OCCURRENCE AND DISTRIBUTION OF AROMATIC <scp>l</scp>‐AMINO ACID (<scp>l</scp>‐DOPA) DECARBOXYLASE IN THE HUMAN BRAIN

Abstract

Abstract —The enzymatic decarboxylation of l ‐DOPA was measured in isotonic dextrose homogenates of different regions of the human brain by estimating 14 CO 2 evolved from tracer amounts of d l ‐DOPA[carboxy1‐ 14 C]. Enzyme activity was linear with respect to tissue concentration and time of incubation. The reaction exhibited a pH maximum at 7·0, was completely dependent upon the presence of high concentrations of pyridoxal phosphate, proceeded at the same rate in an atmosphere of air and nitrogen, and produced dopamine in addition to CO 2 as a reaction product. The enzyme preparation behaved like an aromatic l ‐amino acid decarboxylase: it also decarboxylated o ‐tyrosine and when incubated with 5‐hydroxytryptophan, serotonin was isolated as the reaction product; but it was devoid of activity towards d ‐DOPA[carboxy1‐ 14 C]. Within the human brain, l ‐DOPA decarboxylase was most active in the putamen and caudate nucleus; the pineal gland, hypothalamus, and the reticular formation and dorsal raphe areas of the mesencephalon exhibited considerable activity. Areas of cerebral cortex exhibited very low enzymatic activity and in regions composed predominantly of white matter, l ‐DOPA decarboxylase activity was not significantly above blank values. The activity of l ‐DOPA decarboxylase in the human putamen and caudate nucleus tended to decrease with the age of the patients; in comparatively young subjects (46 yr old) the enzyme activity compared favourably with that found, by means of the same assay technique, in the caudate nucleus of the cat.