AMINOPHENOLS AND PHENYLENEDIAMINES 589 



was sufficient time for complete oxidation under the conditions. The iden- 

 tical inhibitions given by p-benzohydroquinone, ?)-phenylenediamine, and 

 the A' -methylated derivates might point to equivalent inhibitory potencies 



J , ., -^ Concentration „ , . , ., .^. 



Inhibitor , „, % Inhibition 



(mil/) 



of the quinonoid forms, but it may be that the inhibitions were limited by 

 depletion of inhibitor. In any event, there is no evidence from any of this 

 work of marked differences in the potencies of these compounds. 



Other reported comparisons are even less reliable for a variety of reasons. 

 Kensler ct al. (1942 b) used a yeast apozymase preparation supplemented 

 with NAD and utilizing fructose- 1,6-diP, so the exact site of action is not 

 known although it is likely to be the 3-phosphoglyceraldehyde dehydro- 

 genase (see accompanying tabulation). Results of this type lend some sup- 



T 1 u .. Concentration ., _ , ., .,. 



Inhibitor , ,.^ % Inhibition 



(mif ) 



port to the concept that the semiquinone form is active. Cohen et al. 

 (1942) examined heart transaminase at pH 7 and incubated the enzyme 

 with the inhibitors for 15 min (see accompanying tabulation). This enzyme 

 appears to be much less readily inhibited by the quinonediimines than 

 by the quinone. They doubted if reaction with SH groups is involved here 



