92 A SYMPOSIUM ON RESPIRATORY ENZYMES 



not involve the intermediary formation of hydrogen peroxide. This 

 is in opposition to the general views on the mechanism of autoxi- 

 dation and would appear highly improbable, since it postulates a 

 reaction of a very high order. Furthermore, on the basis of Keilin 

 and Hartree's hypothesis, the reaction, when proceeding in nitrogen, 

 should exhibit the characteristics of an autocatalytic reaction be- 

 cause of the production of increasing amounts of oxygen during the 

 process. This, however, is not indicated in the data of these authors. 

 Sumner and Dounce (cf. 68), who also were unable to confirm Keilin 

 and Hartree's observations with respect to the importance of oxygen 

 for catalase action, prefer the following schema, which is based on 

 earlier ideas of Haber, Euler, and Liebermann: 



Step I Fe-OH + H^O^ = Fe-OOH + H2O 



Step II Fe-OOH + H2O2 = Fe-OH + H2O + O2 



In this schema, catalase in its ferric form is represented by the 

 symbol Fe-OH, which is often employed for methemoglobin, a 

 molecule very similar to catalase. The symbol Fe-OOH represents 

 an intermediary catalase peroxide which is assumed to react with 

 a fresh substrate molecule to yield oxygen, water, and the free ferri 

 form of the enzyme. It will be noted that this hypothetical schema, 

 in contrast to that of Keilin and Hartree, does not involve the ferri- 

 ferro cycle and is in agreement with the spectroscopic observation 

 of a catalase-peroxide complex in the enzyme-ethyl hydrogen per- 

 oxide reaction (58, 60). But the writer doubts very much whether 

 so simple a schema is adequate to explain all the features of the 

 enzymatic catalysis. We must not forget that the decomposition of 

 hydrogen peroxide can be catalyzed by a variety of agencies besides 

 the enzyme, such as ultraviolet light, dust, metallic and non-metallic 

 surfaces, colloidal platinum, inorganic ferri and ferro salts, cobalti 

 salts, etc. We are therefore confronted with the necessity of finding 

 an explanation for the reaction mechanism which will be equally 

 applicable to these various catalysts. The central theme is, of course, 

 the mode in which hydrogen peroxide is transfonned into water 

 and oxygen. The most cogent formulation for this central process 

 has been given by Haber and Weiss (18): 



( 1 ) OH -f H2O2 = H2O + HOl 



(2) Ha + HaOj = O2 -(- H2O + OH 



In this schema OH and HO. are monovalent radicals. As Haber em- 

 phasizes, in a paper published posthumously (18), the progress of 



