Characterization of the P‐Type and V‐Type ATPases of Cholinergic Synaptic Vesicles and Coupling of Nucleotide Hydrolysis to Acetylcholine Transport

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Abstract
Both phosphointermediate‐ and vacuolar‐type (P‐and V‐type, respectively) ATPase activities found in cholinergic synaptic vesicles isolated from electric organ are immunoprecipitated by a monoclonal antibody to the SV2 epitope characteristic of synaptic vesicles. The two activities can be distinguished by assay in the absence and presence of vanadate, an inhibitor of the P‐type ATPase. Each ATPase has two overlapping activity maxima between pH 5.5 and 9.5 and is inhibited by fluoride and fluorescein isothiocyanate. The P‐type ATPase hydrolyzes ATP and dATP best among common nucleotides, and activity is supported well by Mg2+, Mn2+, or Co2+ but not by Ca2+, Cd2+, or Zn2+. It is stimulated by hyposmotic lysis, detergent solubilization, and some mitochondrial uncouplers. Kinetic analysis revealed two Michaelis constants for MgATP of 28 μM and 3.1 mM, and the native enzyme is proposed to be a dimer of 110‐kDa subunits. The V‐type ATPase hydrolyzes all common nucleoside triphosphates, and Mg2+, Ca2+, Cd2+, Mn2+, and Zn2+ all support activity effectively. Active transport of acetylcholine (ACh) also is supported by various nucleoside triphosphates in the presence of Ca2+ or Mg2+, and the Km for MgATP is 170 μM. The V‐type ATPase is stimulated by mitochondrial uncouplers, but only at concentrations significantly above those required to inhibit ACh active uptake. Kinetic analysis of the V‐type ATPase revealed two Michaelis constants for MgATP of ∼ 26 μM and 2.0 mM. The V‐type ATPase and ACh active transport were inhibited by 84 and 160 pmol of bafilomycin A1/mg of vesicle protein, respectively, from which it is estimated that only one or two V‐type ATPase proton pumps are present per synaptic vesicle. The presence of presumably contaminating Na+, K+‐ATPase in the synaptic vesicle preparation is demonstrated.

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