PEROXIDES 691 



The ion product (H+) (00H-) is 1.55 X 10-^2 at 20°. Thus it is a very weak 

 acid and the ion OOH" is probably unimportant in its reactions. Hydrogen 

 peroxide can function as both oxidant and reductant. It is a strong oxidiz- 

 ing agent in both acid and alkaline media, but a relatively poor reductant. 

 Although the oxidation-reduction potential would be more favorable for 

 oxidation in acid medium, the rate of oxidation is often greater in alkaline 

 conditions. Hydrogen peroxide, of course, is an unstable substance, especi- 

 ally in the presence of organic material, and this must be considered in its 

 use. Despite its instability ( — AF = 23.4 kcal/mole), it is rather stable in 

 pure solution, but its decomposition is catalyzed by heavy metal ions, and 

 is more rapid in alkaline than acid media. 



Inhibition of Enzymes 



The few results summarized in Table 5-5 are not comparable with each 

 other because the conditions were quite different in the various studies. 

 However, there is no doubt that some enzymes are very sensitive to hydro- 

 gen peroxide. The inhibition develops very slowly in some cases; with yeast 

 /5-fructofuranosidase the inhibition by 2.9 M hydrogen peroxide is 0% at 1 

 min, 10% at 30 min, 47% at 3 hr, and 100% at 21 hr (Myrback, 1957 b). 

 Of course, at this very high concentration one has no idea of the mechanism 

 of the inhibition, and can only marvel at the resistance of this enzyme. The 

 inhibition of ATPase depends on the pH at which the reaction is run: Thus 

 the enzyme was incubated with hydrogen peroxide at pH 7 for 15 min, and 

 the inhibition was found to be 51% when the ATPase reaction was tested 

 at pH 6.3 and 95% when tested at 9.2 (Mehl, 1944). The reason for this 

 strange behavior is unknown. One factor that has not been generally consid- 

 ered is the possible presence of heavy metal ions in the hydrogen peroxide. 

 Holmberg (1939) believed that the inhibition of uricase he observed was 

 due to traces of Cu++, inasmuch as diethyldithiocarbamate prevents the 

 inhibition. It is also possible that some metal ion may be necessary to 

 catalyze the oxidation of the enzyme and that the inhibition is not due 

 to the Cu++ itself. 



The inhibition of /5-galactosidase by hydrogen peroxide is completely re- 

 versible by HgS or cyanide, while that by iodine is not, indicating that here 

 one may oxidize the SH groups more specifically with the peroxide (Knopf- 

 macher and SaUe, 1941). Reactivation of ATPase inhibited by hydrogen 

 peroxide was observed by both Mehl (1944) and Ziff (1944), using cysteine 

 or glutathione, so that specific oxidation of SH groups may occur with this 

 enzjTne. Simultaneously there is a suppression of the interaction of actin 

 and myosin, which is believed to depend on SH groups (Bailey and Perry, 

 1947). Papain inhibited up to 90% by hydrogen peroxide can also be reac- 

 tivated by cysteine, but beyond this there is apparently oxidation beyond 

 the disulfide stage (Sanner and Pihl, 1963). Blocking the SH groups with 



