Photoaffinity labeling of the acetylcholine transporter.

The acetylcholine (AcCh) binding site in the AcCh transporter-vesamicol receptor (AcChT-VR) present in synaptic vesicles isolated from the electric organ of Torpedo was characterized. A high-affinity analogue of AcCh containing an aryl azido group, namely, cyclohexylmethyl cis-N-(4-azidophenacyl)-N-methylisonipecotate bromide (AzidoAcCh), was synthesized in nonradioactive and highly tritiated forms. AzidoAcCh was shown to be a competitive inhibitor of [3H]AcCh active transport and binding of [3H]-vesamicol to the allosteric site. The [3H]AzidoAcCh saturation curve was determined. In all cases the AcChT.AzidoAcCh complex exhibited an inhibition or dissociation constant of about 0.3 microM. Binding of [3H]AzidoAcCh was inhibited by vesamicol and AcCh. AzidoAcCh irreversibly blocked greater than 90% of the [3H]vesamicol binding sites after multiple rounds of photolysis and reequilibration with fresh ligand. Autofluorographs of synaptic vesicles photoaffinity-labeled with [3H]AzidoAcCh showed specific labeling of material exhibiting a continuous distribution from 50 to 250 kDa after sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The result demonstrates that the AcChT has an unexpected structure highly suggestive of the synaptic vesicle proteoglycan.