MECHANISMS OF OXIDATION 143 



cells contain catalase, an enzyme whose function it is to destroy 

 hydrogen peroxide as it is formed. 



In support of his own theory Warburg points out that while 

 it is true that the oxidation of succinic acid in the presence of 

 methylene blue proceeds both in the absence and in the presence 

 of hydrocyanic acid, the latter substance inhibits the reaction 

 when oxygen is employed as the hydrogen acceptor in place 

 of methylene blue ; this, according to him, shows that the 

 oxygen is unable to function when the iron is inactivated 

 by hydrocyanic acid. Wieland, on the other hand, explains 

 the action of hydrocyanic acid as being due to its destructive 

 effect upon catalase which is thereby rendered unable to 

 prevent the accumulation of hydrogen peroxide, which sub- 

 stance is toxic to the cell. To this suggestion Warburg objects 

 on the ground that such an effect would be permanent whereas 

 he has shown the retarding effect of hydrocyanic acid to be 

 temporary. 



A difficulty in the way of accepting the idea that iron 

 is an essential element in any oxidizing mechanism is also 

 presented by the fact that Willstatter has shown that the 

 purest preparations of peroxidase are free from iron. Further- 

 more, it has been pointed out that the action of hydrocyanic 

 acid is not always to depress oxidation ; it may occasionally 

 stimulate * A further obstacle in the way of interpreting 

 the inhibiting action of hydrocyanic acid as being due to its 

 combining with iron, is the observation of Moureu and Du- 

 fraisse f that hydrocyanic acid, in the entire absence of iron, is 

 able to act as a powerful depressant of oxidation ; it is as these 

 authors describe it, an antoxygen. According to the theory 

 of Dufraisse and Moureu, most oxidizable substances remain 

 unattacked by atmospheric oxygen owing to the presence of 

 traces of a negative catalyst or antoxygen ; a substance which 

 is particularly active in this respect is hydroquinone, which, 

 in small quantity, is able to prevent entirely the autoxidation 

 of benzaldehyde through benzaldehyde peroxide to benzoic 

 acid, a reaction which, as is well known, takes place very 



* Buchanan : '* Science," 1927, 66, 23S. 



f Moureu and Dufraisse : " Compt. rend.," 1926, 183, 685. 



