INHIBITORS OF CATALASE 409 



analysis a 20% oxidation of the diainagnetic ferrous compound to 

 the fully ionic ferric azide compound takes place. The results of 

 Keilin and Hartree add further convincing evidence for the ferrous 

 nature of the red azide compound. 



2.3. Inhibitors 



The substances which have been shown to combine with catalase 

 also inhibit its catalytic activity. This is also true of the anions. 

 Only the (FeOH) form is active, not the (FeX) compounds. The 

 inhibition by anions previously observed by Michaelis and Pechstein 

 (1939), by Santesson (2436), and by Agner (28) is thus explained. 

 Ethyl hydrogen peroxide inhibits the action of catalase on hydrogen 

 peroxide competitively (2647) ;* 50% inhibition is caused by 8 X 10~^ 

 M cyanide (3166), and by 3 X 10~^ M sodium hydrosulfide (26^7). 

 According to Zeile the dissociation constant of the cyanide compound 

 agrees with this value for 50% inhibition, but a smaller degree of 

 inhibition by cyanide (50% inhibition at about 5 X 10~ ^ M) has 

 been reported by Stern (2647) and Keilin and Hartree (lJf99). It 

 has also been claimed that the cyanide inhibition does not depend 

 on hydrogen peroxide concentration, which it should do if hydrogen 

 peroxide and cyanide were to compete for the iron. Zeile concludes 

 from this that cyanide combines with a group other than the hematin 

 iron, but this appears to be in contradiction to his earlier experiments 

 quoted above. 



TABLE 11 

 Comparison between Effectiveness and Affinity of Inhibitors for Catalase 



Inhibitor concentration 

 Inhibitor at 50^^ inhibition, M Dissociation constant 



The inhibition by azide and hydroxylamine, on the other hand, is 

 far stronger than would be warranted by their affinity for catalase 

 as determined spectrophotometrically (2H9,U0SJJt87,lJf99,2535). 

 Table II gives the values for the concentrations of inhibitors causing 

 50% inhibition, together with those at which half-dissociation occurs 

 * Cf., however. Chance {'t-iob). 



