OXIDASES, PEROXIDASES, AND CATALASE 89 



or catalase, respectively. This observation is usually interpreted in 

 terms of an irreversible destruction of the active protein component 

 of the enzyme in some side reaction, e.g., by denaturation or as a 

 result of attack by other enzymes present as impurities. While this 

 may be so, it is not always easy to distinguish clearly between a 

 true catalysis, where the catalyst is progressively eliminated by side 

 reactions, and an induced reaction, where the inductor is slowly 

 converted into an inactive form. This is particularly true of experi- 

 ments in biological systems. According to all three theories outlined 

 above, the reaction comes to a standstill once all ferrous iron has 

 been converted into ferric iron. A continuation of the process is 

 obviously possible only if the substrate or some other component 

 in the system is able to reduce Fe^^^ back to Fe^^.* One could 

 visualize "hybrid" processes where the ferrous iron, in the main 

 reaction, acts as an inductor and where the inactive ferric iron 

 is slowly reduced to the active ferrous form by some "outsider" 

 such as a thiol and is thereby enabled to start the induced reaction 

 all over again. Such a situation, if encountered in living cells, would 

 probably defy any attempt to distinguish between true and apparent 

 catalysis, especially if the reducer is constantly replenished from 

 suitable precursors, e.g., glutathione from protein breakdown. 



We go one step further. If iron can break down the potential 

 barrier, shielding stable substrate molecules, either by the ferri- 

 ferro cycle or by the induced reaction mechanisms just mentioned, 

 is it not possible that iron, when linked up in suitable complexes, 

 could bring about changes in certain substrate molecules or initiate 

 chain reactions without itself suffering a change in valency? It is 

 on this possibility that the issue of the mechanism of catalase action 

 largely hinges. It is well established that the iron in catalase exists 

 in a remarkably stable ferri state. The enzyme is invariably isolated 

 from all sources as the ferri foiTn and it defies reduction with 

 activated hydrogen or with hydrosulfite. This property is unique 

 among hemin proteins; it is not shared even by peroxidase, which 

 may be readily reduced to the ferro form by agents such as sodium 

 hydrosulfite. Moreover, it can be shown that the spectroscopically 

 well-defined intermediate arising in the catalase-ethyl hydrogen 



* Theoretically, ferri ion could act catalytically by being reversibly oxidized 

 to a higher stage of valency. Supporting such a view are the spectroscopic 

 observations of Bohnson and Robertson (J. Amer. Chem. Soc, 45, 2493 (1923), 

 on weak acid solutions of ferric salts in the presence of hydrogen peroxide. The 

 color changes occurring in this system were interpreted by these workers to 

 be due to the formation of ferric acid. See, however, Haber and Weiss ( 18 ) . 



