Determination of aromatic-L-amino acid decarboxylase in human plasma

Aromatic-L-aminoacid (dopa) decarboxylase (ALAAD) was determined in human plasma by its ability to form dopamine from the substrate 3,4-dihydroxyphenylalanine in the presence of pyridoxal-5-phosphate as cofactor. Dopamine formed was quantitated by high performance liquid chromatography with electrochemical detection. A preincubation step of plasma with the cofactor and dithioerythritol was necessary to obtain optimal reaction conditions. The assay method showed good linearity and reproducibility. The inhibition pattern of the therapeutically used peripheral dopa decarboxylase inhibitors, carbidopa and benserazide, was studied and appeared to be dependent on whether the inhibitor was added before or after the preincubation step. Mean levels in 40 control subjects, in 40 patients with essential hypertension and in 15 patients with phaeochromocytoma, were 34.6 (SD 12.1), 28.5 (SD 10.9) and 34.7 (SD 18.4) mU/l respectively. In the patients with essential hypertension the enzyme level decreased with age (p less than 0.05). Very high levels were found in plasma of two patients with metastatic phaeochromocytoma and in two patients with untreated neuroblastoma, but not in two patients with neuroblastoma after chemotherapy. The method described can be used for measuring uninhibited ALAAD activity in patients treated with benserazide, as well as for measuring total, i.e. the sum of inhibited and uninhibited, ALAAD activity in patients treated with carbidopa.