Differential inhibition of activated tyrosine hydroxylase

Under conditions of cyclic AMP-dependent protein phosphorylation, tyrosine hydroxylase (EC 1.14.16.2; TH) is activated. Kinetic analysis reveals that, upon activation the affinity of the enzyme cofactor tetrahydrobiopterin, Vmax, as well as the Ki of its putative feedback inhibitor dopamine, are increased. Catecholic inhibitors of rat striatal TH have been assessed for the structural requirements that impart differential sensitivity to activated and control enzyme. By varying cofactor and inhibitor concentrations, Ki's were generated from Dixon plots. Structural analogs of dopamine in which the amino group was fixed in a cis conformation, i.e., 6,7-dihydroxytetrahydroisoquinolines, exhibit the same Ki for activated and nonactivated TH. However, 2-amino-6,7-dihydroxytetralin (ADTN), in which the nitrogen is extended in a fixed trans conformation of the beta-rotamer, exhibited a fourfold increase in Ki upon assaying tyrosine hydroxylase under phosphorylation conditions. By systematically increasing the hydrophobicity of the substituent at C-1 of 1-carboxy-6,7-dihydroxytetrahydroisoquinolines the inhibitory potency was enhanced, suggesting the presence of a hydrophobic region near the catecholic binding site. If the hydrophobic group was rigid as in the catechol estrogens, 2-hydroxy-estradiol and 2-hydroxyestrone, the Ki was relatively low (2 X 10(-5) M) despite the absence of an amino group. Upon activation the Ki increased fourfold. These studies provide insight into the topography of the catecholic binding site on TH and to attendant changes occurring upon activation. The results suggest that the catechol binding site includes both amino group-interacting and hydrophobic regions which are influenced by enzyme activation.