Liver tryptophan 2,3-dioxygenase is a cytosolic enzyme that plays a crucial role in the regulation of circulating levels of tryptophan. Stimulation of the activity of this enzyme by heme enhances the catabolism of tryptophan, making less tryptophan available for uptake into the brain. Melatonin, the major hormone of the pineal gland, is shown to cause competitive inhibition of this enzyme (Ki = 2.70 microM). This structural analog of the substrate L-tryptophan is a negative homotropic cooperative modulator of the enzyme. The enzyme has a Km = 100 microM, and the substrate concentration required for optimum activity was found to be 2.5 mM with substrate inhibition becoming a feature at higher levels of tryptophan.
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