Irreversible Inhibition of Catalase 237 



such a donor substantiates, at least for blood catalase, the ideas of the 

 physiological role of catalase as a peroxidase in tissues rich in the enzyme 

 (Keilin and Hartree, 1945). 



THEORY 



Under conditions in which hydrogen peroxide is continuously supplied 

 at a low concentration, to a mixture containing catalase, an irreversible 

 inhibitor of the AT series and a catalase donor, the following reactions 

 occur : 



(1) The formation of Cat. H^Oa I (Chance, Greenstein and Roughton, 



1952): 



*i 

 E + S ^ ES (a) 



(E^p—q'—q-) («) *2 (p) 



in which E represents the enzyme, S the substrate hydrogen peroxide, and ES 

 represents Cat. HgOo I. The expressions in brackets are the concentrations of 

 the reactants and the symbols represent the concentrations of the total original 

 enzyme in terms of catalase haematin (£), of Cat. H2O2 I (/>), of the enzyme 

 haematin bound reversibly by the inhibitor {q), and of the enzyme haemaiin 

 irreversibly bound by the inhibitor {q"). 



(2) The reaction of the donor with Cat. HgOg I or peroxidatic reaction of 

 catalase (Chance et al., 1952): 



ES + AH2 -- E + SH2 + A (b) 



(V) (a) 



in which AH2 represents the donor and a its concentration. 



(3) The reversible inhibition of catalase by the inhibitor, first reported for 

 AT by Heim et al, (1956) and shown to be due to a reaction of the inhibitor 

 with the free enzyme (Margoliash and Schejter, unpublished results): 



E + I 4 EI' (c) 



(E-^-q'—q") (i)"-' (<?') 



in which I represents the inhibitor, / its concentration and EI' the reversibly 

 inhibited enzyme haematin. The symbol K^ will be used to represent the 

 equilibrium constant for this reaction. 



(4) The irreversible inhibition of catalase (Margoliash et al., 1960): 



ES + I ^ EI" (d) 



in which EI" represents the irreversibly inhibited enzyme haematin. 

 The ratio of the catalase haematin bound by hydrogen peroxide in Cat. 



