590 5. QUINONES 



since iodoacetate does not inhibit. Siillmann (1943) found lipoxidase to 

 be inhibited 35% by p-benzohydroquinone and 76% by p-aminophenol at 

 10 mM, pH 6.3, and 10-min incubation, but we know nothing of the mech- 



T u-u-i. Concentration . -i^-x- 



Inhibitor , T,r. % Inhibition 



(mM) 



anism of inhibition of this enzyme. Boyland and Gallico (1952) reported 

 that A",iV-diethyl-2>-phenylenediamine is around 13 times more inhibitory 

 than p-benzohydroquinone to catalase, but this may be related to the dif- 

 ferences in redox potential rather than to the SH reactivity of the quinonoid 

 forms. 



It is probably true that the semiquinonediimines are usually much more 

 stable than the quinonediimines, and thus are likely to play an important 

 role in the inhibition of enzymes, as has been emphasized by Kensler et al. 

 (1942 a) from studies of yeast pyruvate decarboxylase, which is inhibited 

 well by the A^-alkylated p-phenylenediamines whose free radicals are the 

 most stable. For example, iV,iV,iV',iV'-tetramethyl-j9-phenylenediamine 

 inhibits 96% at 0.5 mM, whereas iV,iV,iV',A^'-tetramethyl-o-phenylene- 

 diamine inhibits only 10% and p-aminophenol only 15%. Elson and Hoch- 

 Ligeti (1946) also studied the effects of phenylenediamines on urease, 

 but allowed the solutions to stand 1-7 days in order to form the oxidation 

 products, so that little information can be obtained from their results, 

 except confirmation of the fact that m-phenylenediamine is a weaker 

 inhibitor than the para and ortJio isomers and shows no increased activity 

 on standing. Their results on liver succinate oxidase are more interpretable 

 because of the rapid oxidation of the compounds through the cytochromes, 

 but differ from those of Potter in that the iV-methylated derivatives appear 

 to be much more potent than p-phenylenediamine. In fact, the inhibitions 

 obtained were roughly one tenth those recorded by Potter, which is odd 

 because the experiments appear to have been conducted identically. Dif- 

 ferences in homogenate concentration may have been responsible, but if 

 they were it is evident that one cannot interpret any of this work quanti- 



