Hydrophobicity of the tetrabenazine-binding site of the chromaffin granule monoamine transporter.

The catecholamine uptake inhibitor tetrabenazine (TBZ) binds to a high affinity site on the chromaffin granule membrane, presumably on the monoamine transporter. The hydrophobicity of the TBZ-binding site was investigated by comparing the potency of drugs to displace [3H]dihydrotetrabenazine (TBZOH), a ligand of the TBZ-binding site, with the lipophilicity of these drugs reflected by their octanol/buffer apparent partition coefficient (P o/b). Drugs tested were five substrates of the transporter, seven TBZ derivatives, and the inhibitors reserpine, haloperidol, and chlorpromazine. The validity of apparent P o/b as an index of lipophilicity was shown by measuring drug partitioning between buffer and chromaffin granule membranes. For most of the inhibitors tested, octanol/buffer and membrane/buffer apparent partition coefficients were correlated. For substrates of uptake and TBZ derivatives, the potency of a compound to displace [3H]TBZOH from its binding site was correlated to its apparent P o/b. This relationship was valid over a range of 5 orders of magnitude. These data are interpreted as indicating that the TBZ-binding site is hydrophobic and is in equilibrium with the ligand present in the membrane phase, and that substrates and TBZ derivatives are characterized by an equal intrinsic affinity for this site of about 1 microM. The 3-fold difference in affinity observed between alpha- and beta-diastereoisomers of TBZOH was accounted for by a similar difference in apparent P o/b. Reserpine, haloperidol, and chlorpromazine have much lower intrinsic affinity for the TBZ-binding site.