CHAPTER LXVH 

 THE METABOLISM OF PROTEIN 



Introductory. The older physiologists believed thai the protein tah 

 with the food was brougb.1 unto a soluble condition by the digestive en- 

 zymes, and thai 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 

 slu.wcd thai this simple theory of Liebig could uol be correct. It v 

 furthermore, found thai when an excess of proteins such ?g albumin 

 gains entry to the blood, pari of the protein appears in an unchang 

 condition in the urine ; and thai enzj mes capable of digesting this protein 

 luii not other varieties make their appearance in the blood. 



After the injection of foreign proteins into the blood, symptoms 

 varying severity often develop, from the almosl instantaneous death 

 produced by snake venom to the slowly developing anaphylactic r< 

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

 cally indistinguishable from those of the blood serum itself. When p 

 tein is taken in the usual amounts by month, these poisonous reacti 

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

 is it possible during digestion of a protein meal i«> det< od protein in 



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

 that the very slow intravenous injection of completely di did 



not produce on the pari of the body any of the reactions that injec 

 protein itself produces, indicating thai perfecl assimilation had occurred 

 Prom these and similar observations it soon became dear that protein 

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

 nil be completely broken <l<in-n into tin amino acids, which are t) nilt 



into tin protein of tin organism. The direcl evidence for this important 



change in belief icerning protein metabolism has been gained by the 



discoveries that: I oitrogen equilibrium can be maintained in animals 

 fed with completely digested protein mixtures; and _* amino acids can 

 he isolated from the bipod. 



Tin' experiments of the firsl group consist, in principle, in breaking 

 down protein until there is no longer the characteristic biurel ind 



then feeding this digestion mixture to animals and observing them from 

 day to day. using as criteria of their nutritional condition Hie body weighl 



