The interactions of the opioid peptide [d-Ala2]methionine-enkephalinamide with di-palmitoylphosphatidylcholine (DPPC) large unilamellar vesicles containing gangliosides GM1, Gdla, and Gtlb and synaptic plasma membranes selectively enriched with dimyristoylphosphatidylcholine (DMPC) and ganglioside Gdla have been investigated by using high-sensitivity differential scanning calorimetry. In the absence of gangliosides, the addition of enkephalinamide in concentrations of up to 10-3 M does not induce any appreciable change in the heat capacity function of DPPC. In the presence of gangliosides, however, changes in the heat capacity function were observed with as little as micromolar concentrations of the enkephalinamide; the same is true for DMPC-Gdla-enriched synaptic membranes. The magnitude and the nature of the enkephalinamide effect depend on the type of ganglioside studied. For DPPC vesicles containing ganglioside GM1 only a slight broadening in the heat capacity function and a small upward shift in the transition temperature were observed. For DPPC vesicles containing ganglioside Gdla the effect was more dramatic; enkephalinamide concentrations as low as 10-5 M caused the appearance of two well-defined peaks in the heat capacity function in contrast to the one peak observed in the absence of enkephalinamide. In the case of DPPC vesicles containing ganglioside Gtlb the enkephalinamide effect was seen at concentrations of 10-4 M or higher. Synaptic plasma membranes were isolated from bovine brain, selectively enriched with exogenous lipid, and their thermotropic behavior was characterized by steady-state fluorescence spectroscopy and differential scanning calorimetry. This lipid enrichment results in the appearance of a membrane phase transition otherwise absent in the intact membrane preparation. Synaptic plasma membranes enriched with DMPC and 10 mol % ganglioside Gdla also exhibited a similar response to the addition of enkephalinamide, suggesting that enkephalins might be able to induce phase separation processes in both synthetic and natural membranes. These results provide evidence for the existence of high-affinity interactions between specific gangliosides and enkephalins and suggest that gangliosides may play an indirect role in the biological functioning of opiate receptors.
ASJC Scopus subject areas