Acute regulation of tyrosine hydroxylase

Research Doctorate - Doctor of Philosopy (PhD)Tyrosine hydroxylase (TH), the rate-limiting enzyme in catecholamine biosynthesis, is regulated acutely by a combination of phosphorylation of three key serine (Ser) residues (Ser19, Ser31 and Ser40), and feedback inhibition by the catecholamines. Phosphorylation of Ser40 directly increases TH activity by relieving feedback inhibition of the enzyme. The phosphorylation of Ser19 or Ser31 can potentiate the phosphorylation of Ser40 in a process known as hierarchical phosphorylation. The 2 major human TH isoforms, hTH1 and hTH2, are differentially regulated by hierarchical phosphorylation in vitro. In this study, the human neuroblastoma SH-SY5Y cell line has been transfected with hTH1 and hTH2, and it has been demonstrated that phosphorylation of Ser31 potentiates the phosphorylation of Ser40 in hTH1. Phosphorylation of the equivalent Ser31 residue in hTH2 was not detectable, and thus this enzyme is not subject to Ser31-mediated hierarchical phosphorylation of Ser40 in situ. This is the first study to demonstrate that hTH1 and hTH2 are differentially regulated in situ. In addition, we have examined the nature of feedback inhibition of TH by the catecholamines. In addition to the high affinity, non-dissociable dopamine binding that is relieved by Ser40 phosphorylation, we have identified a second low affinity, readily dissociable binding site which regulates TH activity both in vitro and in situ regardless of the phosphorylation state of the enzyme. This low affinity binding site responds to changes in cytosolic catecholamine levels in situ in order to regulate TH activity. This work has contributed to our understanding of the complex nature of the regulation of TH activity