Characterization of [125I]RTI-55 binding to dog caudate dopamine transporter

We have characterized binding of [125I]RTI-55 ((-)-2 beta-carbomethoxy-3 beta-(4-iodophenyl)tropane) to the dog caudate membrane and detergent solubilized dopamine transporter sites. The solubilized transporter was assayed by filtration over polyethylenimine treated Whatman GF/B filters. [125I]RTI-55 specifically binds to transporter sites in membranes and solubilized protein with Kd of 0.27 and 0.34 nM, respectively. The binding of [125I]RTI-55 to the membrane and solubilized transporter was inhibited by dopamine, norepinephrine and serotonin uptake inhibitors with rank order and potencies consistent with that of the dopamine transporter. Thus, similar pharmacological properties between membranes and solubilized dopamine transporters were retained upon solubilization. Rapid assay of solubilized protein will aid in monitoring transporter protein purification.