CHAPTER III 



ENYZMES (Continued) 



INTRACELLULAR PROTEASES' (PROTEOLYTIC ENZYMES), INCLUDING 

 A CONSIDERATION OF AUTOLYSIS 



To what extent synthesis of proteins goes on in the body is still a 

 problem; still more uncertain is the part played by reversible action 

 of proteases. There is evidence enough that somewhere in the body 

 the amino-acids can be rebuilt into protein, for several investigators 

 have succeeded in keeping animals in nitrogenous equilibrium by feed- 

 ing them products of proteolysis that contained no proteins whatever, 

 and as the proteins of the animal body are being broken down in- 

 cessantly, it must be that they were replaced by synthesis of the non- 

 protein material fed to the animals. In addition, it has long been 

 questioned whether amino-acids absorbed from the intestines are not 

 resynthesized into proteins while passing through the intestinal wall. 

 Cohnheim found that in the intestinal epithelium there is an enzyme, 

 erepsin, capable of splitting albumoses and peptones into the amino- 

 acids, which enzyme presumably exists for the purpose of securing 

 complete cleavage of all ingested proteins into their ultimate "build- 

 ing stones." This may be looked upon as a provision to reduce all 

 varieties of proteins to their common elements, so that the body by 

 quantitative selection can resynthesize them into its own types of 

 protein, for, as is well known, foreign proteins {e. g. egg-albumin) 

 introduced directly into the blood stream cannot be utilized, but are 

 excreted unaltered in the urine. ^ As was shown for lipase, the as- 

 sumption that such synthesis occurs as a normal physiological process 

 by reverse enzyme action, requires that the proper enzymes be present 

 in the cells throughout the body, and within recent years it has been 

 abundantly demonstrated that such is the case. 



For over half a century it has been known that amebse digest solid 

 proteins within their bodies, but it is only within a few years that 

 proteolytic enzymes have been definitely isolated from them. It has 

 been much the same with the intracellular proteases of the higher 

 organisms. In 1871 Hoppe-Seyler referred to the liquefaction of 



^ As the possibility exists that ferments which digest proteins may be able to 

 perform a certain amount of synthesis of proteins, the term "proteolytic enzyme" 

 seems to be less suitable than the term "'protease," which merely means an enzyme 

 acting on proteins, and does not compel us to accept any particular view as to 

 what the action is. 



^ According to Austin and Eisenbrey (Arch Int. Med., 1912 (10), 305), dogs 

 on a nitrogen-free diet can utilize horse serum injected intravenously. 



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