62 PLANT PHYSIOLOGY 



sometimes seems very probable. Spongy platinum represents a 

 typical inorganic catalyst. In the proce'ss of manufacturing 

 sulphuric acid, spongy platinum may hasten the oxidation of 

 sulphur from the sulphurous to the sulphuric state by offering a 

 suitable surface on which the sulphur dioxide and oxygen can 

 condense in concentrations high enough to speed up their reac- 

 tion. The platinum black is not used up in this reaction. 



Enzymes are considered organic catalysts of a colloidal nature. 

 Because of their dispersed condition, they possess a large adsorb- 

 ing surface and therefore considerably speed up a reaction that 

 is going on in their presence. 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 proba- 

 bility owing to the gradual decomposition or denaturation 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 highly toxic to living protoplasm, e.g., 

 thymol, chloroform, toluol, and others, do not affect harmfully 

 the action of the enzymes; that is why these antiseptics are 

 usually added to solutions of organic substances in studying 

 enzymatic processes, for the purpose of precluding the devel- 

 opment of putrefactive and fermentative microorganisms. 

 Enzymes are thus chemically considerably more stable than 

 the protoplasm that produces them. But many substances, 

 hke salts of heavy metals, formaldehyde, and other toxic sub- 

 stances, will destroy enzymes by causing an irreversible precipi- 

 tation and a loss of their function. 



One of the greatest difficulties to be overcome is the question 

 of the chemical composition and nature of enzymes. The usual 

 methods of precipitation by alcohol or acetone are the same as 

 methods used for precipitating protein substances, with which 

 they usually precipitate. Because of this behavior, enzymes 

 were for quite a while classified with the protein group. But 

 careful investigations long ago created doubts as to the protein 

 nature of enzymes and led to the suggestion that the general 

 behavior in precipitation is due to coagulated proteins carrying 

 along with them the enzymes, just as they are known to carry 

 along dyes of a colloidal nature, fine sediments, etc. 



