Proposed Enzymes of Auxin Biosynthesis and Their Regulation III. Some Properties of Pea Indolylacetaldehyde Oxidase

Indole-3-acetaldehyde oxidase (IAAld-oxidase) occurs in pea in two forms, of which the first, more active enzyme, has its pH optimum at 4.5, while the second, barely half as active, has a pH optimum at 7.0. Only the pH 4.5 oxidase can be resolved from the acetone powder. Besides IAA1d the more stable IA1d was used as substrate in testing the enzymatic activity. The pea enzyme seems not to be a dismutase since indolylmethanol or indolylethanol were not formed as products. Pyridine nucleotide coenzymes did not activate the partially purified enzyme. The pH 4.5 oxidase was inhibited by more than 50 % by IAA > L-asp > tryptophol > indoleacetylaspartic acid > 2,4-D (at 1 mM concentration). The pH 7.0 oxidase was inhibited relatively more weakly, a stronger than 50 % inhibition was caused only by NAA > L-asp. The oxidases were clearly distinguished by the response to L-asparagine (1 mM): the activity of the pH 4.5 oxidase was increased (+ 12 %), while the activity of the pH 7.0 oxidase was decreased (-71 %). In preliminaryin vitro experiments the phytohormones (1 mM) kinetin and GA₃ increased the conversion of IAAld to IAA, while ABA decreased it.