R. L. HINMAN 



and 

 P. FROST 



Union Carbide Research Institute 



A Model Chemical System for the 

 Study of the Oxidation of I ndole-3 -acetic 



Acid by Peroxidase 



The oxidative degradation of indole-3-acetic acid (lAA) by plant ex- 

 tracts was studied intensively during the 1950's (9). Despite the ad- 

 vances made in understanding the enzymatic components, the re- 

 quired cofactors, and the physiological significance of the reaction, 

 elucidation of the complete process has remained elusive, in good 

 part because the changes undergone by the organic substrate, lAA, 

 are still unknown. It is the purpose of work under way in these lab- 

 oratories to determine the identity of the products formed from lAA 

 when acted upon by the lAA oxidase system. We have found that 

 hydrogen peroxide in acidic media converts lAA to products the 

 spectra of which resemble closely those of the products formed in 

 the oxidation of lAA by peroxidase in the absence of added hydrogen 

 peroxide. Since it is now almost certain that the effective component 

 of the lAA oxidase of higher plants is a peroxidase (9), the HoOo/H^ 

 system has considerable significance in relation to the biological ac- 

 tivity of lAA. We are using the chemical system as a model to aid in 

 investigating the chemistry of the oxidation of lAA by peroxidase. 

 Previous attempts to identify the products of the oxidation of lAA 

 by lAA oxidase have depended mainly on isolation of the products 

 either in the free state or on paper chromatograms. Because of the 

 multiplicity of products actually isolated (7, 10), and the possibility 

 that the product formed initially may undergo decomposition under 

 the conditions of isolation, we have chosen to study the nature of the 

 product(s) by following the changes in the ultraviolet spectrum of 

 lAA during its reaction with peroxidase and with the H0O2/H+ sys- 

 tem. Following the course of these reactions by spectrophotometric 

 techniques has one other advantage over techniques in which only 

 residual lAA is determined. It is quite possible that lAA could be 



[205] 



