hiliibition and Retardation of the Oxidation of lAA 159 



fects are caused by agents which interfere in the reaction sequence in 

 fundamentally different ways and which may be described as chain- 

 stopping and chain-transferring agents respectively. 



In the presence of an inhibitor or chain-stopping agent the ini- 

 tiation of autoxidation is prevented. At low inhibitor concentrations 

 the reaction may commence abruptly after a long induction period 

 and attain a rate equal to that of the control in the absence of in- 

 hibitor. Such temporary inhibition or lag-extension indicates that the 

 inhibitor is irreversibly changed during the induction period to a 

 product that is not inhibitory to the reaction. Its destruction must 

 occur at the expense of an essential intermediate of the system, but 

 it also may be aided by side reactions, e.g., by autodestruction. Thus, 

 the system would never operate in the presence of excessive amounts 

 of the inhibitor or if the latter were not destroyed. 



On the other hand, in the presence of retarding agents, the re- 

 action may proceed to completion at a reduced rate without an in- 

 duction phase. This could occur only if the retarder acted as a chain- 

 transferring agent by substituting a sequence of slow reactions within 

 the rapid sequence of the control. In order to exert its retarding 

 effect continuously, the substance must be reformed following re- 

 action with an essential intermediate. Thereby the retarder inserts 

 a shunt into the normal reaction sequence which slows down the 

 speed of propagation. 



Mechanism of Inhibition by Chain-stopping Agents 



According to the foregoing definition, it may be validly assumed 

 that the catechol-type inhibitor is a chain-stopping agent and is de- 

 stroyed by an intermediate prior to or during the first reaction step 

 of lAA oxidation. The length of the catechol-induced lag period is 

 dependent on its concentration, and when an amount of catechol 

 that would normally cause a 45 min. lag phase is added during the 

 oxidation only a brief staggering of the rate resulted. This was also 

 true of the inhibitor found in boiled undialyzed preparations of 

 commercial catalase (8). Such effects w^ould be expected if the de- 

 stroying agent is an essential intermediate present at a higher con- 

 centration after the oxidation has commenced. Catechol, pyrogallol, 

 ascorbic acid, and probably rutin are all readily oxidized and could 

 temporarily inhibit the oxidation of lAA by reducing manganic 

 ions, a cofactor radical, or possibly a skatole radical. 



\A'e have suggested previously (9) that manganic ions arising dur- 

 ing the initiation and propagation reactions preferentially oxidize 

 catechol. This is supported by the findings that MnOo, PbOo, HoO^, 

 or an oxygen atmosphere may partially or completely overcome 

 catechol inhibition since these are known to lead to the formation 



