PHILIP GEORGE 



red in color but with different absorption bands from the 

 ferrimyoglobin compound (5,30). 



More detailed studies have shown that the red intermediate 

 compounds of ferriperoxidase and ferrimyoglobin are not in 

 equilibrium with their parent substrate, peroxide, as a true 

 "enzyme-substrate complex" would be, i.e., E + S ;^ ES; 

 both are formed in irreversible reactions (11,20). Furthermore 

 spectrophotometric titration with reducing agents showed both 

 compounds to be single equivalent oxidation products of the 

 ferric state of the hemoproteins. Hence the peroxide molecule 

 or its anion, which possess two oxidizing equivalents, cannot be 

 a constituent of these compounds. They do not correspond to 

 "enzyme-substrate complexes" nor hemoprotein complexes in 

 the usual sense, but they react as quadrivalent iron compounds 

 would. The titration results do not, however, distinguish 

 whether the site of the oxidation is at the iron atom or some other 

 group in the molecule — a question which is taken up again 

 shortly. In accord with their nature as higher oxidation states, 

 other strong oxidizing agents were found to form them besides 

 peroxides, for example, HOCl, HOBr, HCIO2, CIO2, and O3, 

 etc., with ferriperoxidase, CIO2 and HCIO2 with ferrimyoglobin 

 and most effective of all with both hemoproteins, the simple 

 electron-accepting oxidizing agents, potassium chloriridate and 

 molybdicyanide (12,13,22). 



The change from the green intermediate, compound I, to 

 the red intermediate, compound II, was recognized to be a single 

 equivalent reduction step, and a new mechanism for peroxidase 

 action was put forward on this basis (9) ; later work has con- 

 firmed this (3). Compound I is thus two equivalents above the 

 ferric state, and therefore, since peroxides possess two oxidizing 

 equivalents, on this evidence alone it could have the true 

 peroxide complex structure. But there is good spectroscopic 

 evidence that the majority of oxidizing agents which give the 

 red compound II of peroxidase also form an intermediate, 

 similar to, or identical with, the green compound I formed by 

 peroxides (12). A particularly stable green intermediate is 

 produced with potassium chloriridate, which for this reason 



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