COMPOSITION OF FOOD AND ACTION OF ENZYMES. 659 



undergoes no change, and it is said, therefore, to act by catalysis. 

 Many similar catalytic reactions are known, and the chemists have 

 reached the important generalization that in such reactions the 

 catalyzer, platinum in the above instance, simply hastens a process 

 which would occur without it, but much more slowly. A catalyzer 

 is a substance, therefore, that alters the velocity of a reaction, 

 but does not initiate it. This idea is illustrated very clearly by 

 the catalysis of hydrogen peroxid. This substance decomposes 

 spontaneously into water and oxygen according to the reaction 

 H 2 a = H 2 O + O, but the decomposition is greatly hastened by 

 the presence of a catalyzer. Thus, Bredig has shown that plati- 

 num in very fine suspension, so-called colloidal solution, exerts a 

 marked accelerating influence upon this reaction; one part of the 

 colloidal platinum to 350 million parts of water may still exercise a 

 perceptible effect. Now, the blood and aqueous extracts of various 

 tissues also catalyze the hydrogen peroxid readily, and this effect 

 has been attributed to the action of an enzyme (catalase). The 

 view has been proposed, therefore, that the enzymes of the body 

 act like the catalyzers of inorganic origin : they influence the velocity 

 of certain special reactions. Such a general conception as this 

 unifies the whole subject of fermentation and holds out the hope 

 that the more precise investigations that are possible in the case of 

 the inorganic catalyzers will eventually lead to a better understanding 

 of the underlying physical causes of fermentation. It should be 

 borne in mind, however, that some of the best known of the ferment 

 actions of the body, such as the peptic or tryptic digestion of pro- 

 teid, fit into this view only theoretically and by analogy. As a 

 matter of fact, albumins at ordinary temperatures do not split up 

 spontaneously into the products formed by the action of pepsin; 

 if we consider that the pepsin simply accelerates a reaction already 

 taking place, it must be stated that this reaction at ordinary 

 temperatures is infinitely slow, that is, practically does not occur. 

 At higher temperatures, however, similar decompositions of al- 

 bumin may be obtained without the presence of an enzyme. 



Reversible Reactions. It has been shown that under proper 

 conditions many chemical reactions are reversible, that is, may 

 take place in opposite directions. For instance, acetic acid and 

 ethyl-alcohol brought together react with the production of ethyl- 

 acetate and water: 



CH 3 COOH + C 2 H 6 OH = CH 3 COOC 2 H 5 + H 2 O. 



Acetic acid. Alcohol. Ethyl-acetate. Water. 



On the other hand, when ethyl-acetate and water are brought 

 together they react with the formation of some acetic acid and 

 ethyl-alcohol, so that the reaction indicated in the above equation 



