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Regulatory Properties of Hepatic Tryptophan Oxygenase

From the ¹ From the Institute of Cancer Research and the Department of Biochemistry, College of Physicians and Surgeons, Columbia University, New York, New York 10032

The kinetic properties of highly purified hepatic l-tryptophan oxygenase have been studied at different hydrogen ion concentrations in the absence and presence of dl--methyltryptophan and 3-hydroxyanthranilic acid. At pH 8.0 plots of velocity versus l-tryptophan concentrations indicate classical hyperbolic Michaelis-Menten kinetics, whereas at pH 6.2 saturation by tryptophan is nonhyperbolic with a marked increase in the [Trp]_0.5. This effect of pH is reversible.

The tryptophan metabolite, 3-hydroxyanthranilic acid, is a negative modulator which at pH 7.9 converts saturation of tryptophan oxygenase by tryptophan from hyperbolic to sigmoidal. This regulatory effect is even more marked upon the protonated enzyme. Progressive saturation by 3-hydroxyanthranilic acid is itself sigmoidal, which is expressed only at higher substrate levels.

-Methyltryptophan increases the apparent affinity of hepatic tryptophan oxygenase for l-tryptophan at pH 6.2, but not at pH 8.0. -Methyltryptophan, in concentrations where it is neither a substrate nor a competitive inhibitor, antagonizes the inactivation of the enzyme by sodium dodecyl sulfate.

These findings indicate that hepatic tryptophan oxygenase is subject to allosteric regulation. The allosteric modulation of the enzyme activity can be shown with its substrate, l-tryptophan, as well as with positive, -methyltryptophan, and negative, 3-hydroxyanthranilic acid, effectors. These allosteric properties of tryptophan oxygenase are strongly affected by the hydrogen ion concentration in the assay medium.

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