988 DIGESTION 



in their meaning. The distinction commonly made between them 

 at the present time, is based upon their place of action. Thus, we may 

 speak of an intracellular or endo-enzyme, when it acts in the cells in 

 which it originated, and of an extracellular or exo-enzyme, when it 

 acts outside its mother-cell. There is every reason to believe that 

 the action of yeast is due to the intracellular behavior of its endo- 

 enzyme (zymase), while, for example, saliva owes its chemical power 

 to its extracellular exo-enzyme, ptyalin.^ 



The Nature of Ferments. — Since ferments are not destroyed in 

 the course of the processes incited by them, the medium must contain 

 them even after the reaction has ceased. In the cells themselves 

 they are held in an inactive form which is somewhat different from that 

 of the active derivative. But, the fact that ferments are never pres- 

 ent in abundant amounts is not the only difficulty met with in isolat- 

 ing them in a sufficiently pure state to determine their chemical nature. 

 Many of them are very unstable and are rendered inert at 80°C., 

 and all of them are colloidal or semi-colloidal in their nature and not 

 easily diffusible. While this peculiarity enables them to adhere to 

 other colloidal material as well as to precipitates, it does not materially 

 facilitate their isolation, because the attempt of separating them most 

 generally diminishes their power of producing their characteristic 

 reaction, and hence, destroys practically the only means of detecting 

 their presence and identity. 



Since ferments are formed from Uving matter, they have been 

 considered as belonging to the class of the proteins. But, inasmuch 

 as their separation from these substances cannot be effected with 

 certainty, the fact that many of them give the characteristic reactions 

 of proteins cannot serve as a means of identifying them, because these 

 positive results may be due to the protein material which is still 

 adherent to them. Consequently, it must suffice at this time to desig- 

 nate them as^ organic substances which are derived from proteins 

 and possess a colloidal nature. 



Classification of Ferments. — Ferments are almost universally 

 present in nature. So great is their number, that the present discus- 

 sion must be restricted to those which take a more direct part in the 

 economy of the animal body. Moreover, since we know more about 

 digestion than we do about the processes of cellular assimilation, any 

 enumeration of this kind must be characterized by a preponderance 

 of the digestive enzymes. This fact is largely responsible for the cus- 

 tom of arranging them in accordance with the character of the reaction 

 produced by them, as follows: 



1 Since these topics are exhaustively dealt with in Mathews' and Hammarsten's 

 Textbooks of Physiological Chemistry, I shall discuss these purely chemical data as 

 briefly as possible. More complete references will also be found in Oppenheimer's 

 Handb. der Biochemie, 1910, and "Die Fermente and ihre Wirkungen," 1903; 

 Vernon's "Intracellular Enzymes," 1908; Euler's "General Chem. of the Enzymes" 

 (transl. by Pope) 1912, and Bayliss, "The Nature of Enzyme Action," 1908. 



