Specific stimulated uptake of acetylcholine by Torpedo electric organ synaptic vesicles.

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Abstract
The specificity of acetylcholine uptake by synaptic vesicles isolated from the electric organ of T. californica was studied. In the absence of cofactors, [3H]acetylcholine was taken up identically to [14C]choline in the same solution (passive uptake) and the equilibrium concentration achieved inside the vesicles was equal to the concentration outside. In the presence of MgATP, [3H]acetylcholine and [14C]choline in the same solution were taken up identically, except only .apprx. half as much of each was taken up (suppressed uptake). [3H]Acetylcholine uptake was stimulated by MgATP and HCO3- .apprx. 4-fold relative to suppressed uptake, for a net concentrative uptake of .apprx. 2:1 (stimulated uptake). Uptake of [4C]choline in the same solution remained at the suppressed level. [3H]Acetylcholine taken up under stimulated conditions migrated with vesicles containing [14C]mannitol on analytical glycerol density gradients during centrifugation. Vesicles were treated with 9 protein modification reagents under mild conditions. Two reagents had no effect on, dithiothreitol potentiated and 6 reagents strongly inhibited subsequent stimulated uptake of [3H]acetylcholine. Uptake of acetylcholine is apparently conditionally specific for the transported substrate, is carried out by the synaptic vesicles rather than a contaminant of the preparation and requires a functional protein system containing a critical sulfhydryl group.