Chemical Changes in Animal Organism. 293 



presence of minute quantities of an iron, salt in these 

 cases the platinum and the iron salt peroxide act as the 

 catalysts. 



An enzyme may be regarded as a catalyst produced 

 by a living organism. A very large variety of such sub- 

 stances exist, of which examples will be given below. 

 Some will act on starches, others on fats, others on pro- 

 teins, whereas another class is concerned in various 

 oxidative processes. A convenient terminology has been 

 suggested to indicate the character of the action. The 

 substance which undergoes change under the influence of 

 the enzyme is known as the " substrate," and the termina- 

 tion ase added to a name of the substrate, or an abbrevia- 

 tion of the same, to designate the ferment. Thus, a fer- 

 ment which degrades starch (otherwise called diastase) may 

 be designated an " amylase," a fat-digesting enzyme is a 

 " lipase," and a protein-digesting ferment a " protease," and 

 the digesting actions themselves are designated as amylo- 

 clastic, lipoclastic, or proteoclastic. 



Very little is known of the chemical character of the 

 various enzymes, and none have been prepared in a pure 

 condition, nor is much known of the mechanism of their 

 action. There is a certain amount of evidence that they 

 enter into combination with their substrate, but it would be 

 necessary to extend the discussion beyond the scope of this 

 book in order to enter into detail as to the mode of action. 

 There are, however, certain characteristics of enzymes 

 which are well established. They are complex substances, 

 which for the most part are incapable of passing a parch- 

 ment membrane, and their properties are destroyed when 

 they are heated above a certain temperature. If a solu- 

 tion of an enzyme is boiled, the fermentative properties are 

 destroyed. An enzyme will act furthermore only under 

 certain specified conditions. It will act, for example, only 



