512 TEXT-BOOK OF PHYSIOLOGY 



THE METABOLISM OF THE PROTEINS 



It will be recalled that the protein constituents of the food introduce 

 into the alimentary canal are reduced by the action of the enzymes of th 

 gastric and pancreatic juices to relatively simple compounds, viz. : ammoni; 

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

 the blood. A portion of the ammonia and a portion of some of the aminc 

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

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

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

 from the body in the feces and in part absorbed into the blood and ultimate! 

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

 458 and 488). The ammonia absorbed is converted during its passag 

 through the liver into urea. 



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

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

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

 have undergone metabolism in consequence of their functional activity, an 

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

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

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

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

 thesame time are undergoing an active metabolism, a larger percentage : 

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

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



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

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

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

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

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

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

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

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

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

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

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

 protein diet see pages 121 and 122. 



The tissue protein, sooner or later, also undergoes katabolism in consc 

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

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

 by the methods already alluded to. Nevertheless there are reasons for thin! 

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

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

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

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

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

 development, after unusual muscle activity and in diseases involving 

 destruction of muscle-tissue there is an increase in creatinin eliminatior 

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

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

 of the metabolism of muscle protein. 



