STEREOSELECTIVITY OF THE INHIBITION OF [H-3] HEMICHOLINIUM-3 BINDING TO THE SODIUM-DEPENDENT HIGH-AFFINITY CHOLINE TRANSPORTER BY THE ENANTIOMERS OF ALPHA-METHYLCHOLINE AND BETA-METHYLCHOLINE

In a previous report, we showed that the enantiomers of alpha- and bet a-methylcholine inhibited choline uptake with stereoselectivity, but t hat their transport by the choline carrier of nerve terminals showed s tereospecificity. The present experiments used the same choline analog ues to determine if either of the above characteristics pertains to th eir ability to interact with the [H-3]-hemicholinium-3 binding site pr esent on striatal membranes and synaptosomes. [H-3]Hemicholinium-3 bin ding to striatal membranes could be inhibited stereoselectively by the enantiomers of beta-methylcholine, but R(+)-alpha-methylcholine was l ittle better than its enantiomer in this test. However, [H-3]hemicholi nium-3 binding to striatal synaptosomes was inhibited stereoselectivel y by the enantiomers of both alpha- and beta-methylcholine. This diffe rence between the properties of [H-3]hemicholinium-3 binding to membra nes or to synaptosomes appears related to the presence of two ligand b inding states. The [H-3]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 [H-3]hemicholinium-3 binding site existed in its low-affinity state, binding was inhibited stereoselectively by th e enantiomers of both alpha- and beta-methylcholine, but when shifted to its high-affinity state, it was inhibited stereoselectively only by the enantiomers of beta-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 chang es depending on its affinity state.