Calorimetric and fluorescence characterization of interactions between enkephalins and liposomal and synaptic plasma membranes containing gangliosides.

Abstract

The interactions of the opioid peptide [D-Ala2]methionine-enkephalinamide with dipalmitoylphosphatidylcholine (DPPC) large unilamellar vesicles containing gangliosides GM1, Gd1a, and Gt1b and synaptic plasma membranes selectively enriched with dimyristoylphosphatidylcholine (DMPC) and ganglioside Gd1a 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-Gd1a-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 Gd1a 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 Gt1b 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.(ABSTRACT TRUNCATED AT 250 WORDS)