444 IX. HEMATIN ENZYMES, II 



behave as "anticatalases." A closely related phenomenon is probably 

 the destruction of catalase by oxygen in the presence of such sub- 

 stances (Marks, 1870). Here the hydrogen peroxide which initiates 

 the reaction is formed by the slow autoxidation of the reducing 

 substance. 



There is obviously some similarity between this action of reducing 

 substances on catalase and the action of dihydroxymaleic acid on 

 peroxidase. In both instances there is evidence for a valency change 

 of the hematin iron. First, carbon monoxide inhibits the action of 

 catalase in the presence of sulfhydryl compounds as well as dihy- 

 droxymaleic acid oxidase; second, the enzymes undergo a more rapid 

 destruction than in the absence of reducing substances. 



The latter is explained by the destructive action of hydrogen peroxide on 

 ferrous heme compounds which leads to bile pigment hematin compounds 

 (c/. Chapter X). In the case of catalase there is no evidence as yet that the 

 enzyme can act as oxidase on reducing substances to any greater extent than 

 the equivalent reduction of the ferricatalase to ferrocatalase, nor is this 

 necessarily demanded by the theory. Here the reactions 1 to 4 of Figure 5 

 may occur without the oxidative cycle, reactions 5 to 9. being initiated. It 

 must be left to further research to demonstrate whether or not the oxidative 

 cycle also occurs with catalase in the presence of reducing substances. 



Previously Lemberg and Legge {1705) had considered that the effect of 

 ascorbic acid on the activity of catalase supported Keilin's theory, but it 

 appears more likely that the reduction to the ferrous state is only caused by 

 some reducing substances, and in their absence occurs only to a very slight 

 extent if at all. 



We have seen that certain substances are able to protect catalase from the 

 action of anticatalases (r/. Section 2.4.). Some of these substances included 

 in the term "philocatalases" are oxidizing substances which may either 

 oxidize the reducing anticatalase or may prevent its action by competition 

 with the anticatalase binding group in the catalase protein molecule. 



.\nother group of substances, such as alcohols, may act as philocatalases 

 in the second way. 



4.2.6. "Coupled Oxidation" of Alcohol. In Section 2.7.2. we 

 have discussed the discovery of Keilin and Hartree that catalase can 

 oxidize certain alcohols with hydrogen peroxide, acting as peroxidase. 

 The conditions under which this reaction occurs are the presence of high 

 concentrations of catalase and very low concentrations of hydrogen 

 peroxide, otherwise catalatic destruction of the hydrogen peroxide 

 occurs. We explain the transformation of catalase into a peroxidase in 

 this system in the way described diagramraatically in Figure 7. 



