Biophys J, July 1999, p. 306-318, Vol. 77, No. 1

Divalent Cation-Mediated Interaction Between Cerebroside Sulfate and Cerebrosides: An Investigation of the Effect of Structural Variations of Lipids by Electrospray Ionization Mass Spectrometry

 ^*The Research Institute, The Hospital for Sick Children, Toronto M5G 1X8;  ^#Molecular Medicine Research Center, University of Toronto, Toronto M5S 1A8; and  ^§Department of Laboratory Medicine and Structural Biology, University of Toronto, Toronto, Ontario M5G 1L5, Canada

Divalent cations mediate a carbohydrate-carbohydrate association between the two major glycolipids, galactosylceramide (GalCer) and its sulfated form, cerebroside sulfate (CBS), of the myelin sheath. We have suggested that interaction between these glycolipids on apposed extracellular surfaces of myelin may be involved in the stability or function of this multilayered structure. A mutant mouse lacking galactolipids because of a disruption in the gene that encodes a galactosyltransferase forms myelin that initially appears relatively normal but is unstable. This myelin contains glucosylceramide (GlcCer) instead of GalCer. To better understand the role of GlcCer in myelin in this mutant, we have compared the ability of divalent cations to complex CBS (galactosyl form) with GlcCer or GalCer in methanol solution by using positive ion electrospray ionization mass spectrometry. Because both the -hydroxylated fatty acid species (HFA) and the nonhydroxylated fatty acid species (NFA) of these lipids occur in myelin, we have also compared the HFA and NFA species. In addition to monomeric Ca²⁺ complexes of all three lipids and oligomeric Ca²⁺ complexes of both GalCer and GlcCer, Ca²⁺ also caused heterotypic complexation of CBS to both GalCer and GlcCer. The heterotypic complexes had the greatest stability of all oligomers formed and survived better at high declustering potentials. Complexes of CBS with GlcCer were less stable than those with GalCer. This was confirmed by using the free sugars and glycosides making up the carbohydrate headgroups of these lipids. HFA species of CBS and GalCer formed more stable complexes than NFA species, but hydroxylation of the fatty acid of GlcCer had no effect. The ability of GlcCer to also complex with CBS, albeit with lower stability, may allow GlcCer to partially compensate for the absence of GalCer in the mouse mutant.

Biophys J, July 1999, p. 306-318, Vol. 77, No. 1
© 1999 by the Biophysical Society   0006-3495/99/07/306/13  $2.00

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