Ca(2+)-dependency and substrate specificity of cholinergic synaptic vesicle ATPase

The Mg(2+)-ATPase associated with highly purified cholinergic synaptic vesicles isolated from the Torpedo electric organ can be activated by millimolar concentrations of Ca(2+) only to a maximum of 46%. ATP is hydrolyzed in preference to other tri-nucleotides on Mg(2+)-activation but not on Ca(2+)-activation of the enzyme. Analysis of the Ca(2+)-dependency of vesicle associated ATPase-activities under controlled free Ca(2+)-concentrations (10(?7) to 10(?3)M) shows that vesicles do not contain a Mg(2+)-dependent, Ca(2+)-stimulated (Ca(2) + Mg(2+)) ATPase. Our results suggest that cholinergic synaptic vesicles contain a single ATPase preferentially stimulated by Mg(2+)-ions and that ATP-stimulated vesicular uptake of Ca(2+) is not due to the presence of a Mg(2+)-ATPase stimulated by micromolar Ca(2+)-concentrations.