Aromatic LAmho Acid Decarboxylase*

After the finding of 5-hydroxytryptamine (serotonin) in tissues (2), this laboratory reported that the amine is derived from the dietary amino acid, tryptophan (3), 5-hydroxytryptophan being the immediate precursor. Decarboxylation of 5-hydroxytryptophan was shown to be catalyzed by an enzyme found in many tissues and was originally considered to be distinct from 3,4-dihydroxyphenylalanine decarboxylase! since the ratio of activity towards 5-hydroxytryptophan and 3,4-dihydroxyphenylalanine seemed to change during enzyme purification. It was observed that the decarboxylations of HTP’ and DPA had different pH optima and cofactor requirements (4). In these early studies, it was not possible to obtain separation of the two activities. It was also known that the tissue distributions of DPA and HTP decarboxylase were similar. Yuwiler, Geller, and Eiduson (5) subsequently showed that there was competition between HTP and DPA for decarboxylation and that various inhibitors had similar actions on both decarboxylations. Werle and Aures (6) and Rosengren (7), using more purified preparations, also presented evidence which suggested that the two substrates were acted upon by the same enzyme. These findings again raised the question as to whether mammalian HTP and DPA decarboxylase are the same enzyme. Reports concerning the decarboxylation of the other dietary aromatic amino acids (8, 9) and of histidine (10) by animal tissues have also appeared, although much of this early work has remained uncorroborated. However, the amines of all the dietary aromatic amino acids are known to be excreted in the urine (11). Using 50to loo-fold purified enzyme preparations and specific inhibitors, we have reinvestigated the problem of aromatic amino acid decarboxylation in mammalian tissues. These studies indicate that there is one enzyme present in mammalian tissues that catalyzes the decarboxylation of DPA, HTP, phenylalanine, tyrosine, tryptophan, and histidine. The previous conclusions (4) were based on studies which did not fully consider the fact that DPA is a poor substrate to assay because of nonenzymatic interaction with the coenzyme. In view of these findings, it is suggested that this enzyme now be referred to as aromatic L-amino acid decarboxylase.