PHENOL OXIDASES 297 



oxidase. Although the inhibition of this enzyme is reduced by increasing 

 catechol concentration, her data do not correspond to pure competitive 

 inhibition and the derived K^ seems to be in error, as pointed out by War- 

 ner (1951). A potato enzyme oxidizing p-cresol is inhibited by resorcinol, 

 phloroglucinol, and orcinol (see tabulation), the inhibition being completely 



Inhibitor (10 mM) % Inhibition (p-cresol = 10 mif) 



Resorcinol 78 



Phloroglucinol 43 



Orcinol 20 



reversible by dialysis and apparently competitive (Schneider and Schmidt, 

 1959). 4-Chlororesorcinol is a much more potent inhibitor of the potato 

 enzyme, oxidizing either catechol or p-cresol, and it has been claimed that 

 the initial inhibition is competitive, although progressive inactivation oc- 

 curs (Heymann et al., 1954). A K^ of 0.024 tqM was calculated. However, 

 the double reciprocal plots seem to me to be of the perfectly noncompetitive 

 type. Bonner and Wildman (1946) postulated that the bulk of spinach leaf 

 respiration passes through a polyphenol oxidase. p-Nitrophenol is a rather 

 potent inhibitor of this respiration, 1 voM inhibiting 94%, and of the po- 

 lyphenol oxidase, whether it is oxidizing catechol or p-cresol. On the other 

 hand, o-nitrophenol is only a weak inhibitor of both. It was felt that p-nitro- 

 phenol might well be an analog of naturally occurring substrates, and o- 

 nitrophenol an analog of o-phenols against which the enzyme is inactive. 

 It is interesting, finally, to note that dihydroxymaleate is an inhibitor of 

 catechol oxidase (Florkin and Duchateau-Bosson, 1939), and it was sug- 

 gested that the 



OH OH 



— C = C— 



grouping complexes with the enzyme in a manner similar to the analogous 

 catechol grouping. Most of this work on inhibiting phenolic compounds 

 is unsatisfactory from the quantitative standpoint and clear-cut proofs 

 of competitive inhibition are lacking. Nevertheless, effective analog in- 

 hibition for this class of enzymes has heen demonstrated. 



We shall now turn to a more potent, more interesting, and more 

 thoroughly studied type of inhibitor, namely, the benzoates. The inhibition 

 of mushroom catechol oxidase by benzoate itself was attributed to a com- 

 petition with the substrate for the active center, although no direct evidence 

 for this was adduced (Ludwig and Nelson, 1939; Gregg and Nelson, 1940), 

 but good competitive kinetics for the inhibition by m-hydroxybenzoate 



