210 TEXTBOOK OF PLANT PHYSIOLOGY 



ticipation in the intermediate steps of a reaction is highly probable. 

 A typical inorganic catalyzer is spongy platinum, which, in the pro- 

 duction of sulphuric acid by the contact method, can oxidize an 

 unlimited quantity of sulphur dioxide without itself undergoing 

 any special change. In the main, its behavior is based on the 

 fact that, due to adsorption, it condenses the reacting gases on 

 its enormous surface and thus considerably accelerates the re- 

 action. 



Enzymes are considered to be organic catalyzers of a colloidal 

 nature. Because of their dispersed condition and adsorbing sur- 

 faces, they considerably speed up a reaction going on in their pres- 

 ence. Being colloidal substances, enzymes are unstable in many 

 ways. They coagulate when boiled, thus losing their activity. 

 Solutions of enzymes, if left to stand for a long time, likewise lose 

 their activity, in all probability owing to the gradual breaking down 

 of the enzyme. They are easily precipitated in water solutions by 

 various dehydrating substances, such as alcohol or acetone, but 

 may be redissolved. Many substances, like salts of heavy metals, 

 formaldehyde, and other toxic substances, will destroy enzymes, 

 by causing an irreversible precipitation and a loss of their function. 



The chemical nature of enzymes still remains unsolved. The 

 usual methods of obtaining them from solutions, precipitation by 

 alcohol or acetone, are the same as methods of precipitating pro- 

 tein substances, together with which they usually precipitate. 

 Because of this behavior, enzymes were for quite a while classified 

 with the protein group. This, however, has been completely dis- 

 carded as a result of newer researches. Apparently, the general 

 behavior in precipitation is based upon the fact that in their coag- 

 ulation protein substances carry along w T ith them the enzymes, just 

 as they are known to carry along dyes of a colloidal nature, fine 

 sediment, etc. Upon this ability to remove from a solution various 

 admixtures is based, for example, the well-known use of the white 

 of an egg for the clearing of a turbid liquid. 



As a result of the investigations by Willstatter and his col- 

 leagues, it has been possible during the past few years to under- 

 stand somewhat more clearly the chemical nature of enzymes. 

 Willstatter has based his work largely on the method of selective 

 adsorption of enzymes by certain finely ground substances, such as 

 clay and kaolin. Owing to their extremely high adsorptive capa- 

 city, enzymes are removed from solutions more readily by these 



