Degradation of Indol-3yl-acetic Acid in Homogenates and Segments of Cabbage Roots

Plots of reaction rate versus substrate concentration of the enzymatic decarboxylation of IAA yield sigmoid, rather than the usual, hyperbolic curves, suggesting that the IAA oxidase of cabbage roots is an allosteric enzyme. The quantity of this enzyme in roots is so high that the IAA concentration is likely to limit IAA degradation in intact cells. Thus, variations in the level of this enzyme seem not to be essential for the regulation of the endogenous IAA concentration. Cabbage roots contain substances that can inhibit IAA oxidase. These substances are spatially separated from IAA oxidase in intact cells, but the same inhibitors are able to reach the enzyme when added exogenously to tissue segments. The possibility that added IAA is treated by tissue segments in another manner than endogenous IAA is discussed.