Stereoselectivity of the Inhibition of [ 3 H]Hemicholinium‐3 Binding to the Sodium‐Dependent High‐Affinity Choline Transporter by the Enantiomers of a‐ and β‐Methylcholine

In a previous report, we showed that the enantiomers of α‐ and β‐methylcholine inhibited choline uptake with Stereoselectivity, but that their transport by the choline carrier of nerve terminals showed stereospecificity. The present experiments used the same choline analogues to determine if either of the above characteristics pertains to their ability to interact with the [ 3 H]‐hemicholinium‐3 binding site present on striatal membranes and synaptosomes. [ 3 H]Hemicholinium‐3 binding to striatal membranes could be inhibited stereoselectively by the enantiomers of β‐methylcholine, but R (+)‐α‐methyl‐choline was little better than its enantiomer in this test. However, [ 3 H]hemicholinium‐3 binding to striatal synaptosomes was inhibited stereoselectively by the enantiomers of both α‐ and β‐methylcholine. This difference between the properties of [ 3 H]hemicholinium‐3 binding to membranes or to synaptosomes appears related to the presence of two ligand binding states. The [ 3 H]hemicholinium‐3 binding site could be shifted to a low‐affinity state by ATP treatment and to a high‐affinity state by EDTA washing. When the [ 3 H]hemicholinium‐3 binding site existed in its low‐affinity state, binding was inhibited stereoselectively by the enantiomers of both a‐ and β‐methylcholine, but when shifted to its high‐affinity state, it was inhibited stereoselectively only by the enantiomers of β–methylcholine. We conclude that hemicholinium‐3 interacts with the substrate recognition site of the high‐affinity choline transporter, but that the Stereoselectivity of this site changes depending on its affinity state.