72 PHYSICOCHEMICAL BASIS OF PHYSIOLOGICAL PROCESSES 



plex protein molecule is split up to render it absorbable from the intes- 

 tine, and the tissues appropriate from the blood those of the degradation 

 products that they require for the construction of protoplasm, which, 

 later, they decompose so as to utilize the energy which the organism 

 demands. All these processes are believed to be the work of enzymes. 



The Nature of Enzyme Action 



The changes brought about by enzymes can also be accomplished by 

 ordinary chemical means, but these have often to be of a very energetic 

 nature to accomplish what the enzyme can so quickly and quietly 

 perform. 



It is the custom to regard enzymes as catalysts. A catalyst is a sub- 

 stance which accelerates (or retards) a chemical reaction which in its 

 absence could proceed at a different (usually slower) pace. The action 

 of catalysts has been aptly likened to that of a lubricant. A weight 

 placed at the top of an inclined plane, so held that the weight only slowly 

 slips down, has its velocity greatly increased if its under surface be 

 oiled. The oil accelerates the action but does not affect the ultimate 

 result. Catalysts do not combine with the final products of the reaction, 

 these being, as a rule, the same as they would have been had no catalyst 

 been added. Another characteristic is the tremendous amount of chem- 

 ical change which even a trace of catalyst can induce. There are many 

 examples of catalysts in the inorganic world, among which may be cited 

 the action of spongy platinum on hydrogen peroxide. This substance 

 normally tends to decompose into water and oxygen, but if a small 

 amount of spongy platinum is added to it, the decomposition is greatly 

 accelerated: H 2 2 = H 2 + 0. 



A very good example of the action of an inorganic catalyst is that of 

 the hydrogen ion on cane sugar, or other disaccharides, in the presence 

 of water. It accelerates the hydrolysis. Cane sugar solution at room 

 temperature does not indeed, in sterile solution, undergo any appreciable 

 hydrolysis, but at 100 C. it does, which leads us to believe that, though 

 inappreciable, the action also occurs at room temperature. By adding 

 a little hydrochloric acid, or other acid not having an oxidizing effect 

 on sugar, we greatly accelerate the hydrolysis because of the hydrogen 

 ions present in the acid solution. Within certain limits the rate of hy- 

 drolysis is proportional to the amount of catalyst present. 



Enzymes, like other catalysts, produce their action when present in 

 very small amounts (e. g., sucrase can hydrolyze 200,000 times its weight 

 of cane sugar; diastase can convert starch to sugar in a dilution of 

 1-1,000,000) and there is a distinct relationship between the amount of 

 enzyme present and the rate of the reaction. The final product of the 



