Page:Popular Science Monthly Volume 59.djvu/513

Rh 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 concerned in the production of the famous black varnish used in lacquer work; and urease, an enzyme that converts urea into ammonium carbonate, are among the other better-known enzymes.

There is reason to believe also that the various anti-microbic substances found in the bodies of some artificially immunized and some naturally immune animals are to be regarded as enzymes, as is likewise 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 character 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 decomposed 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 chemical equilibrium, is determined by the chemical forces within the substance itself, the speed at which the change occurs is determined by the enzyme.