CHEMICAL COMPOSITION OF PLANTS 61 



Investigations in recent years, especially those of a most 

 eminent authority in biochemistry, Richard Willstatter, and his 

 followers, have produced great changes in our ideas concerning 

 proteolytic enzymes. At present, they are divided into two 

 large groups: proteinases, affecting unchanged colloidal protein 

 substances; and ereptases, the activity of which is restricted to 

 the decomposition of peptones and polypeptides. Proteinases 

 in their turn are divided into three groups: (1) pepsinases, active 

 in a highly acid medium (pH = 2) (this is chiefly pepsin of the 

 stomach juice); (2) enzymes, such as papain, active in a shghtly 

 acid or neutral medium (pH = 4 to 7); and (3) enzymes, like 

 tryptases, active in an alkaline medium (pH = 8 to 9). This 

 division corresponds to principal differences in the conditions 

 of their activity. Pepsin decomposes cations of protein formed 

 in an acid medium; trypsin, anions of protein formed in an 

 alkaline medium; and papain decomposes isoelectric proteins. 



Peptase and tryptase are found chiefly in animals in the 

 secretions of the digestive glands and are active outside the cells 

 in the digestive tract. In plants, they are found only in excep- 

 tional cases, e.g., in insectivorous plants that catch and digest 

 their prey in special organs or in traplike leaves that secrete a 

 digestive juice from special glands. In most cases, proteolytic 

 enzymes of plants are active inside the cell and are not capable 

 of diffusing into the surrounding medium. For a long time, these 

 enzymes were grouped with tryptases, but recently it has been 

 made clear that they belong to the group of papain. The plant 

 ereptases must also be referred to the group of intracellular 

 enzymes. The animal organism possesses an enzyme correspond- 

 ing to the plant papain, viz., catepsin, which is likewise active 

 inside the cell. 



15. General Properties of Enzymes. Methods of Their 

 Isolation and Purification. — Enzymes are usually defined as 

 organic catalysts produced by living cells but capable of mani- 

 festing their activity outside of the cell and acting on specific 

 substances. 



The term ''catalysts" designates substances that do not enter 

 into chemical reactions as reactants but accelerate the reactions 

 by their physical properties. They are not found in the end 

 products of the reaction, or consumed in the reaction, though 

 their participation in intermediate reactions is not excluded and 



