452 CHEMICAL DISCOVERY AND INVENTION 



by a similar agent originating under certain conditions in the 

 blood itself and called thrombin. 



Yet another class of enzymes possess the power of effecting 

 rapid oxidation, and in reference to this property are called 

 oxydases. The process of oxidation in the tissues is one of great 

 importance, for example in connection with respiration, but the 

 mode of action and origin of this class of enzymes is still very 

 obscure. 



The whole subject of enzyme action is so difficult and its 

 systematic study has been undertaken so comparatively recently 

 that only a few generalisations have as yet been recognised. 



As to the origin of enzymes it appears that, while they all 

 originate in living protoplasm, they do not exhibit in the early 

 stages of existence the characteristic catalytic actions which 

 later they exercise. In this preparatory condition the substance 

 is called a zymogen ; thus pepsin is formed from pepsinogen, 

 trypsin from trypsinogen, etc. 



It appears also that in many cases, of which one example, 

 emulsin, has already been mentioned, two enzymes habitually 

 act together to produce the characteristic change, hydrolytic or 

 other, which either enzyme separately is incapable of bringing 

 about or can at most carry partly into effect. An interesting 

 case of this kind has been described by Dr. A. Harden of the 

 Lister Institute, in the study of yeast juice. It was discovered 

 in 1897 by E. Buchner that the cell of the yeast as a whole is not 

 necessary to alcoholic fermentation. By grinding and subse- 

 quent filtration a juice may be separated which introduced into 

 a solution of sugar sets up this change. Harden's experiments 

 have since shown that the fermentation of glucose and fructose 

 by yeast juice is dependent not only on the enzyme, zymase, 

 but requires the presence of another substance the nature of 

 which is obscure, but which can be separated by dialysis and 

 withstands the temperature of boiling without destruction of its 

 activity. A phosphate is in addition always necessary. These 

 two substances the enzyme and the co-enzyme are incapable 

 separately of causing alcoholic fermentation. 



The co-operation of two colloidal agents in the production of 

 a given effect suggests that there may be bodies which are not 

 mutually helpful, but on the contrary antagonistic. The question 

 why the blood does not clot in the veins, why the stomach is not 

 itself dissolved by the digestive juices which it generates from 



