440 IX. HEMATIN ENZYMES, II 



and that tlie radicals formed in reactions 1 and i initiate only short chains, 

 while reaction 3 breaks the chains. Thus the concentration of the radicals 

 is assumed to remain so small that no noticeable oxidation of hydrogen 

 donors takes place. In fact, platinum metal acting catalatically on hydrogen 

 peroxide likewise does not act as a peroxidase. An unexpected specificity of 

 the action of substances added to act as "chain-breakers" on the catalatic 

 and photolytic destruction of hydrogen peroxide was, however, found by 

 Schwab and collaborators (2511). Their experiments are occasionally quoted 

 as supporting the radical chain theory, but in fact they supply evidence 

 against it, since the radical chain theory fails to explain the observed speci- 

 ficity of these inhibitors. 



Our general objections to the radical chain theory as an explanation 

 of enzymic reactions has been discussed in Chapter VIII, Section 6.2, 



The radical chain theories all presuppose a valency change of the 

 iron in the catalase. While we have seen that there is now general 

 agreement that the iron of peroxidase remains ferric during its action, 

 there is some evidence to support the assumption that the iron of 

 catalase changes its valency. A definite proof, however, that during 

 the normal uninhibited action of catalase such a change occurs, is 

 still lacking. 



4.2.2. Keilin's Theory. It has been shown in Section 2.2. that 

 Keilin and Hartree (1487,1499) have produced strong evidence that, 

 in the presence of azide or hydroxylamine and hydrogen peroxide, a 

 reduction of catalase to a ferrous compound occurs. They assumed 

 that azide ferricatalase is reduced by hydrogen peroxide, but not by 

 dithionite, and that the reduction product is an azide ferrocatalase, 

 which is oxidized by atmospheric oxygen but not by hydrogen 

 peroxide. 



The same mechanism is postulated for the action of catalase on 

 hydrogen peroxide in the absence of azide or hydroxylamine. The 

 inhibitory action of these compounds is explained by their combi- 

 nation with ferrocatalase. 



The reaction mechanism of catalatic action is formulated as follows : 



4 Fe'+ + 2 H2O2 -^ 4 Fe2+ -t- 4 H+ + 2 O2 (1) 



4 Fe2+ + O2 + 4 H+ ^ 4 Fe^+ + 2 H.O (2) 



There are several weak points in this theory. First, carbon monoxide, 

 although it inhibits catalatic action in the presence of small amounts of azide 

 (1499) or in the presence of sulfhydryl compounds (1^90), does not produce 

 an inhibition with pure catalase; second, the mode of action of an enzyme 

 mav be drastically modified by compounds which can react with it. Theorell 



