Tyrosine hydroxylase: a substrate of cyclic AMP-dependent protein kinase.

Abstract

Data demonstrating the direct phosphorylation of tyrosine hydroxylase purified from rat pheochromocytoma by ATP, Mg2+ and cyclic[c]AMP-dependent protein kinase catalytic subunit are presented. The incorporation of phosphate is highly correlated with the activation of the hydroxylase when either the time of preincubation or the amount of protein kinase subunit is varied. The rate of phosphorylation of tyrosine hydroxylase compares favorably with that of H1 histone, a known substrate of protein kinase. Lineweaver-Burk analysis of crude or purified rat pheochromocytoma tyrosine hydroxylase activity, as a function of pterin cofactor concentration, in the absence of ATP, Mg2+ and protein kinase catalytic subunit, yields a curvilinear relationship which can be resolved into 2 lines, suggesting 2 enzyme forms with different affinities for pterin cofactor. A fraction of the hydroxylase present in the tumor exists in the activated state, presumably due to the presence of ATP and endogenous protein kinase activity. When the soluble enzyme is activated by cAMP, ATP, Mg2+ and protein kinase, virtually all of the enzyme is converted to the low Km state. Tyrosine hydroxylase is a substrate of cAMP-dependent protein kinase in vitro and, presumably, in vivo.