Tyrosine hydroxylase activation. Comparison of in vitro phosphorylation and in vivo administration of haloperidol.
Academic Article
Overview
abstract
Rat striatal tyrosine hydroxylase (TH) was assayed 2 hr following treatment with 1 mg/kg haloperidol. TH activity in striata from haloperidol-treated rats (haloperidol TH) was increased significantly relative to control when assayed at pH 7.0, but not at pH 6.0, in the presence of 175 muM tetrahydrobiopterin (BH4). TH was also phosphorylated in vitro, catalyzed by sufficient quantities of the catalytic subunit of bovine heart protein kinase to cause greater than 90% activation after 10 min. TH was activated by phosphorylation at both pH 6.0 and pH 7.0, but the activation was greater at pH 7.0. Haloperidol TH, activated relative to control TH at pH 7.0, was activated by phosphorylation, but there was no difference between haloperidol TH and control TH activity at either pH 6.0 or 7.0 following phosphorylation. Comparison of Lineweaver-Burk plots of nonphosphorylated and phosphorylated TH indicated that activation by phosphorylation was due to a 5-fold change in Km for BH4 and a 2-fold change in Vmax at pH 7.0. Haloperidol TH kinetics were intermediate between those of nonphosphorylated and phosphorylated TH at pH 7.0. Analysis by Lineweaver-Burk, Hanes-Woolf, and Eadie-Scatchard plots suggested that the haloperidol TH kinetic data were the result of a mixture of two forms of the enzyme, with different affinities for cofactor. Theoretical calculations of TH activity of mixtures of nonphosphorylated and phosphorylated TH suggested that the haloperidol data could be explained by postulating a mixture of 25-35% phosphorylated TH molecules with 65-75% nonphosphorylated TH molecules. An hypothesis of the role of TH phosphorylation during conditions of increased neuronal firing rate, such as may occur with haloperidol treatment, is presented.
