150 E. R. IVaygood and G. A. Maclachlan 



a chain-stopping or chain-transferring mechanism. It should be noted 

 that the destruction of lAA differs from standard chain oxidations 

 (7) by being dependent on peroxidase. Consequently, substances that 

 inhibit peroxidase or catalase, e.g., cyanide, also inhibit LA^\ oxida- 

 tion. Such enzyme poisons are to be distinguished from inhibitors 

 and retarders of the chain oxidation of lAA and are not considered 

 in this paper. 



METHODS 



The preparation of wheat leaf extracts and horseradish peroxidase, 

 the reaction conditions, and the experimental techniques used in 

 the present study have been described previously (6, 9). Unless 

 otherwise stated, standard systems with resorcinol or dichlorophenol 

 (DCP) as cofactor have been used to catalyze the oxidation of lAA. 

 These contain the following components: 0.50 ml. wheat leaf en- 

 zyme (ca. 0.2 mg. protein N) ; 3.0 fjii MnCL; 1.5 fJM resorcinol or 

 2,4-dichlorophenol (DCP) ; 150 fjuM orthophosphate, pH 6.0; 6.6 fuM 

 lAA, ammonium or sodium salt, pH 6.0 z= 158 ^1. Oo: \ol. 3.0 ml.; 

 29.5° C. 



EXPERIMENTAL RESULTS 



Typical progress curves of the oxygen uptake during L\A oxida- 

 tion catalyzed by extracts from winter-grown wheat leaves exhibited 



80 120 



TIME. min. 



Fig. 1. Retardation and inliihiiion of indole-3-acetic acid oxidation. .S\sttins 

 standard with: A, no additions; B, 0.03 fiM (10"* M) catechol; C, 0.63 mM (2.1 x ^^~* 

 M) riboflavinphospliatc; D, 0.015 fiM (1.5 x 10"'' A/) livthocjninonc. 



