16 INTRODUCTION. 



a cleavage process is connected with a decomposition of water and a 

 taking up of its constituents it is called a hydrolytic cleavage. As an 

 example of such a cleavage we can mention the decomposition of starch 

 into sugar and the splitting of neutral fats into the corresponding fatty 

 acids and glycerin: 



C 3 H 5 (OH) 3 + 3 (Ci 8 H 36 O 2 ) . 



Tristearin Glycerin Stearic acid 



As a rule the hydrolytic cleavage processes as they occur in the ani- 

 mal body may be performed outside of it by means of higher tem- 

 peratures with or without the simultaneous action of acids or alkalies. 

 Considering the two above-mentioned examples, we know that starch 

 is converted into sugar when it is boiled with dilute acids, and also 

 that the fats are split into fatty acids and glycerin on heating them 

 with caustic alkalies or by the action of superheated steam. The heat 

 or the chemical reagents which are used for the performance of these 

 reactions would cause immediate death if applied to the living body. 

 Consequently the animal organism must have other means at its dis- 

 posal which act similarly, but in such a manner that they may work 

 without endangering the life or normal constitution of the tissues. The 

 animal body has in the enzymes such means and especially the second 

 chief group which have a hydrolytic action. 



Among the hydrolytic enzymes we must mention, in, the first place 

 the proteolytic, or those which dissolve protein, whose best studied repre- 

 sentatives, pepsin and trypsin, occur in the animal kingdom; the lipolytic 

 or fat-splitting; and the amylolytic enzymes, or diastases which act 

 upon the complex carbohydrates. In this group we include the 

 amylases, which act upon starch, and the invertases, which split the 

 disaccharides into simpler forms of sugar. In close relation to those 

 enzymes we may mention the glucoside-splitting enzymes, which occur 

 especially in the higher plants. Among the hydrolytic enzymes of the 

 animal kingdom we must also include arginase, which splits arginine into 

 urea and ornithine ; the two deamidizing enzymes adenase and guanase, 

 which convert the two bodies adenine and guanine, with the splitting 

 off of ammonia, into hypoxan thine and xan thine respectively; and the 

 hippuric-acid-splitting histozyme and the urea-splitting urease. The 

 protein-coagulating enzymes, chymosin or casein-coagulating, and thrombin 

 or blood-coagulating enzyme, belong to a special though not clearly 

 defined group. These enzymes are also included by some investigators 

 among the proteolytic enzymes. 



Many enzymes are secreted by the cells as such or as proenzymes. 

 They act outside of the cells in which they were formed, or they act after 



