CHEMISTRY OF ENZYMES 349 



2. FERMENTING ENZYMES. 



(a) Alcoholic fermentation of glucose, levulose, mannose, etc., by zymase. 



(b) Lactic acid fermentation of lactose by lactic acid bacteria. 



(c) Butyric acid fermentation of lactose by the butyric bacteria, etc. 

 H COAGULATING ENZYMES. 



Rennin (Chymosin) which curdles milk. 

 Thrombin which coagulates blood. 

 Pectase soluble pectic bodies. 



4. OXIDIZING ENZYMES. 



(a) Oxidases which oxidize alcohols to acids, e.g., the action of Myco- 



derma aceti, etc., etc. 



(b) Catalases or peroxidases which set free oxygen from hydrogen per- 



oxide, or other peroxides, causing these substances to blue 

 guaiacum resin. 



METHODS EMPLOYED IN ISOLATION OF ENZYMES. 



The material from which the enzyme is to be extracted is 

 ground up with water or dilute alcohol together with a little 

 toluene to act as an antiseptic ; sometimes the material to be 

 extracted is previously dried by gently warming or by 

 dipping in absolute alcohol ; in some cases it is necessary to 

 destroy the cell walls, before extraction, by grinding up with 

 glass or Kieselguhr (infusorial earth) or by autolysis. 



From aqueous solutions enzymes may be precipitated in 

 the form of amorphous powders by the addition of an excess 

 of alcohol. 



Details for the isolation of certain enzymes are given 

 below. 



CHEMICAL CONSTITUTION. 



The chemical constitution and nature of enzymes is, as 

 yet, largely a matter of speculation, owing to the fact that it 

 is very difficult to obtain an enzyme in a pure condition ; 

 attempts at purification generally end in the diminution or 

 complete destruction of the activity of the material under ex- 

 amination. Owing to their tendency to be withdrawn (ad- 

 sorbed) from solution by precipitates formed in their presence, 

 it is difficult to purify them from proteins by any means 

 which involve the precipitation of the latter ; this may, to 

 some extent, account for the fact that all enzymes were 

 formerly supposed to be of a protein nature. According to 

 Pekelharing* pepsin is in some way related to the nucleo- 



* Pekelharing : " Zeit. physiol. Chem.," 1885, 9, 577. 



