Sidedness of (Calcium, Magnesium) Adenosine Triphosphatase of Purified Torpedo Synaptic Vesicles

Abstract: Purified Torpedo synaptic vesicles contain ouabain-insensitive Mg²⁺_τ and Ca²⁺-stimulated ATPase activity. The sidedness of the ATPase on the vesicular membranes was investigated. Addition of ATP and Mg²⁺ or Ca²⁺ to intact vesicles results in activation of the ATPase. Exposure of the vesicles to low concentrations of Lubrol-PX and Triton X-100, which do not solubilize the activity, results in the concurrent release of the vesicular contents and in an increase of the Mg²⁺-ATPase activity, whereas the Ca²⁺-dependent activity is drastically decreased. p -Chloromercuribenzene sulphonate (PCMBS) almost completely inhibits the activity of detergent-treated vesicles whereas that of the native material is only slightly affected. Tryptic digestion of intact vesicles and of vesicular ghosts results in partial reduction of the ATPase activity. These results suggest that the vesicles contain an outward oriented Ca²⁺/Mg²⁺ ATPase activity which can be modulated by detergents.