Ferrous ion activates the less active form of human adrenal tyrosine hydroxylase

The less active form of human tyrosine hydroxylase has been previously reported but its physiological role is unknown. We partially purified the less active form of tyrosine hydroxylase from human adrenals, and examined differences in the properties of the active and less active forms. We succeeded in activation of the less active form of human tyrosine hydroxylase by addition of 100 ?M Fe(2+). Fe(2+) decreased the K(max) for pteridine cofactor in both the active and less active forms, but increased the V(max) only in the less active form. Fe(2+) changed the V(max) but not the K(m) of the less active form for tyrosine. These results suggest that Fe(2+) may regulate tyrosine hydroxylase activity in vivo as a result of activation of its less active form.