46 PHYSIOLOGICAL CHEMISTRY 



They are responsible for the breaking down or building up of 

 many classes of compounds, but apparently they only change 

 the speed of reactions which, if given sufficient time, would go 

 on of themselves. Substances which shown this behavior are 

 spoken of as catalytic agents, and are said to act by catalysis. 

 Enzymes have been denned as substances produced by living 

 cells, which act by catalysis. 



It was formerly believed that the enzyme of yeast which 

 breaks down sugar into alcohol and carbon dioxide acted only 

 within the living cell, and that the vital activities of the cell 

 were intimately connected with its action. Organisms of the 

 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. 



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 



