The effects of phosphorylation at Ser40 of rat tyrosine hydroxylase on the affinities of catechols have been determined with both the ferric and ferrous forms of the enzyme. Phosphorylation had no effect on the K(i) value for the inhibition of the ferrous enzyme by either dopamine or DOPA when the initial rate of turnover was measured in assays. However, phosphorylation of the ferric enzyme resulted in a 17-fold decrease in affinity for DOPA and a 300-fold decrease in the affinity for dopamine, while the affinity for dihydroxynaphthalene was unchanged. The changes in binding affinity for the two catecholamines were almost exclusively due to large increases in the dissociation rate constants upon phosphorylation. These results support a novel mechanism for regulation in which phosphorylation affects binding of catecholamines to the catalytically inactive ferric form of the tyrosine hydroxylase.
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