MECHANISM OF METABOLISM. 135 



exo-enzymes (secreted) is not exact, either, since invertase and lactase are 

 retained in the cells of some organisms and secreted by others. 

 The following classification is used in the further discussions: 



I. Hydrolytic Enzymes. 



1. of carbohydrates: cellulase (cytase), diastase (ptyalin, amylase), invertase, 

 lactase, maltase. 



2. of fats: lipase (steapsin). 



3. of proteins: 



a. proteolytic (proteases): pepsin (peptase), trypsin (tryptase), erepsin 

 (ereptase). 



b. coagulating: thrombase, rennet (chymosin). 

 II. Zymases. 



1. of carbohydrates: alcoholase, lactacidase. 



2. of other nitrogen-free bodies: vinegar-oxidase. 



3. of proteins: endo-tryptase, autolytic enzymes, amidase, urease. 



III. Oxidizing Enzymes. 



Vinegar-oxidase, tyrosinase. 



IV. Reducing Enzymes. 



Katalase, reductases of nitrates, sulphur, sulphites, telluric salts, methy- 

 lene blue, litmus. 



Several names have been given to some of the enzymes; these are 

 found in parentheses in the above classification. 



The general action of enzymes being explained in the preceding pages, 

 it remains to describe more in detail the different enzymes of microbial 

 origin. 



HYDROLYTIC ENZYMES. 



ENZYMES OF CARBOHYDRATES. Enzymes which decompose carbo- 

 hydrates are very commonly found in nature, because carbohydrates 

 constitute a very extensive and common group of organic matter. By 

 far the largest part of the dry plant consists of cellulose, starch and sugar. 

 To decompose them, enzymes are necessary. The chemical reaction 

 of these enzymes is hydrolytic; in other words, the larger molecule is 

 broken into smaller ones by the simple addition of water. Thus, the 

 cellulose-destroying enzyme, called cellulase or cytase, decomposes the 

 cellulose into soluble sugars after the following formula: 



C 6 H 10 5 +H 2 0=C 6 H 12 6 



or, considering that the cellulose molecule is really many times C 6 H 10 O 5 , 

 the formula will be more accurately written 



(C fl H 10 6 ) n +nH 2 = nC fl H 12 B 



