434 IX. HEMATIN ENZYMES, II 



Theorell and collaborators {26,2722,2772,2775,2786) (cf. also m) 

 then showed that animal and plant peroxidases can act as oxidases 

 with dihydroxymaleic acid, although horse-radish peroxidase (in con- 

 tradistinction to horse-radish paraperoxidase) occasionally required 

 hydroquinone as mediator. While dihydroxymaleic oxidase is 

 generally strongly inhibited by 10"^ M cyanide, the horse-radish 

 peroxidase -hydroquinone -dihydroxymaleic acid system is not inhib- 

 ited. The enzyme of some plants is not cyanide sensitive; in others 

 it is inhibited by cyanide {134)- Manganese increases the activity of 

 the enzyme. 



Dihydroxymaleic acid oxidase is strongly inhibited by catalase. 

 Its action has therefore been explained by assuming that hydrogen 

 peroxide is formed by autoxidation of dihydroxymaleic acid and that 

 the peroxidase catalyzes the oxidation of the acid by the hydrogen 

 peroxide. It will be shown in Section 4 that the mechanism of its 

 action cannot be thus sufficiently explained. 



It is of particular importance that carbon monoxide, which does 

 not inhibit peroxidases, inhibits dihydroxymaleic acid oxidase and 

 that the carbon monoxide inhibition is reversed by light (2722,2778). 

 There is thus definite evidence that the valency of the hematin iron 

 in the peroxidases undergoes a change when they react with dihy- 

 droxymaleic acid. As should be expected from this (cf. Chapter X), 

 the prosthetic group undergoes irreversible oxidation under these 

 conditions (2791). Ascorbic acid also inhibits the oxidase; it can 

 evidently combine with it without being oxidized itself. 



3.9. Biological Function of Peroxidases 



About the biological function of peroxidases even less is known than 

 about that of catalase. In addition to the problem of hydrogen 

 peroxide formation in the cell, we have to find the biological hydrogen 

 donors which react with peroxidase. The study is further complicated 

 by the presence of heat-stable hematin compounds in the tissues 

 which are able to catalyze the oxidation of benzidine by hydrogen 

 peroxide. 



3.9.1. Plant Peroxidases. The presence of peroxidase in plants has been 

 demonstrate^ by Bach and Chodat (109) and by Onslow (2077). 



Plants can be divided into two classes: polyphenol oxidase plants and 

 peroxidase plants. The tissues of the first group become brown to black if 

 damaged by the oxidation of polyphenol to quinoid pigments by the action of 

 the copper-containing enzyme polyphenol oxidase, while the tissues of the 



