PEROXIDASE AND DIHYDROXYMALEIC ACID OXIDASE 437 



and can be considered as an iron peroxide. The formation of iron 

 peroxides had been assumed many years ago by Manchot to play a 

 role in the peroxidative action of ionic iron, and more recently by 

 Polonovski and Jayle {llfll,2163) as the explanation of the peroxi- 

 dative action of hem/globin. In the reaction schemes discussed 

 below, the peroxidase hydrogen peroxide compound is therefore 

 formulated as (FeOOH), but this makes no essential difference from 

 the earlier formulation as (Fe"*" • H2O2). 



The investigations of Chance (cf. Section 3.2.4.) lead to the follow- 

 ing simple picture of peroxidase function shown by Figure 4. The 



Fe»+OH Fe'+OH Fe'+OOH „ , Fe'+OH , „ ^iia 



I H2A I H2A H:0, I H2A I +H2O+A 



I ^ » I X > I M » I X 



Fig. 4. Mechanism of action of peroxidase. 



enzyme unites with hydrogen peroxide through its ferric heraatin iron 

 and with a hydrogen donor through a group in its protein. The 

 reduction of hydrogen peroxide to water by the hydrogen donor then 

 takes place as an intracomplex reaction. Finally, the oxidized sub- 

 strate dissociates and is replaced by a fresh molecule of substrate. 



4.1.2. Mode of Action of Dihydroxymaleic Acid Oxidase. It 



remains to deal with the ability of peroxidases to function as oxidases 

 of dihydroxymaleic acid with molecular oxygen. This is a much more 

 complex problem. Any theory which could claim to be considered as 

 satisfactory must explain: first, the increase of oxygen uptake on 

 addition of peroxidase to dihydroxymaleic acid, second, the inhibition 

 by carbon monoxide, and third, the inhibition by catalase. 



The only theory so far discussed {1^22,2287,2722) assumes that 

 hydrogen peroxide is formed by autoxidation of dihydroxymaleic 

 acid and that the peroxidase accelerates the oxidation of the acid by 

 the peroxide thus formed. If H2M is written for dihydroxymaleic 

 acid: 



O2 + H2M -> H2O2 + M 



H2O2 + H2M ?!:^"!!> 2 H2O + M 



This theory explains the inhibition by catalase, but fails to account 

 for the increased uptake of oxygen and for the carbon monoxide 

 inhibition. 



