Inhibition and Retardation of the Oxidation of lAA 161 



in which it is reduced back to hydroquinone and indirectly causes 

 the further production of Mn^'. The only readily oxidizable compo- 

 nent that could take part in such a reversible reaction is the phenolic 

 cofactor. Resorcinol forms an insoluble oxidation product merely on 

 standing with p-quinone. Dichlorophenol and maleic hydrazide did 

 not show any visible reaction, probably because their oxidized radi- 

 cals (RO-) do not condense readily to colored products. Neverthe- 

 less, when p-quinone was incubated with pyrophosphate or citrate, 

 the addition of either maleic hydrazide or dichlorophenol resulted in 

 the oxidation of manganese. Manganic ions were rapidly produced 

 on warming and were detected as pink manganipyrophosphate or 

 orange manganicitrate. This is essentially the Kenten-Mann reaction 

 for the oxidation of manganese (4), but differs in that quinone re- 

 places peroxide plus peroxidase. Assuming that hydroquinone 

 [ O (—OH) 2 ] competes with lAA for Mn+3, then the mechanism of 

 this retardation may be considered due to the partial substitution in 

 the chain of the following series of slower reactions, where ROH repre- 

 sents the monohydric phenolic cofactor. 



Q (—OH), + Mn-' -^ Q (=0). + Mm^ + 2H- 

 O (=0)o +2ROH ^ O (—OH), + 2RO- 

 RO- + Mn-2 ^ H- ^ ROH + Mn-^ 



Riboflavin. In contrast to hydroquinone, riboflavin (Rb) is added 

 in the oxidized form only. Consequently, if it were not involved in 

 a reversible redox system, it would have no effect on the system, but 

 in this respect it resembles p-benzoquinone. We have implied pre- 

 viously that the redox system (Rb ^ Rb-2H) is established and 

 that most probably the oxidized form indirectly generates Mn^^ by 

 reacting with the phenolic cofactor and reduced riboflavin (Rb-2H) 

 would compete with lAA for M.\Y'\ This would have the effect of 

 retarding the oxidation from the outset by a chain transfer mecha- 

 nism. 



The evidence in support of the manganigenic properties of 

 riboflavin is as follows. Riboflavin partially overcomes the catechol- 

 induced lag period of lAA oxidation in the same manner as man- 

 ganiversene, MnOo, PbOo, HoOo, and oxygen which effects have 

 been interpreted as manganigenic. Riboflavin, as well as these other 

 agencies, also overcomes the inherent lag period of resorcinol oxida- 

 tion catalyzed by Mn-peroxidase system (6). This oxidation appears 

 to be dependent on the generation of Mn+s since it is completely 

 inhibited by pyrophosphate and citrate as is lAA oxidation when 

 catalyzed by Mn-peroxidase-resorcinol systems (9). The only readily 

 oxidizable component of the lAA oxidation system is the phenolic 



