436 5. QUINONES 



Reactions with Thiols 



Quinones do not possess aromatic characteristics and behave chemically 

 more like open-chain or, /^-unsaturated ketones, reaction with many sub- 

 stances being by 1,4 addition: 



I I I 



o=c— c=c— 



12 3 4 



It is thought that direct addition to the ethylenic bond is not common 

 and is biologically unimportant. Quinones may either oxidize thiols or 

 react with them by 1,4 addition, and it is often difficult to distinguish 

 between these reactions in enzyme work. It is often thought that simple 

 oxidation of SH groups should be reversible upon removing the quinone 

 and adding a reductant, whereas 1,4 addition should be irreversible. How- 

 ever, in the case of proteins or enzymes it is not at all necessary that oxida- 

 tion be readily reversible since secondary structural changes may occur 

 which prevent subsequent reduction. Furthermore, although most addi- 

 tion compounds are reasonably stable, it is not at all certain that the reac- 

 tion is irreversible. Hydrolysis of the thioquinones could result in the 

 restoration of the thiol, with formation of hydroxy quinones. Indeed, Bray 

 and Garrett (1961) have shown that the addition compound of menadione 

 and glutathione is hydrolyzed by liver glutathionase. Thus one cannot 

 use the criterion of reversibility with assurance in attempting to distinguish 

 between these two types of reaction. 



The primary 1,4 addition of a thiol to a quinone may be represented by 

 the reactions: 



I I + HS-R 



