462 DIGESTION 



Enzymes. — If a sterile suspension of starch and water is protected from 

 contamination by micro-organisms, it may be allowed to stand indefinitely with- 

 out conversion of any detectable amount of the starch into sugar. In order 

 to accomplish such a hydrolysis in the laboratory it is necessary to treat the 

 starch suspension with a strong acid, and to subject it to a relatively high 

 temperature. In living organisms, however, this starch to sugar transforma- 

 tion, and other similar hydrolytic reactions, occur in media which are seldom 

 strongly acid, and at temperatures which are usually not greatly different 

 from those of the environment. This is made possible by the presence in 

 living organisms of substances known as enzy?nes which partake of the nature 

 of catalysts. In fact enzymes are frequently spoken of as "organic catalysts." 

 Catalysts are substances which accelerate or retard the rate of a reaction, yet 

 are themselves found to be chemically unchanged at the end of that reaction. 

 In most of the known examples of catalysis, including enzymatic reactions, 

 the effect of the catalyst is to accelerate rather than to retard the reaction. 

 It has often been considered that catalysts cannot start a reaction but that they 

 merely influence the speed of a reaction which is already in progress, although 

 perhaps only at an infinitesimal rate. Many contemporary authorities, how- 

 ever, incline to the view that catalysts, including enzymes, can actually initiate 

 reactions. 



Among the well known inorganic catalysts are such substances as platinum 

 black, metallic oxides, and various metals in the colloidal state. Many cata- 

 lysts appear to owe their properties largely to the possession of an enormous 

 specific surface at which free secondary valences are present. These cause 

 the interacting compounds to be adsorbed on the surface of the catalysts. In 

 this adsorption process compounds are brought into such intimate contact on 

 the surface of the adsorbent that reactions are initiated which would not 

 otherwise occur, or at least would proceed at a much slower rate. The com- 

 pounds produced by such reactions are not, in accordance with the principles 

 of adsorption equilibria, retained in quantity at the surface of the adsorbent. 

 Hence the products of the reactions are released from the surface of the 

 catalyst almost as rapidly as they are formed. The catalytic action of enzymes 

 is also believed to depend on an adsorption mechanism. 



Enzymes, as far as is known, are produced only by the protoplasm of 

 living cells. It is not know whether there are definite centers of enzyme syn- 

 thesis in the protoplasm, or whether this property is possessed by protoplasm 

 generally. 



Cells which lack a specific enzyme in its active form are frequently found 

 to contain a chemical precursor of the enzyme which under certain condi- 

 tions is converted into the enzvme. Such substances are termed zymogens. 



