Solubilization and characterization of kappa opioid binding sites from guinea pig cerebellum

Solubilization of opioid binding sites from guinea pig cerebellum by digitonin, in the absence and presence of NaCl, resulted in very similar yields (25-30%) of [3H]bremazocine binding. Saturation curves of [3H]bremazocine binding give linear Scatchard plots for both soluble and membrane-bound binding sites yielding similar Kd's and Bmax's. Soluble kappa sites seem to resemble closely their membrane-bound counterparts and retain high affinity and selectivity for various kappa opioid ligands. The apparent molecular weight of soluble kappa sites is ca. 4 X 10(5). Results from this study, along with our previous findings with toad and guinea pig brain, indicate that kappa sites (unlike mu and delta) can be solubilized in good yield by digitonin even in the absence of NaCl. This supports the hypothesis that kappa sites may represent molecular species different from those of mu and delta sites.