Is Cocaine Protonated When it Binds to the Dopamine Transporter?

Abstract

There has been much controversy about whether the well-known alkaloid and tertiary amine base cocaine (pKa = 8.5) binds to the human dopamine transporter (DAT) in its protonated form. Most potent DAT inhibitors are also strong amines─yet there are some noteworthy examples where neutral cocaine analogues have high affinity, while the quaternary ammonium analog of cocaine, cocaine methiodide, is a comparatively poor inhibitor. In this paper, we show that a fluorescent cocaine analog, with a lower pKa than cocaine, becomes protonated in the DAT binding site and conclude that similar behavior must be expected from cocaine. By determining the pKa of the aspartate residue in DAT believed to interact with the amine of cocaine, we are able to explain the apparently contradictory structure-activity data of cocaine analogues.