METABOLISM 517 



THE METABOLISM OF THE PROTEINS 



It will be recalled that the protein constituents of the food introduced into 

 the alimentary canal are reduced by the action of the enzymes of the gastric, 

 pancreatic and intestinal juices to relatively simple compounds, viz. : ammonia, 

 amino- and diamino-acids which are for the most part at once absorbed into 

 the blood. A portion of the ammonia and a portion of some of the amino- 

 acids pass into the large intestine where they give rise under the destructive 

 action of bacteria to substances of a simpler character such as indol, skatol, 

 and phenol. These and other putrefactive substances are in part discharged 

 from the body in the feces and in part absorbed into the blood and ultimately 

 eliminated in combination with potassium sulphate by the kidney (see pages 

 462 and 493). The ammonia absorbed is converted during its passage 

 through the liver into urea. 



The absorbed amino-acids, after passing through the liver and gaining 

 entrance into the general circulation, are distributed to the tissues where 

 they are in part utilized for the reconstruction of the tissue molecules that 

 have undergone metabolism in consequence of their functional activity, and 

 in part separated by the action of tissue enzymes into the amine element 

 NH 2 , and a carbonaceous radical. The percentage of the amino-acids used 

 for the former purpose varies at different periods of life and under varying 

 circumstances. In youth when tissues are developing and growing and at 

 the same time are undergoing an active metabolism, a larger percentage is 

 utilized for repair and growth than in adult life when the body-growth at 

 least has ceased, though metabolism continues more or less actively. 



The amine element, NH 2 , and the carbonaceous radical arising from 

 the cleavage of the amino-acids not utilized for tissue growth and repair are 

 disposed of as follows. The NH 2 is combined with hydrogen and then with 

 CO 2 to form ammonium carbonate which, perhaps in the tissues, certainly 

 in the liver is transformed into urea; the carbonaceous radical is oxidized 

 to CO 2 and water with the liberation of heat. The heat-producing power 

 of protein 4 Calories per gram is almost entirely due to this oxidation. 

 Since scarcely more than half the protein consumed is utilized for tissue 

 repair the question has been raised as to whether a smaller amount than 

 that usually consumed viz.: 100 to 120 grams, would not be equally satis- 

 factory. For statements regarding the advantages of a low- and high- 

 protein diet see pages 121 and 122. 



The tissue protein, sooner or later, also undergoes katabolism in conse- 

 quence of functional activity, and the natural supposition would be that it 

 too is reduced to its constituent amino-acids which are then disposed of 

 by the methods already alluded to. Nevertheless there are reasons for think- 

 ing that this is not wholly the case. Folin has presented a series of facts 

 that lead to the supposition that this portion of the protein is represented 

 in part by creatinin. Thus it has been shown that while the urea excreted 

 rises and falls with the protein consumed, the amount of creatinin excreted 

 remains more or less stationary; again, in individuals of pronounced muscle 

 development, after unusual muscle activity and in diseases involving a 

 destruction of muscle-tissue there is an increase in creatinin elimination. 

 In the opposite conditions the amount excreted is low. For these and other 

 reasons creatinin is regarded as a specific final end-product and an indicator 

 of the metabolism of muscle protein. 



