CHAPTER VII. 

 THEORIES OF PROTEIN METABOLISM. 



MANY hypotheses have been advanced to account for the various 

 changes observed during the course of protein metabolism, but until 

 about five years ago only two were really considered. These two 

 were put forward by Voit in 1867 and by Pfliiger in 1893 re- 

 spectively. Previously the theory advanced by Liebig was almost 

 universally accepted. Liebig considered that the protein of the food 

 was the one essential material, that it entered the organism without 

 having undergone any very serious change during digestion, and that 

 it immediately and directly replaced the effete material of the tissues. 



Voit. 



Voit (VII) put forward the view that the protein of the food after 

 absorption circulated in the tissue fluids became "circulating pro- 

 tein " and was utilized (catabolized) by the living tissues without first 

 becoming an integral part of them. This " circulating protein " was 

 readily broken down, whereas the "tissue protein" was resistant. A 

 certain amount of the tissue protein constantly died and was excreted, 

 and was replaced by material drawn from the "circulating" or food 

 protein. No chemical difference existed between the "circulating" 

 and the " tissue " protein. One of the facts which led Voit to the 

 conclusion that protein existed in two forms was that during starva- 

 tion a bare i per cent, of the tissue protein of the body was broken 

 down per diem, whereas, if protein were fed in amount equal to 12 

 per cent, of the body protein, the breakdown was fifteen times greater 

 than in hunger. 



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