SOLUBLE FERMENTS OR ENZYMES. 503 



or bitter principle also split off has a protective value and prevents 

 injury to the plant by animals. Besides emulsin, a few other glucoside- 

 splitting enzymes are known. 



Lipase, a fat-splitting enzyme; laccase, an oxidizing enzyme con- 

 cerned in the production of the famous black varnish used in lacquer 

 work; and urease, an enzyme that converts urea into ammonium car- 

 bonate, are among the other better-known enzymes. 



There is reason to believe also that the various anti-microbic sub- 

 stances found in the bodies of some artificially immunized and some 

 naturally immune animals are to be regarded as enzymes, as is like- 

 wise the substance that is found in the blood of typhoid patients and 

 that has a 'clumping' or 'agglutinating' action upon the typhoid bacilli. 

 Eesearch in this direction has not, however, proceeded far enough to 

 enable us to offer anything more than a conjecture as to the real char- 

 acter of the 'agglutinines' and lysines.' 



The precise mode of action of enzymes has been the theme of 

 much speculation. Perhaps the simplest and most natural view of 

 some cases is to suppose that the enzyme combines first, for example, 

 with a molecule of water, and then attaches itself to the body upon 

 which it acts. This new compound, meeting with another molecule of 

 the same substance, is then decomposed into the body which the enzyme 

 produces and the enzyme itself. The enzyme thus acts as a simple 

 intermediary, bringing the molecule of water or oxygen in closer contact 

 with the fermentable substance. This view has certain arguments in its 

 favor, as for instance the fact that the enzyme does not exhaust itself in 

 the course of the changes that it produces. If it is unceasingly decom- 

 posed and reconstituted the reason for this is clear. 



Such an explanation, however, is hardly valid for the action of 

 zymase upon sugar and for the reversible action of maltase. Now 

 there are certain facts regarding the action of mineral acids upon 

 sugars and proteids, of various salts upon the phenomena of clotting 

 and oxidation and of other changes brought about by inorganic 

 substances which render it difficult to set enzyme action apart as a' 

 thing by itself. The action of an enzyme is essentially 'catalytic,' 

 that is, it is able to exert an influence wholly out of proportion to its 

 quantity, and itself remain unaltered at the end of the process. It has 

 been pointed out that the influence of an enzyme or indeed any catalytic 

 agent is simply to retard or accelerate changes which ordinarily take 

 place more slowly or more rapidly. In other words, an enzyme simply 

 influences the rate of change, not the final condition of the substance 

 upon which it acts. The nature of the change, the final state of chemi- 

 cal equilibrium, is determined by the chemical forces within the sub- 

 stance itself, the speed at which the change occurs is determined 

 by the enzyme. 



