Equilibrium binding and transport by vesicular acetylcholine transporter.

The method describes production and the selection of neurosecretory PC12A123.7 cells stably transfected with human vesicular acetylcholine transporter (hVAChT). Transfected cells provide postnuclear supernatant used to characterize equilibrium binding of the neurotransmitter acetylcholine (ACh), the pH dependence for transport of ACh, and the rate behavior for dissociation of the allosteric, high-affinity inhibitor vesamicol. Retention of radiolabeled ACh or vesamicol, mediated by hVAChT in synaptic-like microvesicles of postnuclear supernatant, is measured using filter assays. The procedure for regression analysis of data also is described.

Multiple protonation states of vesicular acetylcholine transporter detected by binding of [3H]vesamicol.

Vesicular acetylcholine transporter (VAChT) is inhibited by (-)-vesamicol [(-)-trans-2-(4-phenylpiperidino)cyclohexanol], which binds tightly to an allosteric site. The tertiary alkylamine center in (-)-vesamicol is protonated and positively charged at acidic and neutral pH and unprotonated and uncharged at alkaline pH. Deprotonation of the amine has been taken to explain loss of (-)-vesamicol binding at alkaline pH. However, binding data deviate from a stereotypical bell shape, and more binding occurs than expected at alkaline pH. The current study characterizes the binding of (-)-vesamicol from pH 5 to pH 10 using filter assays, (-)-[3H]vesamicol (hereafter called [3H]vesamicol), and human VAChT expressed in PC12(A123.7) cells. At acidic pH, protons and [3H]vesamicol compete for binding to VAChT. Preexposure or long-term exposure of VAChT to high pH does not affect binding, thus eliminating potential denaturation of VAChT and failure of the filter assay. The dissociation constant for the complex between protonated [3H]vesamicol and VAChT decreases from 12 nM at neutral pH to 2.1 nM at pH 10. The simplest model of VAChT that explains the behavior requires a proton at site 1 to dissociate with pK1 = 6.5 +/- 0.1, a proton at site A to dissociate with pKA = 7.6 +/- 0.2, and a proton at site B to dissociate with pKB = 10.0 +/- 0.1. Deprotonation of the site 1 proton is obligatory for [3H]vesamicol binding. Deprotonation of site A decreases affinity (2.2 +/- 0.5)-fold, and deprotonation of site B increases affinity (18 +/- 4)-fold. Time-dependent dissociation of bound [3H]vesamicol is biphasic, but equilibrium saturation curves are not. The contrasting phasicity suggests that the pathway to and from the [3H]vesamicol binding site exists in open and at least partially closed states. The potential significance of the findings to development of PET and SPECT ligands based on (-)-vesamicol for human diagnostics also is discussed.