694 5. OXIDANTS 



p-mercuribenzoate prevents their reaction with hydrogen peroxide. The 

 single SH group of papain is about 5 times as susceptible to oxidation by 

 hydrogen peroxide as the SH group of phosphoglyceraldehyde dehydrogen- 

 ase. a-Chymotrypsin has two methionine residues, one being 3 residues 

 away from the active site serine and the other 15 residues removed. The 

 Met-3 is oxidized by hydrogen peroxide specifically, whereas both SH groups 

 are oxidized in the urea-denatured enzyme (Schachter et at., 1963). It is 

 obvious that disulfide bonds cannot be formed intramolecularly in the na- 

 tive enzyme and it was shown that methionine sulfoxide is the product. 

 Glutathione is also oxidized to the sulfoxide by hydrogen peroxide (Utzin- 

 ger et al., 1963). 



Hydrogen peroxide generated during oxidation in enzyme preparations 

 or cells is sometimes inhibitory and for many years it has been assumed 

 that at least one function of catalase is to protect cells against it. Dixon 

 (1925) was the first to demonstrate this with a purified enzyme system. 

 When purines are oxidized by oxygen in the presence of xanthine oxidase 

 there is a progressive inactivation of the enzyme, which is due to hydrogen 

 peroxide formed, since it can be prevented by catalase. Hydrogen peroxide 

 was claimed to stimulate xanthine oxidase at very low concentrations (be- 

 tween 0.00001 mM and 0.01 mM) (Table 5-5), and to inhibit above 0.1 mM 

 (Bernheim and Dixon, 1928). The rate of activation is slow, maximal effects 

 of 0.001 mM hydrogen peroxide occurring in around 100 min. The mecha- 

 nism for this is unknown; metal impurities seem unlikely because they would 

 be at extremely low concentrations. The inhibition of Aspergillus aconitase 

 depends on the strain of the organism, the sensitivity varying over at least 

 a 4-fold range of concentration (Bruchmann, 1961 a). Certain strains tend 

 to accumulate citrate under specified conditions and this is believed to be 

 due to the hydrogen peroxide formed and the particular sensitivity of aco- 

 nitase in these strains (Bruchmann, 1961 b). Adding exogenous hydrogen 

 peroxide augments the accumulation of citrate (Bruchmann, 1961 c). 



Succinyl peroxide is a radiomimetic substance and has been found to 

 inhibit several SH enzymes while having much less action on non-SH en- 

 zymes (Wills, 1959). The enzymes inhibited are amylase, /5-fructofuranosi- 

 dase, urease, succinate dehydrogenase, phosphoglyceraldehyde dehydrogen- 

 ase, papain, tyrosinase, and cholinesterase. Urease, for example, is inhib- 

 ited completely in 8 min by 0.033 mM succinyl peroxide. Reversal of inhi- 

 bitions by cysteine is obtained only if the period of exposure to the peroxide 

 is brief. It is questionable if the action is exerted by succinyl peroxide itself, 

 since it is immediately hydrolyzed to persuccinate in aqueous solution, and 

 peracids have long been known to oxidize SH groups (Freudenberg and 

 Eyer, 1932; Swan, 1959). The inhibition of catalase by monoethyl peroxide 

 is probably not due to SH group oxidation but to an analog type of inhi- 

 bition (Blaschko, 1935). 



