Activity of tyrosine hydroxylase is regulated by feedback inhibition and inactivation by catecholamines, and activation by protein phosphorylation. In this article, reaction mechanisms for the conversion of tyrosine hydroxylase to an inactive/stable form by catecholamines. and activation of tyrosine hydroxylase by phosphorylation Lit Ser-40 are discussed. Inactivation may be induced by sub-stoichiometric amounts of catecholamines, and activation by phosphorylation of Ser-4 0 may require phosphorylation of three or all four subunits of a tyrosine hydroxylase molecule. Cooperative phosphorylation Lit Ser-40 in the subunits is also discussed. (c) 2005 Elsevier Inc. All rights reserved.