DIHYDROXYMALEIC ACID OXIDASE 433 



Theorell (rf. Table IV): I. ef^^^ = 1^2.9; II, e'^f^f = 11..5: and III. e^f^j = 89. 

 One mole combines with one mole of hydrogen peroxide, the dissociation 

 constant of the complex being 10-«; 3 X 10-* M cyanide inhibits its action 

 completely. 



The enzyme catalyzes only the peroxidaiive oxidation of cyto- 

 chrome c, and is inactive toward pyrogallol, while horse-radish 

 peroxidase does not oxidize cytochrome c. The kinetics of the 

 cytochrome c oxidation are described by the equation: 



dt 



= k(Fe2+)(E) 



where (Fe^"^) is the concentration of ferrocytochrome c and (E) is 

 that of the enzyme. The amount of enzyme which gives the value 1 

 for the expression {— d log Fe^"'")/c?/, where t is in minutes, is taken as 

 the unit. The number of units per milligram of enzyme was found 

 to be 700-800. 



While Barron believes that this peroxidase may onlj' arise during 

 the prolonged autolysis of yeast during the preparation, the speci- 

 ficity of the enzyme makes a biological function probable. Hogness 

 and collaborators find about 250 times more cytochrome peroxidase 

 than catalase in yeast, so that, in this organism at least, the enzyme 

 may compete with catalase for the substrate. One must also keep in 

 mind the possibility of structurally organized reactions. 



3.8. Dihydroxymaleic Acid Oxidase 



Banga, and Szent-Gyorgyi and collaborators (133-135,2287) have 

 discovered an enzyme in horse-radish, as well as in a great number 

 of other plants, which catalyzes the oxidation by atmospheric oxygen 

 of dihydroxymaleic acid, H02CC(OH)=C(OH)C02H. Dihydroxy- 

 maleic acid is of some interest because of the chemical similarity of 

 its oxidizable — C(0H)=C(0H) — group to the same group in 

 ascorbic acid, (cf. also Wieland and Franke, 3071). The occurrence 

 of dihydroxymaleic acid in grapes is probable {98 If). 



Robeznieks (2287) has shown that the enzyme also works with 

 hydrogen peroxide as hydrogen acceptor. The reaction was acceler- 

 ated by hydroquinone and benzidine, but not by benzopyrane dyes 

 with two vicinal hydroxyl groups or by catechol. According to Huszak 

 (1375), these latter substances accelerate the peroxidative oxidation 

 of ascorbic acid as hydrogen carriers. 



