Energetics of Reserpine Binding and Occlusion by the Chromaffin Granule Biogenic Amine Transporter

The energetics of reserpine binding to the bovine adrenal biogenic amine transporter suggest that H⁺ ion translocation converts the transporter to a form which binds reserpine essentially irreversibly. Reserpine binding to bovine adrenal chromaffin granule membrane vesicles is accelerated by generation of a transmembrane pH difference (ApH) (interior acid) or electrical potential (Δψ) (interior positive). Both components of the electrochemical H⁺ potential (Δ~μ_H+) must be dissipated to block reserpine binding, and generation of either one stimulates the binding rate. Reserpine binding is less dependent than amine transport on the ΔpH, suggesting that translocation of fewer H⁺ ions is required to expose the high-affinity site than are required for net transport. Bound reserpine dissociates very slowly, if at all, from the transporter. Binding is stable to 1% cholate, 1.5% Triton X-100, 1 M SCN⁻, and 8 M urea, but sodium dodecyl sulfate (0.035%) and high temperatures (100 °C) released bound resperine, indicating that binding is noncovalent. The results raise the possibility that the transporter, by translocating one H⁺ ion outward down its concentration gradient, is converted to a form that can either transport a neutral substrate molecule inward or occlude reserpine in a dead-end complex.