342 S. M. Siegel and F. Porto 



to the balance of oxidants (electron acceptors) and antioxidants 

 (electron donors) in the cell tissue or organism: 



A. Definition and properties of antioxidants. 



1. Any electron donor which inhibits the oxidation of suitable 

 labile substrates with high stoichiometric efficiency is an anti- 

 oxidant. 



2. Antioxidants may operate by: 



a. Reaction with an intermediate which is the electron-de- 

 ficient member of an oxidation-reduction equilibrium. 



b. Trapping or deactivation of radicals or ions. 



c. Reaction wuth an oxidant which would otherwise attack a 

 labile substrate. 



d. Reaction or interaction with the electron-deficient form of 

 an oxidation (electron-transporting) catalyst. Both electron- 

 transfer and TT-complex formation are included. 



3. Antioxidant efficiency derives from the following mechanisms: 



a. Participation in a cycle wherein a suitable electron source 

 can regenerate the antioxidant from its oxidized form. 



b. Trapping of radical or ionic initiators of chain reactions. 



c. Reducing the efficiency of an electron-transport catalyst 

 with which it interacts. 



B. Biochemical activities of antioxidants derive from the following: 



1. Antioxidants protect labile cellular components against attack 

 by primary oxidants such as O2 or derivative oxidants including 

 H0O2 and other peroxides, oxidizing radicals, ions, or such 

 moieties when bound in macromolecules. 



a. Non-specific protection entails a general shift of oxidation- 

 reduction equilibria toward the reduced conditions. 



b. Specific protection may be conferred by thermodynamic 

 and geometric restrictions which fix the antioxidant in 

 electron transfer chains or localize it on selectively adsorb- 

 ing surface sites of macromolecules or membranes. 



2. Antioxidants may influence or regulate specific metabolic path- 

 ways: 



a. By reductive activation of the inactive oxidized forms of 

 enzymes (e.g., Enz - SS - R -[- e -f- H^^ ^ Enz - SH -[- ' ^ ' ^)- 



b. By blocking electron-transport catalysts, antioxiciants may 

 "shut down" specific metabolic pathways (e.g., inhibition 

 of flavoproteins by formation of a 7r-complex between anti- 

 oxidant and riboflavin). 



c. As mobile electronic systems by serving themselves as electron 

 carriers or carrier moieties of oxidizing enzymes. 



