Reactivity of glycoconjugates in membranes. I. Determination of transbilayer distribution of gangliosides in lipid vesicles by chemical methods.

Two simple chemical methods are described for the determination of the transbilayer distribution of gangliosides GD1a and GM1 in phosphatidylcholine vesicles. The data presented here show an increase in the percentage of GD1a exposed on the outer surface of vesicles with increasing mole fraction of GD1a. The percentage of GD1a exposed on 1:50 and 1:5 GD1a-dipalmitoylphosphatidylcholine vesicles were found to be 67% and 83%, respectively. The same trend is seen for vesicles with dimyristoylphosphatidylcholine and distearoylphosphatidylcholine. Extrapolation of the data to infinite dilution gives 65% of GD1a exposed on the surface of GD1a-dipalmitoylphosphatidylcholine vesicles. The results indicate that composition-dependent changes in transbilayer distribution of GD1a can only partly account for the observed increase in the reactivity of GD1a in vesicles towards neuraminidase from clostridium perfringens as the ratio of GD1a to phosphatidylcholine increases.