MECHANISM OF METABOLISM. 137 



cannot ferment lactose; it produces no lactase, and therefore cannot 

 attack the lactose molecules; they would be easily attacked, if besides the 

 yeast, some lactase were added. Certain lactic bacteria cannot ferment 

 saccharose, because they do not form invertase. 



Invertase is, like diastase, a very common enzyme in green plants. 

 It is also produced by the larger number of molds and yeasts, and also of 

 bacteria. Maltase is not quite so common, and lactase is limited to a 

 few species of microorganisms. A few organisms are known which do 

 not secrete these enzymes but retain them within the cell. This is especi- 

 ally true of lactase, but is also known, in a few instances, of invertase. 

 The enzyme can be obtained from the broken cells. Such enzymes are 

 called endo-enzymes. 



The decomposition of carbohydrates has been followed from the most 

 complex representatives to the simplest ones, the monosaccharides. If 

 these are decomposed further, the resulting product is no longer a carbohy- 

 drate. The simplest sugars are decomposed by zymases, inside the mi- 

 crobial cell, into compounds which are generally called fermentation 

 products; these may result from alcoholic, lactic, butyric fermentations or 

 some other. 



Emulsin is an enzyme which is able to hydrolyze glucosides. Gluco- 

 sides occurring in plants are complex bodies which contain a sugar-radical. 

 Emulsin splits glucosides liberating the sugar, usually dextrose. The 

 typical example for emulsin action is the hydrolysis of amygdalin to 

 hydrocyanic acid, benzaldehyde and dextrose. 



C 20 H 27 O U N+ 2H 2 O= C 6 H 5 COH+ 2 C 6 H 12 O 6 + HCN. 



Amygdalin Benzaldehyde Dextrose Hydrocyanic acid 



Emulsin is found in many molds and bacteria, and recently has 

 been found in yeasts. Glucoside-splitting enzymes play an important 

 role in the fermentations of coffee-beans, cocoa, mustard and indigo. 

 In most of these fermentations, however, the emulsin is probably not 

 formed by microorganisms, but by the plant, from which the ferment- 

 ing material is derived. 



ENZYMES OF FATS. All the enzymes, acting on fat, decompose it 

 in the same manner; the fat molecule takes up three molecules of water, 

 breaking up into glycerin and three molecules of fatty acid, as indicated on 

 page 112. It is possible that there are several fat-splitting enzymes, but 

 the result of the cleavage process is always the same. The name formerly 

 assigned to enzymes of fat is steapsin, but this term is now almost exclu- 



