Molecular Pharmacology, Vol 13, 251-258, Copyright © 1977 by the American Society for Pharmacology and Experimental Therapeutics

Immobilization of Opiates by Membrane Lipids

¹ Center for Brain Research, University of Rochester School of Medicine and Dentistry, Rochester, New York 14642

The polarization of the intrinsic fluorescence of levorphanol and related morphinans was used to investigate the physical and chemical nature of opiate-lipid complexes in both organic solvents and aqueous buffers. Phosphatidylserine, but neither phosphatidylethanolamine nor stearic acid, produced a large fluorescence polarization of levorphanol in a cyclohexane solvent system, suggesting that the drug became immobilized by phosphatidylserine micelles. Aqueous suspensions of phosphatidylserine and phosphatidylinositol produced a maximal fluorescence polarization of levorphanol, whereas the same concentrations of cerebroside sulfate and phosphatidic acid were much less effective and phosphatidylethanolamine, not at all. Thus phosphatidylserine and phosphatidylinositOl greatly restricted the rotational mobility of the bound opiate while the other lipids did not. A modified Scatchard treatment of the data indicated single binding sites with dissociation constants of 8.8 x 10^-5 and 5.7 x 10^-5 M for the serine and inositol phosphatides, respectively. The accessibilities of chloroform and I^- to the drug bound to phosphatidylserine liposomes were determined from fluorescence quenching experiments. Chloroform was about 10 times more accessible than I^-, suggesting that levorphanol was intercalated in the hydrophobic interior of the lipid bilayer and remote from the aqueous/lipid interface. Stereospecificity was absent, since the (+) enantiomer of levorphanol, dextrorphan, had the same binding affinities as levorphanol for both phosphatidylserine and phosphatidylinositol. A hydrogen bond between phosphatidylserine and the phenolic hydroxyl of opiate derivatives was demonstrated by infrared techniques in an organic solvent. The finding that the fluorescence of dextromethorphan could be polarized by the addition of phosphatidylserine liposomes demonstrated that hydrogen bonding between the phenolic hydroxyl and the lipid was not required for immobilization of the bound drugs. It is suggested that phosphatidylserine or phosphatidylinositol could be a site for the nonspecific binding of opiates and related compounds to nerve membranes.