CHAPTER III 

 ENZYMES (CONTINUED) 



Intracellular Proteases 1 (Proteolytic Enzymes), Including a 

 Consideration of Autolysis 



To what extent synthesis of proteids goes on in the body is 

 still a problem ; still more uncertain is the part played by rever- 

 sible action of proteases. If the possibility of resynthesis of 

 fats by lipase is still unsettled, the possibility of resynthesis of 

 proteids by proteid-splitting enzymes must be still more open to 

 question. There is evidence enough that somewhere in the body 

 the amino-acids can be rebuilt into proteid, for Loewi, 2 and 

 since him several others, has succeeded in keeping animals in 

 nitrogenous equilibrium by feeding theui products of proteol- 

 ysis that contained no proteids whatever, and as the proteids of 

 the animal body are incessantly being broken down, it must be 

 that they were replaced by synthesis of the non-proteid material 

 fed to the animals. In addition, it has long been known that 

 amino-acids absorbed from the intestines do not reappear iu the 

 blood coming from the intestines, indicating that they are resyn- 

 thesized into proteids while passing through the intestinal wall. 

 Cohnheim 3 found that in the intestinal epithelium there is an en- 

 zyme, erepsin, capable of splitting album oses and peptones into the 

 amino-acids, which enzyme presumably exists for the purpose of 

 securing complete cleavage of all ingested proteids into their ulti- 

 mate " building stones." This may be looked upon as a provision 

 to reduce all varieties of proteids to their common elements, so 

 that the body by quantitative selection can resynthesize them into 

 its own types of proteid, for, as is well known, foreign proteids 

 (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 assumption that such synthesis occurs 



1 As long as the possibility still exists that ferments which digest proteids 

 may be able to perform a certain amount of synthesis of proteids, the term 

 " proteolytic enzyme " seems to be less suitable than the term " protease," 

 which merely means an enzyme acting on proteids, and does not compel us to 

 accept any particular view as to what the action is. 



2 Centr. f. PhysioL, 1902 (15), 590. 



3 Zeitschr. f. physiol. Chem.,1901 (33), 451 ; 1902 (35), 134. 



