OXIDANTS 659 



vate, lactate, fructose, and succinate (Dickens, 1946 a). It was suggested 

 that the most sensitive system is perhaps the pyruvate oxidase due to the 

 involvement of SH groups. This inactivation is not mediated through the 

 H2O2 formed, inasmuch as the concentrations are never great enough due 

 to the catalase present. The following enzymes are inactivated by high Og 

 tensions: succinate dehydrogenase, phosphoglyceraldehyde dehydrogenase, 

 choline oxidase, phosphoglucomutase, and other SH enzymes (Dickens, 

 1946 b). Lactate dehydrogenase, malate dehydrogenase, D-amino acid oxi- 

 dase, and yeast hexokinase are resistant. It is interesting that malonate 

 (1 mM) and Mn++ (0.25 mM) protect succinate dehydrogenase against oxi- 

 dation by O2, and that NAD protects phosphoglyceraldehyde dehydrogen- 

 ase, indicating that either O2 or some intermediate must react directly 

 with the enzyme SH groups. Haugaard (1946) also found a good correlation 

 between the SH nature of enzymes and their susceptibility to O2, the follow- 

 ing sensitive enzymes being added to the list above: a-ketoglutarate oxidase, 

 pyruvate oxidase, glutamate dehydrogenase, and xanthine oxidase. Dickens 

 stated that succinate dehydrogenase is irreversibly inactivated by 0^, but 

 Haugaard found reactivation by cysteine or glutathione, indicating a simple 

 disulfide formation. Inactivation of certain enzymes during extraction and 

 purification is probably due to oxidation by Og since metal-chelating agents, 

 such as ethylenediaminetetraacetate (EDTA), are able to protect against 

 such inactivation. 



The cytochrome system may be involved in the inactivation of enzymes 

 by O2. Since cysteine is oxidized by O2 through the cytochrome system to 

 form cystine, and since cystine will in turn oxidize certain enzyme SH groups 

 (see page 661 ), enzyme extracts in which cysteine is present may be unstable. 

 Thus cysteine inhibits succinate dehydrogenase (Potter and DuBois, 1943), 

 but in mouse kidney homogenates the inhibition shows a lag period which is 

 interpreted as due to the necessary oxidation of cysteine to cystine by the 

 cytochrome system (Ames and Elvehjem, 1944 a, b). 



Various Minor Oxidants 



A number of strong oxidants, such as permanganate, perchlorate, dichro- 

 mate, and related compounds, have been used in the past to oxidize enzyme 

 groups. Most of these have dropped out of use because they were felt to 

 lack specificity toward SH groups, or other groups. Actually none of these 

 oxidants has been studied thoroughly with respect to what enzyme groups 

 are oxidized, or to the optimal conditions for achieving specificity. It is 

 quite possible that, at certain pH's and concentrations and temperatures, 

 these reagents may be specific oxidants. Certainly some enzymes are quite 

 susceptible and others very resistant, and it would be interesting to know 

 the reasons. For example, permanganate at 10 mM inhibits a-amylase 

 > 75% (Di Carlo and Redfern, 1947), at 0.05 mM inhibits /^-amylase 85% 



