lAA Oxidase-Peroxidase of Peas 



145 



corresponds closely to that for the photosensitized oxidation of lAA 

 by riboflavin and to the absorption spectrum of riboflavin (3). It has 

 therefore been postulated by Galston et al. (5) that a flavine enzyme 

 is involved in the light-promoted step. And because light overcomes 

 the inhibitory effect of added catalase, and in view of the well-known 

 production of hydrogen peroxide upon autoxidation of flavines, it 

 was proposed (5) that the oxidase step, i.e., the peroxide-generating 

 step, was mediated by a flavine enzyme. 



Other workers (9, 20) interpret the light promotion as being due 

 to photolysis of a natural inhibitor, for the promotion occurs only 

 on undialyzed preparations. 



Notwithstanding the activation by light and by a dialyzable co- 

 factor, it has been demonstrated (5, 13) that crystalline horseradish 

 peroxidase alone can catalyze the oxidation of lAA. (Acting on its 

 classical substrates, this enzyme requires no cofactor, nor light.) Oxy- 

 gen is consumed, and peroxide is again obligatorily involved. There 

 may be a lag phase in the reaction, which can be abolished by adding 

 hydrogen peroxide, or natural cofactor, or DCP. The reaction 

 scheme evolved by Kenten (13), and generally supported in its es- 

 sentials by others, is seen in Figure 1. 



Manganous ions have been variously reported to promote (9, 13, 19, 

 20) or inhibit (4, 8, 10) the oxidation of lAA. It now appears (11, 14) 



LIGHT 



Fig. 1. Reaction scheme evolved by Kenten for the enzymatic oxidation of indole- 

 3-acetic acid. 



