RESUMO
[3H]Dihydrotetrabenazine bound to a single class of binding sites in bovine striatal synaptic vesicles with an apparent dissociation constant of 3-9 nM. This is comparable to the inhibitory potency of dihydrotetrabenazine in catecholamine transport assays. In contrast to these results, [3H]dihydrotetrabenazine bound to at least two classes of sites in all other subsynaptic fractions investigated. The higher affinity class of sites was comparable in affinity to that of synaptic vesicles, whereas the lower affinity sites exhibited an apparent dissociation constant of 95-400 nM. Higher affinity sites were most abundant in the synaptic vesicle fraction, and little higher affinity binding was observed in mitochondrial and myelin fractions, or in highly purified synaptic plasma membranes. Lower affinity binding was not enriched in any subsynaptic fraction and was the only class of binding sites detected in homogenates of liver and diaphragm. The distribution of the presynaptic vesicle marker synaptophysin corresponded with that of higher affinity but not lower affinity binding. These results are consistent with the expectation that the higher affinity sites are associated primarily with synaptic vesicles and other neuronal entities that are in communication with these organelles.