Effect of Ferrous Ion on Tyrosine Hydroxylase of Bovine Adrenal Medulla

We have studied the effect of Fe2+ on a partially purified tyrosine hydroxylase, prepared after solubilizing with trypsin the crude enzyme sedimented from bovine adrenal medulla homogenate. An approximately 2-fold activation was consistently observed following preincubation of the enzyme with Fe2+ and mercaptoethanol in pH 8.6 Tris buffer. In addition to activating freshly prepared enzyme, the preliminary incubation step restored almost fully the activity of purified tyrosine hydroxylase preparations that had lost 80 to 90% of their activity upon standing overnight at room temperature in pH 6.0 buffer. Catalase or peroxidase could replace Fe2+ in the enzyme assay per se, indicating that the effect of Fe2+ in this step was probably due to its ability to remove H2O2. However, Fe2+ appeared to be specifically required in the activation step; it could not be replaced by catalase, peroxidase, or cations other than Fe2+, either in the presence or absence of mercaptoethanol. Mercaptoethanol could not be replaced in the activation step by glutathione, cysteine, dithiothreitol, or ascorbic acid. These studies indicate that Fe2+, under appropriate conditions, affects tyrosine hydroxylase in a unique manner.