. CHAPTER LXVII 

 THE METABOLISM OF PROTEIN 



Introductory. The older physiologists believed that the protein taken 

 with the food was brought into a soluble condition by the digestive en- 

 zymes, and that it was then absorbed into the blood and directly incor- 

 porated with the tissues. The discovery of the enzymes trypsin and 

 erepsin and of free amino acids in the gastrointestinal contents clearly 

 showed that this simple theory of Liebig could not be correct. It was, 

 furthermore, found that when an excess of proteins such as egg albumin 

 gains entry to the blood, part of the protein appears in an unchanged 

 condition in the urine ; and that enzymes capable of digesting this protein 

 but not other varieties make their appearance in the blood. 



After the injection of foreign proteins into the blood, symptoms of 

 varying severity often develop, from the almost instantaneous death 

 produced by snake venom to the slowly developing anaphylactic reac- 

 tions which follow the injection into the blood of many proteins chemi- 

 cally indistinguishable from those of the blood serum itself. When pro- 

 tein is taken in the usual amounts by mouth, these poisonous reactions 

 do not supervene, even snake venom is harmless when swallowed, nor 

 is it "possible during digestion of a protein meal to detect food protein in 

 the blood by means of the precipitin reaction. Finally it was discovered 

 that the very slow intravenous injection of completely digested flesh did 

 not produce on the part of the body any of the reactions that injected 

 protein itself produces, indicating that perfect assimilation had occurred. 

 From these and similar observations it soon became clear that protein 

 can not be absorbed as such from the alimentary canal, but must first of 

 all ~be completely broken down into the amino acids, which are then rebuilt 

 into the protein of the organism. The direct evidence for this important 

 change in belief concerning protein metabolism has been gained by the 

 discoveries that: (1) nitrogen equilibrium can be maintained in animals 

 fed with completely digested protein mixtures; and (2) amino acids can 

 be isolated from the blood. 



The experiments of the first group consist, in principle, in breaking 

 down protein until there is no longer the characteristic biuret test, and 

 then feeding this digestion mixture to animals and observing them from 

 day to day, using as criteria of their nutritional condition the body weight 



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