Regulation of opioid receptor binding; possible mechanisms of sulfhydryl groups in the binding site

Opioid receptor bindings of four different ligands, dihydromorphine (DHM), D-Ala2-D-Leu5-enkephalin (DADLE), ethylketocyclazocine (EKC) and phencyclidine (PCP), were investigated with the treatment of 5,5'-dithiobis-(2-nitrobenzoic acid), DTNB, and 5,5'-dithiobis-(2-nitro-N-2'-hydroxyethylbenzamide), DTNHEB; a relative positive charged analog of DTNB. DTNB and DTNHEB effectively inhibited the binding of DHM and DADLE. Despite the presence of maximally effective concentrations of DTNB for DHM and DADLE, the receptor binding of EKC decreased intermediately, like effect of a partial agonist. DTNHEB inactivated the binding of EKC in a similar fashion to that of DHM. DTNB did not alter the intensity of the decrease of EKC binding by DTNHEB, even given concurrently. It suggests that an anionic center of the receptor has multiple active sulfhydryl sites. The ability of GTP to inhibit DADLE binding to the receptor disappeared by the pre-treatment of DTNB, and DTNB-induced inactivation of opioid agonist binding was potentiated in the presence of NaCl. DTNB-sensitive site may couple a mechanism of ligand binding that GTP regulated. The receptor binding of PCP was not influenced by DTNB and/or DTNHEB.