Simultaneous immunological titration of DOPA and 5-hydroxytryptophan decarboxylase activities, from a number of tissues of various species, showed that the two activities were not distinguishable with a monospecific antiserum to hog kidney decarboxylase. Together with previous findings, these data firmly establish thb concept that in mammalian tissues the two enzyme activities are associated with a single protein, namely aromatic L-amino acid decarboxylase.The enzymatic synthesis of the biogenic amines serotonin, dopamine, and norepinephrine involves decarboxylation of the corresponding amino acids, 5-hydroxytryptophan and 3.4-dihydroxyphenylalanine (DOPA). DOPA decarboxylase (EC 4.1.1.26) was the first enzyme of the catecholamine biosynthetic pathway to be discovered (1). After the isolation and identification of serotonin (2, 3), Clark, Weissbach, and Udenfriend (4) found significant DOPA decarboxylase activity in their most highly purified preparations of 5-hydroxytryptophan decarboxylase (EC 4.1.1.28) from guinea pig kidney. For a number of reasons, they considered that the two activities represented distinct enzymes. However, as analytical procedures improved; reports from many laboratories made it appear more likely that a single enzyme acts on both substrates (5-16). In view of their evidence that a partially purified enzyme from guinea pig kidney decarboxylated a large number of aromatic -amino acids, including DOPA and 5-hydroxytryptophan, Lovenberg, Weissbach, and Udenfriend (i4) proposed that the enzyme be named "aromatic i-amino acid decarboxylase".Because the amiies resulting from decarboxylation are localized differently and are thought to have distinct physiological functions, many investigators have found it difficult to accept the hypothesis that the same enzyme is involved in both pathways. The recent demonstration that a single, homogeneous enzyme from hog kidney decarboxylates all the naturally occurring aromatic amino acids (17) is a necessary, but not a sufficient, condition for the "single enzyme" hypothesis. The possibility remains that a second enzyme may also exist, at least in some tissues. Recently, Sims and Bloom (18) have reported that, after intracisternal administration of 6-hydroxydopamine to rats, DOPA decarboxylase activities were substantially decreased in certain areas of the brain, while the 5-hydroxytryptophan decarboxylase activities were no different from controls.In the present report, evidence is presented in support of the "single enzyme" hypothesis. It is shown that DOPA decarboxylase and 5-hydroxytryptophan decarboxylase activities are not immunologically distinguishable. DOPA decarboxylase activity was assayed as described (17). The availability of carboxyl-labeled 5-hydroxytryptophan made it possible to assay 5-hydroxytryptophan decarboxylase by a basically similar method. All conditions were the same as those for the DOPA decarboxylase assay, with the following exceptions: the buffer was Tris * HCl (pH 8.
