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Updated: Oct 9, 2026

Pull-down of Calmodulin-binding Proteins
Published on: January 23, 2012
Role of calmodulin in the activation of tryptophan hydroxylase
Abstract:
Tryptophan hydroxylase can be activated 2.0- to 2.5-fold in vitro by ATPa dn Mg2+. This apparent phosphorylation effect is not dependent on cyclic nucleotides but is dependent on the presence of calcium. The activation of tryptophan hydroxylase by ATP-Mg2+ reduces the apparent Km of the enzyme for its cofactor, 6-methyltetrahydropterin, from 0.21 to 0.09 mM. The addition of certain antipsychotic drugs known to bind to calmodulin in a phosphorylation reaction mixture prevents the activation to tryptophan hydroxylase by ATP-Mg2+ in the concentration-dependent fashion. External addition of purified calmodulin protects the enzyme from the drug-induced effects. Preparation of calmodulin-free tryptophan hydroxylase by affinity chromatography on fluphenazine-Sepharose 4B yields an enzyme that is no longer activated by ATP-Mg2+, whereas the readdition of calmodulin to a calmodulin-free enzyme restores the responsiveness of tryptophan hydroxylase to ATP-Mg2+. This restoration is dependent on Ca2+. Taken together, these results indicate that the activation of tryptophan hydroxylase by phosphorylating conditions is dependent on both calcium and calmodulin.
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