2 PHYSIOLOGICAL CHEMISTRY 



about the identical fermentative processes which, up to this time, had 

 been deemed possible only in the presence of the active, living yeast cell. 



Buchner's manipulation of the yeast cells consisted in first grind- 

 ing them with sand and infusorial earth, after which the finely divided 

 material was subjected to great pressure (300 atmospheres) and yielded 

 a liquid which possessed the fermentative activity of the unchanged 

 yeast cell. 1 This liquid contained, zymase, the principal enzyme of 

 the yeast cell. Later the lactic-acid- and acetic-acid-producing bac- 

 teria were subjected by Buchner to treatment similar to that accorded 

 the yeast cells, and the active intracellular enzymes were obtained. 

 Many other instances are on record in which a soluble, active agent has 

 been isolated from ferment cells, with the result that it is pretty well 

 established that all the so-called organized ferments elaborate sub- 

 stances of this character. 



Enzymes act by catalysis and hence may be termed catalyzers or 

 catalysts. A simple rough definition of a catalyst is "a substance 

 which alters the velocity of a chemical reaction without undergoing 

 any apparent physical or chemical change itself and without becoming 

 a part of the product formed." It is a well-known fact that the veloc- 

 ity of the greater number of chemical reactions may be changed 

 through the presence of some catalyst. For example, take the case of 

 hydrogen peroxide. It spontaneously decomposes slowly into water 

 and oxygen. In the presence of colloidal platinum, 2 however, the de- 

 composition is much accelerated and ceases only when the destruction 

 of the hydrogen peroxide is complete. Without multiplying instances, 

 suffice it to say that there is a close analogy between inorganic catalysts 

 and enzymes, the main point of difference between the enzymes and most 

 of the inorganic catalysts being that the enzymes are colloids. The 

 great majority of enzymes are hydrolytic in character. 



We may define an enzyme as an organic catalyst which is elaborated 

 by an animal or vegetable cell and whose activity is entirely independent 

 of any of the life processes of such a cell. According to this definition 

 the enzyme zymase elaborated by the yeast cell is entirely comparable 

 to the enzyme pepsin elaborated by the cells of the stomach mucosa. 

 One is derived from a vegetable cell, the other from an animal cell, yet 

 the activity of neither is dependent upon the integrity of the cell. 



Inasmuch as each of the enzymes has an action which is more or less 

 specific in character, and since it is a fairly simple matter, ordinarily, to 

 determine the character of that action, the classification of the enzymes 



1 In later investigations the process was improved by freezing the ground cells with 

 liquid air and finely pulverizing them before applying the pressure. 



2 Produced by the passage of electric sparks between two platinum terminals immersed 

 in distilled water, thus liberating ultra-microscopic particles. 



