588 5. QUINONES 



Inhibition of Enzymes 



It is impossible to compare the inhibitory potencies of jj-benzoquinone, 

 j)-quinoneimine, and p-quinonediimine from the data available, since the 

 latter compounds have not been tested directly. Comparisons of the relative 

 activities of the reduced forms are complicated by two factors: the different 

 rates of oxidation to the quinonoid forms (which will depend on the pH), 

 and the relative stabilities of the quinonoid forms. If one assumes that only 

 the quinonoid form is inhibitory, the different degrees of inhibition observed 

 may relate more to the pseudosteady-state concentrations of the quinonoid 

 forms, rather than to their inherent inhibitory potencies. In other words, 

 we never know the true concentration of the active form present.* The 

 results obtained by Potter (1942) on urease at pH 5, where oxidation pro- 

 ceeds slowly, are thus useless for comparative purposes, and show only 

 that the quinonoid forms are active (see accompanying tabulation). Oxida- 



T u-u-^ Concentration ., ^ ..... 



Inhibitor , ..^ % Inhibition 



(mM) 



tion by bromine at a concentration one tenth that of the inhibitor increases 

 the inhibition by p-benzohydroquinone but does not significantly affect 

 that by p-phenylenediamine or the A^, A^-dimethyl derivatives, which is 

 unexpected. The results obtained on succinate oxidase are more pertinent 

 relative to the actions of the quinonoid forms, since the pH was 7.4 and 

 the cytochrome system was present (see accompanying tabulation) (Pot- 

 ter, 1942; Potter and DuBois, 1943). Since the succinate was added 20 min 

 after the inhibitor and readings were taken 30-40 min after that, there 



* If the cytochromes are present in the enzyme preparation, the relative rates at 

 which the reduced forms are oxidized will to some extent determine the inhibitions 

 observed. Horio (1958) has shown that p-benzohydroquinone and p-phenylenediamine 

 are oxidized at equivalent rates by cytochromCgb and cytochrome oxidase from Pseu- 

 domonas aeruginosa, but that jj-aminophenol is oxidized faster by cytochromegb and 

 slower by the oxidase. 



