FERMENTS AND FERMENTATION. 93 



sidered in trying to explain why the pancreas is not digested 

 by its own trypsin, or in general why any organ is not digested 

 by the ferments contained in it. This explanation of the 

 immunity of tissues against their own ferments can hold in 

 only a few cases, however. For other enzymes the existence 

 of proferments has not been so definitely established. 



There seems to be little doubt but that proferments exist 

 for rennin (caseinase), steapsin (lipase), and pepsin (acid- 

 proteinase). The existence of a preferment of acid-proteinase 

 in the gastric mucosa is demonstrated by the following facts: 

 If the minced mucous membrane of the stomach of an animal 

 is extracted with a dilute alkaline solution and the whole is fil- 

 tered, a clear solution is obtained which does not act on proteins. 

 If this solution is acidulated with hydrochloric acid it digests 

 proteins rapidly. But if this acid solution is rendered alka- 

 line once more, and is then again acidulated with hydro^- 

 chloric acid, the digestive properties of the solution for pro- 

 teins will have been lost. These experimental facts are 

 explained on the assumption that pepsin is readily destroyed 

 in an alkaline medium, while its^zyjnogen is not, and that the 

 conversion of the zymogen into the ferment is accomplished 

 through the acid.. 



The relation that ferments bear to their proferments is 

 not understood. The conversion of the preferment into 

 the ferment may at times be accomplished through the 

 action of a specific substance, such as enterokinase. But 

 the character of this substance, which by some is regarded 

 as itself a ferment (whence the name), by others as a sub- 

 stance differing markedly from this group of compounds, 

 is also not understood. In the majority of instances dilute 

 acids are able to bring about a conversion of the preferment 

 into the ferment. Whether this represents more than a 

 simple change in the physical state of a colloid is still un- 

 known. 



2. Inorganic Ferments. Beyond what has already been 

 said we cannot here enter intc a discussion of the various 



