Identification of indole-3-acetaldehyde and indole-3-acetaldehyde reductase in Chinese cabbage

Indole-3-acetaldehyde (IAAId) was identified as a natural compound in Chinese cabbage ( Brassica campestris L. ssp. pekinensis cv. Granat) seedlings by chemical conversion to indole-3-acetaldoxime (1AOX) followed by mass spectroscopy. The lAAId reductase (EC 1.2. 3.1), an enzyme with a molecular mass of 32 kDa, was extracted, purified 5-fold and characterized. The enzymatic IAAld reduction showed a pH optimum at 6–7 and a marked preference for NADPH as cofactor The K_m value for IAAld was 125 μ M , for NADPH 36 μ M . The enzyme reaction was inhibited at high NADPH concentrations (>200 μ M ) and modulated by IAA and indole-3-ethanol (IEt). Sulfhydryl reagents inhibited IEt formation, suggesting the participation of SH-groups in the reaction. Phenylacetaldehyde and benzaldehyde were competitive substrates, while acetaldehyde acted partly as an inhibitor, and partly as an activator on the IAAld reduction. IAAld reductase activity was also detected in other Brassica species. The importance of this enzyme is discussed with respect to the possibilities of IAA biosynthesis in the Brassicaceae.