Tyrosine Hydroxylase Activation and Inactivation by Protein Phosphorylation Conditions

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Abstract
Tyrosine hydroxylase, the rate‐limiting enzyme in catecholamine biosynthesis, catalyzes the conversion of tyrosine to DOPA. Cyclic AMP‐dependent protein phosphorylation conditions alter tyrosine hydroxylase activity in rat striatal homogenates. In agreement with other laboratories, we find that short‐term pre‐incubation (3 min) of extracts under phosphorylating conditions (Mg. ATP, cAMP) increases enzyme activity two‐ to tenfold over control as measured during a subsequent 15‐min assay. We now report that pre‐incubation under phosphorylating conditions for longer periods (30 min) results in a loss of activity to levels equal to or below that of the control enzyme. Addition of purified bovine brain protein kinase catalytic subunit and Mg. ATP enhances activation and increases the rate of inactivation. To demonstrate that inactivation is not associated with proteolytic degradation or irreversible de‐naturation, the inactivated form of the enzyme can be reactivated. The protein kinase inhibitor protein decreases the activation process and prevents inactivation of the enzyme to below control values. The sedimentation coefficient is not changed by phosphorylation conditions (S = 8.8 ± 0.1). Although the apparent Km, of the enzyme for the 6‐methyltetrahydropterine (6‐MPH4) cofactor is reduced (0.86 mM, control; 0.32 MM, activated), it is also reduced in the inactivated form (0.38 mM). The Ki, for dopamine is increased from 4.5 μM for the control to 28 μM for the activated enzyme, whereas the inactivated form of the enzyme exhibits a Ki of 10 μM. Removal of catecholamines by gel filtration fails to alter activity and the apparent cofactor Km. Moreover, both the activated and the inactivated states persist following gel filtration. It therefore appears that the activation‐inactivation process is not mediated solely by the modulation of enzyme feedback inhibition or changes in the Km, for 6‐MPH4. We also describe a coupled decarboxylase assay in which labeled dopamine is resolved from the precursors tyrosine and DOPA by low‐voltage paper electrophoresis.