Regulation of enzymic oxidation of indole-3-acetic acid by phenols: Structure-activity relationships

Mono- and diphenols were tested for their effects on the decarboxylation of [1-¹⁴C]IAA catalysed by purified horseradish peroxidase (EC 1.11.1.7) in the presence or absence of 2,4-dichlorophenol (DCP). The number of hydroxyl groups and their position relative to each other and the nature and position of other substituents on the aromatic ring were found to affect the activity. Although the effects were complex, the following generalizations may be made. (1) Monophenols produce activation when no other cofactor is present. p-Substituted monophenols are more active than o- or m-compounds. In the presence of DCP, the activity varies from slight activation to strong inhibition. (2) m-Diphenols also produce activation in the absence of other cofactors while o- and p-diphenols, with the exception of 3,4-dihydroxyacetophenone and 3,4-dihydroxypropiophenone, produce strong inhibition in the presence or absence of DCP. The o-diphenolsare degraded in the IAA-oxidizing enzyme system and thus produce only a temporary inhibition. (3) m-Diphenols and 3,4-dihydroxyacetophenone produce a sustained inhibition in the presence of DCP. (4) Substitution at position 2 significantly alters the activity of m-diphenols. (5) O-Methylation alters the activity of most o-diphenols. In the absence of DCP, o-methoxyphenols and certain other phenols such as 3,4-dihydroxyacetophenone and 2,6-dihydroxyacetophenone either promote or inhibit IAA oxidation depending on concentration.