658 5. OXIDANTS 



(Philpot and Small, 1938; Weill and Caldwell, 1945 a). Hypochlorite oxidizes 

 a number of amino acids, only glycine being resistant, and spectral changes 

 occur with proteins indicating oxidation of tyrosine and tryptophan residues 

 (Lieben and Bauminger, 1933 b); in addition it chlorinates amino groups 

 (Wright, 1926). Sizer (1942 b) noted in his work with many oxidants that 

 SH groups are by no means the only susceptible groups on enzymes, the 

 tyrosine residues being particularly oxidizable. These results point to the 

 importance of exercising great caution in the choice of oxidants and condi- 

 tions for treatment of enzymes if specific oxidation of SH groups is desired. 

 The need for characterizing well the enzyme changes — e.g., disappear- 

 ance of SH groups as determined by the standard methods, or alterations 

 in the ultraviolet spectrum — upon oxidation is also indicated. 



Oxidation of Enzymes by Molecular Oxygen 



Thiols and enzyme SH groups are not oxidized by Og unless certain metal 

 ions are present. Thus papain is oxidized by Og in the presence of Cu++ or 

 Fe+++ and the consequent inactivation of the enzyme is readily reversed by 

 glutathione (Hellerman and Perkins, 1934). Enzymes such as papain and 

 urease were the earliest studied with respect to the effect of oxidation on 

 their catalytic activities, and this work led to the concept wherein the redox 

 state of SH groups is an important regulating mechanism in cell metabolism 

 (Hellerman, 1939). The initial over-all reaction may be written as: 



2 R— SH + O2 ±5 R— S— S— R + H2O2 



but the hydrogen peroxide can produce further oxidation: 



2 R— SH + H2O2 ^ R— S— S— R + 2 H^O 



or it can oxidize other components present. The kinetics of the Cu++- and 

 Fe+++-catalyzed oxidations are complex and the mechanism is not comple- 

 tely understood. One theory involves the formation of a Fe++-thiol radical 

 from a Fe+++-thiol complex; two such radicals would combine to form the 

 disulfide and free Fe++, which is reoxidized by O2 (Williams, 1956). A sec- 

 ond theory postulates a Fe++ (thiol )2 chelate complex, which is oxidized 

 by O2 to the ferric complex, within which electron transfer occurs to form 

 the disulfide and Fe++ (Martell and Calvin, 1952). Since some type of com- 

 plex between metal ion and thiol must occur, it is evident that the suscepti- 

 bility of various SH groups to this type of oxidation must vary greatly. 

 It should be noted that the rates of such oxidations depend on the nature 

 of the buffer used and the pH. 



The toxic effects of high tensions of Og on cell metabolism may depend 

 on the oxidation of enzyme SH groups. Brain respiration is slowly inhibited 

 by O2 and there is increasing inability of the tissue to oxidize glucose, pyru- 



