ENZYMES AND THEIR ACTION. 5 



shows the greatest activity at a certain temperature called the 

 optimum temperature; there is also a minimum and a maximum 

 temperature for each specific enzyme. Their action is inhibited by 

 sufficiently lowering the temperature, and the enzyme, if in solution, 

 is entirely destroyed by subjecting it to a temperature of 100 C. 

 The best known enzymes, whether derived from warm-blooded or 

 cold-blooded animals, are most active between 35-45 C. The 

 nature of the surrounding media alters the velocity of the enzymatic 

 action, some enzymes being more active in acid solution whereas 

 others require an alkaline fluid. 



Many of the more important enzymes do not occur preformed 

 within the cell, but are present in the form of a zymogen or mother- 

 substance. In order to yield the active enzyme this zymogen must 

 be transformed in a certain specific manner and by a certain specific 

 substance. This transformation of the inactive zymogen into the 

 active enzyme is termed activation. For instance, the zymogen of 

 the enzyme pepsin of the gastric juice, termed pepsinogen, is acti- 

 vated by the hydrochloric acid secreted by the gastric cells (see 

 p. 119), whereas the activation of the trypsinogen of the pancreatic 

 juice is brought about by a substance termed enterokinase 1 (see 

 p. 141). These are examples of many well known activation pro- 

 cesses going on continually within the animal organism. The 

 agency which is instrumental in activating a zymogen is generally 

 termed a zymo-exciter or a kinase. In the cases cited hydrochloric 

 acid would be termed a zymo-exciter and enterokinase would be 

 termed a kinase. 



After filtering yeast juice, prepared by the Buchner process (see 

 p. 2), through a Martin gelatin filter, Harden and Young showed 

 that the colloids left behind and the filtrate were both inactive 

 fermentatively. Upon treating the colloid material (enzyme) with 

 some of the filtrate, however, the mixture was shown to be able to 

 bring about pronounced fermentation. It is believed that a co- 

 enzyme present in the filtrate was the efficient agent in the trans- 

 formation of the inactive enzyme. It is necessary to make frequent 

 renewals of the co-enzyme in order to maintain continuous fermen- 

 tation. It was further shown that this co-enzyme, in addition to 

 being diffusible, was not destroyed by boiling and that it disappeared 

 from yeast juice when this latter was fermented or allowed to 

 undergo autolysis. The exact nature of this co-enzyme of zymase 



1 According to Delezenne trypsinogen may be rapidly activated by soluble 

 calcium salts. 



