Consequences of administration of indoleacetic acid 



known to give rise to H2O2 upon reoxidation by molecular oxygen, we have 

 inferred that the oxygen known to be required for the oxidation of lAA by 

 the oxidase is involved in such a reaction. 



(c) The enzymatic activity is inhibited by cyanide, azide, and other heavy- 

 metal inhibitors (Tang and Bonner, 1947; Wagenknecht and Burris, 1950). 

 This fact, together with the apparent requirement for H2O2, led us to con- 

 jecture that the oxidase was in fact composed of two moieties, a peroxide- 

 generating system (presumably flavo-protein) and a peroxidase (Galston, 

 Bonner, and Baker, 1953). 



This conjecture has been supported by the construction of an analogous 

 system consisting of xanthine oxidase and horseradish root peroxidase, 

 which will oxidize lAA when some substrate for the flavo-protein moiety is 



aor- 



Figure 6. The inhibition of lAA- 

 oxidase activity by crystalline catalase. 

 At each arrow, the reaction mixture 

 was divided, and catalase added to one 

 aliquot. 



I 

 I 



so 



'^0 



20- 



Cafalase 



Catalase 

 Catalase 



80 100 120 ifO 

 min 



supplied to the system (Galston et al., 1953). Since, in the lAA oxidase, no 

 such additional substrate is required, we have further postulated that lAA, 

 like dioxymaleic acid (Swedin and Theorell, 1940), is capable of giving rise 

 to a peroxide used in its own peroxidation. This conjectured peroxigenic 

 action of lAA was first directly demonstrated by Andreae and Andreae (1953) 

 and later confirmed in our own laboratory (Siegel and Galston, 1955; 

 Pilet and Galston, 1955). 



(d) One final fact concerning the enzyme needs to be recorded here, 

 though its full significance is not yet completely appreciated. The action of 

 the oxidase is greatly enhanced by certain cofactors, among which 2:4- 

 dichlorophenol (DCP) (Goldacre, Galston, and Weintraub, 1953) and the 

 manganous ion (Wagenknecht and Burris, 1950) are most effective. We 

 have obtained direct evidence that both of these cofactors operate by 

 increasing the effective peroxide level (Siegel and Galston, 1955), though 

 Lockhart (1955) believes the effect of DCP to be directly on the peroxidase. 



With this picture of the chemical nature of lAA-oxidase in mind, let us 

 return to the etiolated pea seedling and inquire as to the functional signi- 

 ficance, if any, of this auxin-destroying system. Miss Dalberg and I discovered 

 two facts which seemed to us highly interesting (Galston and Dalberg, 1954). 

 In the first place, roots, which are known to be much more sensitive to avixins 



225 



