ENZYMES. 181 



one of dextrose. This enzyme is found in a large number of 

 yeasts and moulds, and also in a few bacteria. The diastases 

 also belong to this group. They convert starch into dextrin 

 and maltose, and occur in fungi, such as Mucor and Asper- 

 gillus. Cytase acts on cellulose, etc. 



(2) The glucoside-splitting enzymes, to which group 

 emulsin belongs, an enzyme discovered by Liebig and Wohler 

 in 1837. This enzyme changes amygdalin, a glucoside occur- 

 ring in bitter almonds, into benzaldehyde, hydrocyanic acid, 

 and glucose. 



(3) The fat-splitting enzymes (lipases), which decompose 

 fats into glycerine and fatty acids. 



(4) The albumen-splitting or proteolytic enzymes (pro- 

 teases), including pepsin, occurring in gastric juice, which 

 converts albuminoids into albumoses and peptone ; trypsin 

 in the intestinal secretions, which brings about further changes, 

 and, more in our domain, the endotryptase of yeast. 



II. The enzymes in this class have quite a different action 

 from the foregoing. They include the oxydases, the effect of 

 which is to split up molecular oxygen, and thus render it active. 



Buchner and Meisenheimer have shown that oxydase 

 occurs in acetic acid bacteria, and also in yeast. 



III. Reducing-enzymes (catalases). These appear to occur 

 in yeast and in certain bacteria. 



IV. A special group, the fermentation enzymes, or zymases, 

 the chemical action of which is entirely different. They 

 bring about true fermentation, which has been defined by 

 H. Fischer as an intro-molecular readjustment of oxygen, 

 accompanied both by oxidation and reduction of the several 

 carbon atoms, and an increase in the compounds of carbon 

 and oxygen, the usual result of the process being the breaking 

 up of a single molecule into several. In addition to the true 

 zymase (or alcoholase), this group includes the enzyme of 

 lactic acid fermentation. A further description of the former 

 will be found in the chapter on alcoholic yeasts. 



The activity of enzymes is influenced to a very considerable 

 degree by temperature. The optimum for zymase lies at about 

 30, for pepsin about 40, and for the proteolytic enzymes 

 (e.g., barley malt) at about 60 C. 



