62 PHYSIOLOGICAL CHEMISTRY 



type of yeast were called ferments. Another class of reactions 

 was known to be independent of the cell by which the active 

 principle was produced; substances responsible for such reac- 

 tions were called unorganized ferments. Among them were 

 the various digestive enzymes. In 1897 this fallacy was cor- 

 rected by Buchner, who ground up yeast cells with sharp sand, 

 thus tearing the cells and allowing the cell fluids to escape. 

 The entire mixture of sand and broken cells was then pressed 

 in a powerful press. A small quantity of liquid was obtained 

 and was filtered through a porous porcelain filter which held 

 back any fragments of cells. The clear liquid was found to 

 decompose sugar in the same way as the original yeast cells 

 had done. The activities of the enzyme were thus in no way 

 dependent upon the vital processes going on in the cell. It is 

 not to be supposed from this that the enzyme activities are of 

 no value to the cell. Quite the contrary is the case, for undoubt- 

 edly the greater number of the chemical reactions which go on 

 in the cell, and which to a large extent make up the sum total 

 of what we call its vital activities, depend upon simple chemical 

 reactions which are directed or controlled by enzymes. We 

 still know certain types of enzymes which thus far have not 

 been isolated from the cells in active form, but possibly this 

 will be accomplished in the course of time. 



Enzymes which normally act within the cell by which they 

 are produced are called intracellular, or endoenzymes. Those 

 which normally are secreted, and act outside the parent cell are 

 called extracellular, or exoenzymes. The former are of great 

 interest in pathological processes. The latter include the en- 

 zymes which carry out the processes of digestion in the alimen- 

 tary tract. 



Nomenclature and Classification. The nomenclature of the 

 enzymes is somewhat irregular, as several of the substances 

 were named before any regular plan had been adopted. The 

 ending "ase" is now employed to indicate that a substance is 

 an enzyme, and the remainder of the word usually indicates 

 either the substance upon which the enzyme acts, the nature of 



