Phosphoryl Group Transfer by a Fraction of the Soluble Proteins of Catecholamine Storage Vesicles

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Abstract
The terminal phosphate group of ATP was transferred to ADP by an enzyme present in the soluble core proteins of adrenal medulla catecholamine storage vesicles. It was purified 10–30‐fold by DEAE Sephadex chromatography (Fraction I). The enzyme required divalent metal ions for activation; Mn2+ was almost as effective as Mg2+, but Ca2+ was only a weak activator. Activation by Mg2+ took place over a very narrow concentration range (0.5–3 mm). The specificity of the enzyme activity to nucleoside triphosphates was broad, to the nucleoside diphosphates narrow, favouring adenosine diphosphate. In dependence on the pH the activity increased from pH 4 to pH 7 and remained constantly high between pH 7 and 9. The Arrhenius plot was linear between 5 and 70°C, with an activation energy of 11.1 kcal/mol. The phosphoryl group transfer reaction depended on the function of thiol groups; p‐hydroxymercuribenzoate inhibited 50% of the enzyme activity; dithioerythritol reactivated it completely. Gel electrophoresis revealed that in Fraction I, a protein of molecular weight about 45,000, was enriched compared with the total soluble proteins. The enzyme‐enriched Fraction I differed significantly in its relative amino acid composition from that of the total soluble proteins; in general, the acidic amino acids were reduced and the more basic acids enhanced.