ON THE NATURE OF HEMOPROTEIN REACTIONS 



was thought originally to differ in some way from compound I 

 in the peroxide systems, possibly by stabilization through com- 

 plex formation with the chloriridite ion (13). However, a 

 simpler kinetic explanation for the greater stability is to be 

 preferred, for peroxides unlike chloriridate cannot readily 

 oxidize compound II back to compound I, and hence, with 

 chloriridate, compound I will persist all the time excess chlor- 

 iridate is present. The simplest interpretation of these observa- 

 tions is that compound I is also a higher oxidation state, which 

 reacts as a quinquevalent iron compound would. In view of 

 spectroscopic and other similarities, the same oxidation-reduction 

 relationship is probably true for compounds I and II of catalase, 

 which has recently been found to react with hypochlorous acid 

 to give intermediate compounds similar to, or identical with, 

 compounds I and II formed by peroxides (14). The specificity 

 shown by peroxidase and catalase evidently follows from minor 

 differences in the reactivity of the two higher oxidation states, 

 and a mechanistic interpretation in terms of a "reaction cage" 

 has been put forward recently (10). 



These higher oxidation states, which can be represented in 

 chemical equations by the symbols Fepe^, Fcpg^ and FcMb? etc., 

 could result either from electron or hydrogen atom abstraction 

 from the porphyrin ring system, or from oxidation at the iron 

 atom itself. The latter is more likely for the following reasons. 

 Experiments in which cyanide or fluoride together with peroxide 

 are added to ferrimyoglobin (24), or ferricatalase, indicate 

 competition between the reagents for the iron atom. The 

 absence of any higher oxidation states of ferricytochrome c is 

 additional evidence that an exposed iron atom is required, and 

 furthermore that the oxidation process is more complicated than 

 electron transfer. This is also substantiated by the observation 

 that none of the intermediate compounds forms a series of 

 derivatives with various ligands, as do the ferric and ferrous 

 oxidation states. This last observation also suggests that both 

 the primary (electrovalent) and the coordination valencies of 

 the iron atom are fully utilized in tlie highier oxidation states. 



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