BRITTON CHANCE 



POSSIBLE RATE-LIMITING STEPS IN CATALASE ACTION 



Since we are not at present able to make spectroscopic 

 studies of catalase in 5 Af peroxide, one must consider hypo- 

 thetical explanations for the hyperbolic relationship of Figure 2, 



1. That the activity is inhibited by the formation of an 

 inhibitory compound not in the reaction sequence (complex II) 

 (10). 



2. That the activity is limited by the formation of a 

 ternary complex consisting of complex I and a second peroxide 

 molecule (11,17,56). 



3. That a short-lived compound of catalase and peroxide 

 precedes complex I. 



Each of these three possibilities leads to hyperbolic relation- 

 ships between reaction velocity and hydrogen peroxide concen- 

 tration and cannot be distinguished on this basis. Ogura has, 

 however, shown that inhibition of catalase by cyanide, fluoride, 

 or formate becomes a competitive effect at very high peroxide 

 concentrations. This observation rules out mechanism (3), 

 which should not show a competitive effect of the kind observed. 

 Thus these studies indicate that no intermediate of a life-time 

 longer than 2 X 10~^ sec. precedes complex I. 



RATE CONSTANTS FOR A REACTION SEQUENCE 

 INVOLVING A TERNARY COMPLEX 



If we accept mechanism (2) above as more likely than mech- 

 anism ( 7), and there is some evidence in favor of this at the present 

 time (55), the molecular statistics of catalase reactions would be 

 as follows : 



, , ^^ ^ Ai = 0.5 X 10' A/-isec.-' , , 



catalase + H2O2 * complex I (2) 



, , , .-^ ^ ^4' = 1.5 X IC M-i8ec.-> ^ ^ 



complex I + H2O2 > ternary complex (3) 



A;3 = 3 X 10' 8ec. "i , ^ 



ternary complex > catalase + O2 + 2H2O (4) 



That the ternary complex is catalase complex III (42) is un- 

 likely because catalase complex III requires complex II for 



314 



