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Plant Physiology 51:739-743 (1973)
© 1973 American Society of Plant Biologists

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Articles

Indole-3-ethanol Oxidase

Kinetics, Inhibition, and Regulation by Auxins 1

Frank W. Percival, William K. Purves and Larry E. Vickery2

a Department of Biological Sciences, University of California, Santa Barbara, California 93106

We report the further characterization of indole-3-ethanol oxidase from cucumber seedlings. The effects of various inhibitors suggest that the enzyme may be a flavoprotein with a metal ion and sulfhydryl groups required for full activity. Indole-3-acetaldehyde, a product of the reaction, inhibits the enzyme. This inhibition is overcome by O2 but not by indole-3-ethanol, indicating that the kinetic mechanism of the enzyme is a ping-pong Bi-Bi. The enzyme undergoes cooperative interactions with indoleethanol, yielding Hill coefficients as high as 2.96. Gibberellins are without effect on the enzyme, but it is inhibited by several acidic indoles possessing growth-promoting activity and by two synthetic auxins, 2,4-dichlorophenoxyacetic acid and 2,4,5-trichlorophenoxyacetic acid. Increasing concentrations of indoleacetic acid (IAA) brought about a slight reduction in the indoleethanol concentration producing halfmaximal velocity. Increasing levels of indoleethanol decreased the concentration of IAA required for half-maximal inhibition. At low concentrations of indoleethanol, low levels of IAA activated rather than inhibited. The effect of IAA was not overcome at higher levels of indoleethanol. These results may be interpreted as showing that IAA is a noncompetitive inhibitor which binds to that conformation of the enzyme which also binds indoleethanol. The significance of these interactions for the regulation of IAA biosynthesis is discussed.

2 Present address: Chemical Biodynamics Laboratory, University of California, Berkeley, Calif. 94720.

1 This research was supported by National Science Foundation Grant GB-13219 to W. K. Purves, a National Science Foundation Predoctoral Fellowship to F. W. Percival, and a National Defense Education Act Predoctoral Fellowship to L. E. Vickery.






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ASPB Publications PLANT PHYSIOLOGY® THE PLANT CELL
Copyright © 1973 by the American Society of Plant Biologists