Free and protein-bound gangliosides in Triton X-100 extracts from rat brain

Interaction of soluble and membrane proteins with monolayers of glycosphingolipids

1. The interactions of four proteins (albumin, myelin basic protein, melittin and glycophorin) with eight neutral or acidic glycosphingolipids, including sulphatides and gangliosides, five zwitterionic or anionic phospholipids and some of their mixtures, were studied in lipid monolayers at the air/145 mM-NaCl interface. 2. In lipid-free interfaces, the surface pressure and surface potential reached by either soluble or integral membrane proteins did not reveal marked differences. 3. All the proteins studied showed interactions with each of the lipids but the maximal interactions were found for basic proteins with acidic glycosphingolipids. 4. Surface-potential measurements indicated that different dipolar organizations at the interface can be adopted by lipid-protein interactions showing the same value for surface free energy. 5. The individual surface properties of either the lipid of protein component are modified as a consequence of the lipid-protein interaction. 6. In mixed-lipid monolayers, the composition of the interface may affect the lipid-protein interactions in a non-proportional manner with respect to the relative amount of the individual lipid components.

Neurotransmitter movements in nerve endings. Influence of substances that modify the interfacial potential

Polysialogangliosides, sulphatides, glycerylmonooleate, unsaturated fatty acids, myelin basic protein and sucrose inhibit the Na+-coupled uptake and induce a Ca2+-dependent release of dopamine from nerve endings. Substances chemically related to those referred to above, such as monosialogangliosides, neutral glycosphingolipids, glycerylmonostearate, saturated fatty acids and albumin, do not show these effects. Mixtures of polysialogangliosides or sulphatides with myelin basic protein or albumin inhibit, to different degrees, the effects of the individual components. The decreased uptake induced by sucrose reverted to control levels upon reduction of the concentration of the perturbing agent. The restoration of the uptake was probably mediated by the Na+-pump reconstituting the transmembrane Na+-gradient necessary for the Na+-coupled cotransport of dopamine. It is suggested that the effects of uptake inhibitor or release inducer agents derive from their ability to decrease the surface potential and modify the molecular organization of phospholipid interfaces which can result in changes of the membrane ionic permeability.