Decarboxylation of p‐Tyrosine: A Potential Source of p‐Tyramine in Mammalian Tissues

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Abstract
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.