and from phenyl- 

 alanin, this observation constitutes a convincing proof that these 

 amino-acids are produced from tissue-proteins. A striking illus- 

 tration of the fact that the amino-acids of the tissues are not simply 

 a store of reserve material derived directly from the alimentary canal 

 is the failure of a period of starvation to make any impression upon 

 their amount. They normally constitute 2 to 4 per cent, of the dry 

 weight of the tissues, and if anything, tend somewhat to increase 

 in the tissues of a starving animal. 



Fate of Amino- Acids in the Body. The problem of the kata- 

 bolism of proteins is thus reduced to the question, What becomes of 

 the amino-acids ? Where, how, by what stages, and to what end- 

 products are they decomposed ? Some, possible or probable steps 

 in their metabolic history have been already suggested in dealing 

 with the intermediary metabolism of carbohydrates (p. 542). 

 Something more will have to be said of the where and the how of 

 their chemical degradation in treating of the place and manner of 

 urea formation. As to the end-products by which they are repre- 

 sented in the final balance-sheet of the bodily economy, the answer 

 is easy. The amino-acids, whatever intermediate stages they may 

 pass through, whatever cleavages, oxidations, or reductions they may 

 undergo, yield eventually carbon dioxide, water, and comparatively 

 simple nitrogen-containing substances, which after further changes 

 appear in the urine principally as urea, and in birds and reptiles as 

 uric acid. When amino-acids are fed to mammals or introduced 

 parenterally, a very large proportion of the nitrogen appears in the 

 urine as urea. The same is true when, instead of simple amino- 

 acids, polypeptides, like glycyl-glycin, alanyl-alanin, or leucyl-leucin, 

 are given. When amino-acids are administered to birds, the great 

 bulk of the nitrogen is excreted in the form of uric acid. Whether 

 in mammals, and if so to what extent, uric acid is also one of the 

 nitrogenous end-products of the decomposition of ordinary proteins 

 or of the amino-acids which they yield, are moot questions. In 

 any case, the most important and characteristic source of the uric 

 acid in mammals and the other groups of animals whose chief 

 nitrogenous end-product is urea, is not the ordinary proteins, but 

 the nucleins which form constituents of the nucleo-proteins. 



We have no definite information as to the production of water from 

 the hydrogen of the tissues, except what can be theoretically deduced 

 from the statistics of nutrition (p. 620). A few words will be said 

 a little farther on about the production of carbon dioxide from 

 proteins; we have now to consider the seat and manner of formation 

 of the nitrogenous metabolites. And since in man and the other 

 mammals urea contains, under ordinary conditions, by far the 



