Characterization of the cocaine binding site on the sigma-1 receptor

The cocaine photoaffinity label 3-iodo-4-azidococaine ([I-125]IACoc) binds to the sigma-1 receptor with an affinity that is 2-3 orders of magnitude higher than the parent compound cocaine [Kahoun, J. R., and Ruoho, A. E. (1992) Proc. Natl. Acad. Sci. U.S.A. 89, 1393-1397]. In the present study, the binding properties of several cocaine derivatives to the guinea pig liver sigma-1 receptor were determined. The results from assessing the affinity of various derivatives of cocaine which were substituted on the phenyl ring indicated that an important determinant of binding to the guinea pig sigma-1 receptor binding site may be the development of a dipole in the ring in which the pi electron density of the phenyl ring is reduced. This implies that an electron-rich source is present in the sigma-1 receptor binding site, such as the pi system of an aromatic ring or other electron-rich side chains, which interact with the phenyl ring of cocaine. The precise [I-125]IACoc derivatization site in the guinea pig sigma-1 receptor was identified using chemical cleavage and purification of the resulting labeled peptides. Cyanogen bromide cleavage of the [I-125]IACoc photolabeled sigma-1 receptor followed by radiosequencing identified Asp188, which is located in the putative steroid binding domain-like II (SBDL II) near the carboxyl terminus, as the site of [I-125]IACoc insertion. Systematic truncation of the C-terminus indicated the requirement for the last 15 amino acid residues of the receptor for [I-125]IACoc photolabeling.