PETER M. RAY 



University of Michigan 



The InterpretatLon of Rates of I n.dote-3 -acetic 



Acid Oxidation 



Although several fundamentally different mechanisms have been 

 proposed to explain enzymatic oxidation of indole-3-acetic acid (lAA) 

 by peroxidase-containing preparations, a feature common to all of 

 these is that the oxidation is interpreted as a cyclical process involving 

 several reactions. Goldacre (2) and Galston, Bonner, and Baker (1) 

 suggested that HgOo is consumed in a peroxidase-catalyzed step, and 

 reformed from Oo in an oxidase-catalyzed step. In the scheme pro- 

 posed by Maclachlan and Waygood (3), lAA is supposed to be oxi- 

 dized by Mn^3^ giving a radical which reacts with Oo; the resulting 

 oxidant is used by peroxidase to oxidize a phenol, which in turn 

 reoxidizes Mn+2 to Mn^^. Another possible mechanism, suggested 

 recently by Yamazaki and Souzu (6), is that lAA is oxidized directly 

 by peroxidase in a one-electron oxidation (V2 HoOo), giving a radical 

 which reacts with oxygen to regenerate HoOo. 



I do not feel that any of the specific proposals about the mechanism 

 of enzymatic lAA oxidation has been established unequivocally, and 

 I think it is not yet possible to say whether some of the different 

 enzyme preparations which have been studied actually operate by 

 different reaction mechanisms. It may even be that some or all of the 

 reactions of the formally distinct mechanisms actually go on together 

 in a single reaction medium, and contribute to varying degrees to the 

 over-all oxidation depending upon the conditions, the addition of 

 phenols or of Mn, and so forth. In all of the suggested mechanisms, 

 and very likely in any others which may be conceived, the reaction 

 rates must depend upon concentrations of intermediates such as HoOo 

 and radicals or other partially oxidized products derived from lAA. 

 The purpose of this discussion is to explain the important bearing 

 of this upon the meaning of lAA oxidation rates, and of progress 



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