Efecto de fenoles y Mn.^2x sobre la degradación de ácido indole-3-acético catalizada por peroxidasa

Abstract

The peroxidase catalyzed oxidation of indole-3-acetic acid is either activated or inhibited by phenolics compounds. The overall effect of these cofactors depends on their concentrations. Low concentrations of phenols such as 2,4-dichlorophenol or resorcinol greatly promote auxin degradation, whereas high concentrations inhibit it. Mn²⁺ salts prevent the inhibition observad when high monophenol concentrations are used. Activation is such remarkable that the reaction rate can be enhanced as much as a hundred times. Since indole-3-acetic acid is a relatively slow substrate for peroxidase, the coupled oxidation of phenols can thus be viewed as a very effective regulatory system for plant growth. These reactions ha ve thus been investigated using several techniques including spectrophotometry, chromatography, polarography and rapid mixing (stopped-flow spectroscopy). Activation occurs when peroxidase Compound II is the enzymatic compound responsible for indole-3-acetic acid oxidation. The reaction becomes inhibited under conditions where peroxidase Compoimd III is formed. Oxidation of indole-3-acetic acid or indole-3-butyric acid by peroxidase, in the presence of H₂O₂ is affected by monophenols such as 2,4 dichlorophenol. The enhancement in the reaction rate is very similar to that observed for the reaction in the absence of H₂O₂. Since in the presence of H₂O₂the only enzymatic compounds formed are Compounds I and II, these results confirm the fact that for the reaction carried out in the absence of H₂O₂ it is peroxidase Compound II which is responsible for promotion of indole-3-acetic acid oxidation and not peroxidase Compound III. Both indole-3-acetic acid, indole-3-butyric acid, and monophenols are substrates of peroxidase in the presence of H₂O₂ and at neutral pH valúes 2,4-dichIorophenol reacts about 100 times faster with Compounds I and II thant does indole-3-butyric acid. This would suggest that the reaction between enzymatic compounds and monophenols produces phenoxy radicáis able to oxidize indole-3-acetic acid or indole-3-butyric acid. 2,4-dichlorophenol and indole-3-butyric acid bind competitively to the same site of peroxidase which might in part explain the inhibition of indole-3-acetic acid oxidation, in the absence of H₂O₂, observed when high phenol concentrations are used. Reaction mechanisms based upon the preferential reactivity of the phenols for the various peroxidase compounds are proposed to explain these events.