The Interpretation of Indole-3-acetic Acid Oxidation 201 



and inhibitors. This is an inescapable source of possible confusion 

 in comparative experiments on lAA oxidation. 



The gradual increase in reaction rate which occurs during the 

 induction phase must be ascribed to some process by which the total 

 concentration of oxidation intermediates in the medium increases. 

 The proposals mentioned concerning the lAA oxidation mechanism 

 do not provide for such an increase in concentration, since every inter- 

 mediate must be consumed and produced in equal amounts to main- 

 tain the catalytic cycle and account for the reaction stoichiometry. 

 Two principal types of processes can be considered in looking for an 

 explanation of why the reaction rate increases during the induction 

 phase: (aj a minor reaction occurs, separately from the main lAA 

 oxidation process, which forms an lAA oxidation intermediate as a 

 product, or (b) the lAA oxidation reaction itself forms a product 

 which is or can give rise to one of the intermediates, so that as the 

 cycle operates one of the intermediates is formed in excess of the 

 amount consumed. We shall call processes such as (a) or (b) initiation 

 reactions; they initiate a catalytic cycle dependent upon oxidation 

 intermediates. Types (a) and (b) are fundamentally different kinetic- 

 ally, for (b) is tied to the occurrence of the lAA oxidation reaction 

 while (a) is not. With process (b) the reaction would appear to be 

 autocatalytic, whereas with (a) it would not. With (a) the amount of 

 lAA destroyed at time t after the start of reaction should be a linear 

 function of t-; with (b) the log of the amount of lAA destroyed at 

 time t should increase linearly with t once an appreciable rate of re- 

 action is observed. We have concluded that the Omphalia enzyme 

 reaction is in fact autocatalytic, that is, alternative (b). It should be 

 noted that a positive distinction between initiation reactions (a) and 

 (b) is not easy to make, as it must be based upon close study of the 

 early part of the induction phase, because further complications in 

 the kinetics, to be discussed below, appear as the reaction rate in- 

 creases. 



By quantitative comparison between the kinetics of the induction 

 phase and the rate effects of added HoOo, one can get some indication 

 as to the concentrations of intermediates which must actually be 

 formed dming induction. It appears that the amounts formed are 

 much less than the amount of lAA which undergoes oxidation during 

 this period, which suggests that the autocatalytic effect does not arise 

 from a major product of lAA oxidation, but rather by a minor side 

 reaction in the oxidation cycle. 



The lAA oxidation rate does not continue to increase indefinitely, 

 but leads gradually into a steady maximum rate of oxidation which 



