Measurement of aromatic L-amino acid decarboxylase--a technical comment

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.

Decarboxylation of p-tyrosine: a potential source of p-tyramine in mammalian tissues

The question of the existence of a p-tyrosine decarboxylase pathway for the formation of p-tyramine in mammalian tissues remains unresolved. Development of a sensitive and specific assay for p-tyrosine decarboxylase has permitted demonstration of this activity in rat tissues and human kidney. Tyrosine decarboxylase was purified to electrophoretic homogeneity by pH 5.0 precipitation, ammonium sulfate precipitation, gel filtration, phenyl-Sepharose chromatography, DEAE-Sephacel chromatography, and preparative isoelectric focusing. A specific rabbit antiserum to tyrosine decarboxylase was also obtained. Purified tyrosine decarboxylase possessed a narrow pH dependency with an optimum at 8.0. Benzene and certain other organic solvents dramatically stimulated tyrosine decarboxylase activity of purified enzyme. Purified tyrosine decarboxylase activity also decarboxylated L-DOPA, 5-hydroxytryptophan, 3,4-dihydroxyphenylserine, o-tyrosine, m-tyrosine, phenylalanine, histidine, and tryptophan, which suggested that the purified enzyme was aromatic L-amino acid decarboxylase. This conclusion was supported by a constant ratio of 5-hydroxytryptophan decarboxylase to tyrosine decarboxylase throughout the purification scheme and by parallel immunoprecipitation of decarboxylase activities by the specific antityrosine decarboxylase antisera. Thus, we report that p-tyrosine is decarboxylated by aromatic L-amino acid decarboxylase and that this metabolic transformation may be an important source of p-tyramine in mammalian tissues. In conclusion, neuronal tissues that synthesize catecholamines or serotonin should now be considered capable of synthesizing p-tyramine and other biogenic amines.

Assay of tyrosine 3-monooxygenase using the coupled nonenzymatic decarboxylation of dopa

Dopa was found to be decarboxylated nonenzymatically by ferricyanide over the range of pH 4.5 to 10. On the basis of this finding, a new assay method of tyrosine 3-monooxygenase using the coupled nonenzymatic decarboxylation by ferricyanide was developed. The new method was excellent not only in sensitivity and reliability, but also in convenience.

Accurate assay of dopa decarboxylase by preventing nonenzymatic decarboxylation of dopa

The nonenzymatic decarboxylation of dopa was completely blocked by both 2-mercaptoethanol and EDTA together over the wide range of pH. This finding made it possible to measure the activity of dopa decarboxylase precisely even at an alkaline pH value. The pH optimum of dopa decarboxylase was found to be pH 7.0 and the Km value for dopa was determined to be 4 X 10(-5) M.

[Purification and properties of aromatic L-amino acid decarboxylase (4.1.1.28) of rat brain]

L-aromatic aminoacid decarboxylase has been purified more than thousand times from homogenates of rat brain, in several steps : centrifugation, DEAE-cellulose, CM cellulose, hydroxylapatite, DEAE sephadex. Its properties have been studied, most of them on an intermediate fraction of the purification, because of the instability of the purified enzyme in spite of the addition of different stabilizing agents : the enzyme decarboxylates 5-hydroxytryptophan (5 HTP) and DOPA in a ratio constant throughout the purification but does not decarboxylate tryptophan, tyrosine, histidine at a measurable rate. Optimum pH, Km, Vm, have been measured with 5 HTP and DOPA as substrates. The enzyme has a molecular weight of 115.000, an apparent isoelectric point of 6,4-6,5. It is inhibited by serotonin, dopamine, some cations : Cu++, Fe++, Ni++ by N-ethylmaleimide, sodium dodecylsulfate. Some pyridoxal-5 phosphate (PLP) remains strongly bound to the enzyme. For relatively weak concentrations of substrate, the enzyme is inhibited by an excess of PLP ; for weak concentrations of PLP, the enzyme in inhibited by an excess of substrate, particularly of DOPA. We also observe a spontaneous decarboxylation of the substrates that reaches a plateau and is enhanced by high concentrations of PLP, by serotonin, dopamine, Cu++ and reduced by mercaptoethanol and the presence of crude or boiled homogenates. Several possible explanations of the spontaneous decarboxylation and of the enzymic inhibitions by an excess of PLP and by the substrates are given.