Studies on Tyrosine Hydroxylase from Bovine Adrenal Medulla

Abstract Approximately 90% of the total tyrosine hydroxylase activity of bovine adrenal medulla homogenates was localized in the particulate fraction of the homogenate. The particulate enzyme was solubilized by incubation with trypsin, and the solubilized enzyme was partially purified. This enzyme was found to behave like the tyrosine hydroxylase which had been purified from the supernatant fraction by Nagatsu, Levitt, and Udenfriend, in that a tetrahydropteridine is required as cofactor and the enzyme is inhibited by certain tyrosine analogues and catechol derivatives. Partially purified tyrosine hydroxylase requires the addition of Fe2+, and maximum activity is found when the enzyme has previously been incubated with Fe2+. In contrast, incubation of the enzyme with Fe2+ results in almost complete loss of activity, although both forms of iron are active without prior incubation. The enzyme is markedly inhibited by sulfhydryl reagents, indicating the presence of an essential sulfhydryl group on the enzyme. Nevertheless, prior incubation of the enzyme with mercaptoethanol results in inhibition of hydroxylation.