2QO ESSENTIALS OF PHYSIOLOGY. 



organic catalysts, and some suggestions as to their mode of action 

 may be obtained from the study of the methods by which the simpler 

 inorganic catalysts produce their effects. Two examples of the latter 

 may be taken : ( 1 ) the effect of spongy platinum in bringing about the 

 combination of hydrogen and oxygen to form water, and (2) the 

 oxidation of indefinite quantities of S0 2 to form S0 3 by the inter- 

 mediation of * nitrogen peroxide. The platinum acts as a catalyst 

 because of its physical properties. Its low surface tension leads to a 

 concentration of the gases at its surface, whereby the molecules are 

 brought into close contact and their chemical interaction is favoured. 

 With this may be compared the first stage in the action of an enzyme, 

 in which a physical union takes place between enzyme and substrate 

 by the process known as adsorption. 



The nitrogen peroxide, on the other hand, forms an intermediate 

 chemical compound. After yielding part of its oxygen to oxidise S0 2 , 

 and thus becoming reduced to nitric oxide, the latter substance takes 

 up oxygen from the air to form nitrogen peroxide once more, and so 

 the process is repeated indefinitely. 



= N0 2 . 



The enzymes known as oxidases possibly act in a somewhat similar 

 way to the nitrogen peroxide in this reaction. Many oxidases are 

 believed to consist of an organic peroxide combined with a peroxidase. 

 The latter splits off oxygen from the peroxide, which again takes up 

 oxygen, and the process is repeated. In the case of enzymes it is found 

 that, with a limited amount of ferment, the same amount of substrate 

 is acted upon in a given time whether the observation is made early or 

 late in the process. It would, therefore, appear that the mode of 

 action of the organic catalyst is comparable with that of the inorganic 

 catalyst which acts chemically, the enzyme probably being adsorbed by 

 the substrate, which is then capable of taking up water and splitting 

 up, with the result that the enzyme is once again set free. 



In the case of most enzyme reactions the velocity of the process 

 tends to diminish after it has gone on for a time. An enzyme has not 

 only a specific affinity for its particular substrate, but it may also have 

 an affinity for the products of the reaction, and by forming com- 

 binations with these it may be put out of action. By-products are also 

 formed in some enzyme reactions, and these, for example acids or 

 alkalies, may ^either increase or decrease the power of the enzyme itself 

 and so modify the rate of the process. In some cases these by- 

 products destroy the enzyme and so bring the reaction to an end. 



