448 CHEMICAL DISCOVERY AND INVENTION 

 CHAPTER XXXI 



ENZYMES 



THE process by which, grape and other saccharine vegetable 

 juices are converted after much frothing, turbidity, and ultimate 

 clarification, into an intoxicating drink is as old as the history 

 of mankind. But the true nature of the change which goes on 

 was established only after a long controversy, in which the repre- 

 sentatives of a purely mechanical or physical theory originated 

 by Liebig were finally defeated by Pasteur, who established the 

 dependence of ordinary alcoholic fermentation on the action of 

 the living yeast cell. 



This was fifty years ago, but since that day investigations into 

 the phenomena of fermentation have reached a new stage, in 

 which attention is concentrated on the agents by which the cell 

 accomplishes its own growth and development at the same time 

 that it brings about chemical changes in the surrounding medium. 

 These agents are called enzymes, a term which was brought into 

 use so recently as 1878 by the German physiologist W. Kiihne. 



Enzymes are not organisms like moulds or bacteria, but may 

 be described as unorganised, colloidal, nitrogenous substances 

 universally present in living animal and vegetable tissues and, 

 though lifeless themselves, are at present producible only from 

 living matter. They are distinguished by a remarkable catalytic 

 action on carbon compounds, especially carbohydrates, fats, and 

 proteins. Their actions are in some cases selective, but not 

 always, and they are coagulated and rendered inactive by a 

 temperature below that of boiling water. 



The general nature of enzyme action will be understood if a 

 few individual cases are described. 



One of the earliest to be recognised and one of the most im- 

 portant of these substances is diastase. Early in the nineteenth 

 century it became known that an aqueous extract of malt 

 possessed the power of changing soluble starch very rapidly into 

 dextrin and sugar. Pay en and Persoz attempted in 1833 to 

 isolate the active constituent of malt, but with no great success. 

 Forty years later O'Sullivan described a method which was as 

 follows : finely ground pale barley malt was mixed with sufficient 

 water just to cover it, and after three or four hours the extract 



