Intracellular Localization of the Tryptophan-indoleacetate Enzyme System.

Abstract

The intracellular distribution of the enzyme that converts tryptophan to indoleacetic acid was determined for the mung bean seedling (Phaseolus aureus, Roxb.). Hypertonic sucrose-PO4 homogenates of 7- and 15-day-oldseedlings were separated by differential centrifugation into fractions containing nuclei, plastids, mitochondria, microsomes, and "soluble" protein. The last, in turn, was sub-fractionated into two fractions: the ultracentrifugally homogeneous component with an S[image]20 of 19 Svedberg units, and a heterogeneous fraction containing components with-S''20 of 0 to 4. (These fractions correspond, respectively, to the Fractions I and II of Wildman and co-workers (Ann. Rev. Plant Physiol. 3:131-148, 1952).) Enzyme activities were determined at pH 7.4 at substrate saturation levels. The indoleacetic acid formed was resolved by chromatography and assayed either colorimetrically or by Avena curvature test. It is concluded that the tryptophan-to-indoleacetate enzyme occurs as a soluble component of the cytoplasm, and that it is in the group of heterogeneous macro-molecules with sedimentation coefficients between 0 and 4 Svedberg units.