Mammalian histidine decarboxylase. Stability studies with special reference to the effect of salts

Histidine decarboxylase (EC 4.1.1.22) prepared from a murine mastocytoma is activated up to six-fold when the concentration of phosphate in the assay medium is increased from 1 mM to 150 mM. Chloride and sulfate, on the other hand, are inhibitory and appear to interfere with the binding of pyridoxal phosphate to the enzyme. The inhibition by chloride is relatively less pronounced at high than at low concentrations of phosphate. The enzyme is inhibited by heavy metal ions and to some extent by alkylation and oxidation, but also by strong reduction. The histidine decarboxylase activity is stablized by 150 mM potassium phosphate, 1 mM dithiothreitol and 10 micrometers pyridoxal phosphate when stored at 6--8 degrees C. This holds true for both crude extract enzyme and enzyme purified by molecular sieving and hydrophobic interaction chromatography.