Indoleacetaldehyde Reductase of Cucumis sativus L. Kinetic Properties and Role in Auxin Biosynthesis

Indoleacetaldehyde reductase catalyzes the conversion of indoleacetaldehyde to indole ethanol in extracts of Cucumis sativus L., with reduced pyridine nucleotide required as co-substrate. NADH and NADPH result in markedly different enzyme behavior, as reflected in reaction kinetics and in responses to inhibitors and activators. It is not yet clear whether there are two separate enzymes, one specific for NADH and the other for NADPH, or whether there is a single enzyme differentially influenced by the two co-substrates. In the presence of NADH, the indoleacetaldehyde reductase activity was inhibited by NaCl and displayed hyperbolic kinetics under all conditions tested. However, in the presence of NADPH the enzyme was activated by NaCl at concentrations up to 0.1 molar. Under certain conditions with NADPH as co-substrate, the enzyme showed kinetics sigmoidal with respect to indoleacetaldehyde concentration and was strongly inhibited by high concentrations of NADPH. It is possible that this substrate inhibition of the NADPH-linked indoleacetaldehyde reductase activity by NADPH, as well as the sigmoidicity with respect to indoleacetaldehyde concentration, may function in the regulation of auxin biosynthesis.