Stereospecific morphine adsorption to phosphatidyl serine and other membranous components of brain

In the course of investigating various membranous components for morphine binding, it was found that the major substance responsible was phosphatidyl serine. Other acidic lipids, such as phosphatidic acid and phosphoinositides bind to a considerably lesser extent, while neutral lipids, glycolipids and other phosphatides bind slightly or not at all. Total lipid extracts from a number of regions of rat brain exhibited different degrees of binding to (-)-morphine, such regions as the cerebral cortex and thalamus being the greatest. From an examination of the pH curve for binding and the effect of ionic strength, it was concluded that the binging was largely electrostatic. The method employed was the radioactive measurement of 14C-morphine adsorption to surface films of phosphatidyl serine. When the phosphatide was dispersed in a nonionic detergent near the critical micelle concentration, adsorption was maximal, attaining a value of 1 molecule of morphine adsorbed per molecule of phosphatidyl serine in the surface micelle. The relation between binding affinity and biological potency was not consistent. Morphine adsorption occurred with films prepared from a dispersion of the phosphatide and a hydrophobic protein from synaptic membranes. With the use of levorphanol and dextrorphan it could be shown that binding of morphine was stereospecific.