Use of Radiolabeled Monofluoromethyl-Dopa To Define the Subunit Structure of Human l-Dopa Decarboxylase

Human l-Dopa decarboxylase (L-aromatic amino acid decarboxylase, DDC) has been purified from pheochromocytoma tissue, a benign tumor of the catecholamine-synthesizing cells of the adrenal medulla. The binding characteristics of a new radiolabeled enzyme-activated suicide inhibitor of DDC ([³H]monofluoromethyl-Dopa, [³H]MFMD) have been established, and the covalent linkage of the inhibitor to the enzyme has been used to identify that human DDC exists as a dimer of a 50-kDa subunit. An antibody to human DDC identically precipitates the enzyme activity from different human, rat, and mouse tissues. Our data demonstrate the value of [³H]MFMD for probing the structure of DDC and facilitating the purification of this enzyme, and further emphasize the high degree of conservation of the DDC molecule over a wide variety of species.