Fig. 2.

Stereoselective and nonstereoselective binding of a labeled opioid active (−)-ligand (filled reverse L) and the displacement of its binding from various types of sites in tissue by excess unlabeled opioid inactive (+)-ligand (open L) or excess unlabeled (−)-ligand (open reverse L). A, the binding of the labeled (−)-ligand that will participate in all the possible kinds of binding interactions. B, binding of excess unlabeled (+)-ligand and labeled (−)-ligand will result in blockade of labeled (−)-ligand entering the nonstereoselective but saturable binding sites. Thus, the difference of A minus B represents nonstereoselective but saturable binding. C, binding of excess unlabeled (−)-ligand will result in blockade of labeled (−)-ligand entering the saturable by nonstereoselective and stereoselective binding sites. Thus, the difference of B minus C represents saturable stereoselective binding. [Adapted from Goldstein A, Lowney LI, and Pal BK (1971) Stereospecific and nonspecific interactions of the morphine congener levorphanol in subcellular fractions of mouse brain. Proc Natl Acad Sci USA 68:1742–1747. Copyright © 1971 United States National Academy of Sciences. Used with permission.]