Abstract

Addition of CaCl₂ to soluble preparations of tyrosine hydroxylase from rat medulla pons produces a marked activation of the enzyme assayed with subsaturating concentrations of tyrosine (10 µM) and pteridine cofactor (2-amino-4-hydroxy-6,7-dimethyl-5,6,7,8-tetrahydropteridine, 100 µM). While some increase in activity occurs with Ca⁺⁺ concentrations as low as 10 µM, activation is maximal at 50 µM Ca⁺⁺ and remains unchanged up to 1.0 mM. BaC1₂ produces similar although less pronounced effects. MgCl₂ prevents the activation of the enzyme if added to the reaction mixture before CaCl₂. Alone MgCl₂ has no effect in concentrations up to 1 mM. Ethylene glycol bis(β-aminoethyl ether)-N,N'-tetraacetic acid has no direct effects on the enzyme but completely antagonizes the activation produced by Ca⁺⁺. The activation of tyrosine hydroxylase by Ca⁺⁺ is reflected in changes in the kinetic properties of the enzyme. The K_m for tyrosine decreases from 58.1 to 10.3 µM, the K_m for pteridine cofactor decreases from 673 to 125 µM, and the K_i for norepinephrine increases almost 20-fold, from 0.34 to 6.27 mM, in the presence of Ca⁺⁺. Thus norepinephrine is a much less effective inhibitor of the Ca⁺⁺-activated enzyme. The proposal is made that Ca⁺⁺ which enters the nerve terminal during nerve stimulation may enhance norepinephrine synthesis by activating tyrosine hydroxylase in a manner similar to the activation observed in vitro with CaCl₂. Similar findings are reported for tyrosine hydroxylase isolated from rat cerebral cortex.