Calcium Effects on the Solubilization of Membrane-Bound Histidine Decarboxylase in the Rat Brain

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Abstract
In a previous work we have shown that histidine decarboxylase (HD) activity is found in a soluble and a membrane-bound form. A major part (82%) of the membrane-bound HD activity in the crude mitochondrial fraction (P2) was present in the synaptic plasma membrane-containing subfraction. Physiological concentrations of Ca2+ had no direct effect on HD activity but caused a solubilization of approximately 50% of membrane-bound HD in the P2 fraction. Mg2+ had similar but lower effects (20% solubilization) than Ca2+. Incubation with depolarizing concentrations of K+ in the presence of 1 mM CaCl2 caused a significant (30%) solubilization of HD.