INVESTIGATION OF THE CALCIUM-MEDIATED ASSOCIATION BETWEEN THE CARBOHYDRATE HEAD GROUPS OF GALACTOSYLCERAMIDE AND GALACTOSYLCERAMIDE I-3 SULFATE BY ELECTROSPRAY-IONIZATION MASS-SPECTROMETRY

Calcium has been shown previously to cause aggregation of phosphatidyl choline/cholesterol liposomes containing galactosylceramide (GalCer) w ith similar liposomes containing cerebroside sulfate (galactosylcerami de I-3 sulfate) (CBS), suggesting that it mediates a carbohydrate-carb ohydrate association between these two glycolipids. In order to determ ine if such an association occurs, the noncovalent complexes formed on addition of calcium chloride to GalCer and CBS in methanol were exami ned by positive and negative ion spray mass spectrometry, Monomeric Ca 2+ complexes of both lipids were observed. In addition, Ca2+ also caus ed oligomerization of GalCer. Oligomerization of CBS anion was not see n, but dimers would not have been observed, as they would be neutral. However, Ca2+ caused heterotypic complexation of GalCer and CBS, Altho ugh these heterotypic complexes were of low abundance in methanol comp ared with the other monomeric and homotypic oligomeric positive ions f ormed at low declustering potentials, the heterotypic dimer [GalCer . CBS . Ca2+-H](+) had the greatest stability of all oligomers formed an d was the only one to survive at high declustering potentials. Na+ did not cause oligomerization of GalCer in methanol indicating that the c omplexes of GalCer with Ca2+ are not caused by van der Waals interacti ons between the lipid moieties. GalCer and CBS are present in high con centrations in myelin, This Ca2+-mediated carbohydrate-carbohydrate in teraction, which can bridge apposing bilayers, may be involved in adhe sion of the extracellular surfaces of the myelin sheath.