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Volume 271,
Number 7,
Issue of February 16, 1996 pp. 3496-3499
©1996 by The American Society for Biochemistry and Molecular Biology, Inc.
Investigation
of the Calcium-mediated Association between the Carbohydrate Head
Groups of Galactosylceramide and Galactosylceramide I Sulfate by Electrospray Ionization Mass Spectrometry (*)
(Received for publication, August 28,
1995; and in revised form, November 20, 1995)
Kalavelil M.
Koshy
(1),
Joan M.
Boggs
(1) (2)(§)From the
(1)From The Research Institute, Hospital for
Sick Children, Toronto, Ontario M5G 1X8, Canada and the
(2)Department of Clinical Biochemistry, University
of Toronto, Toronto, Ontario M5GIL5, Canada
ABSTRACT
INTRODUCTION
EXPERIMENTAL PROCEDURES
RESULTS AND DISCUSSION
FOOTNOTES
ACKNOWLEDGEMENTS
REFERENCES
ABSTRACT
Calcium has been shown previously to cause aggregation of
phosphatidylcholine/cholesterol liposomes containing galactosylceramide
(GalCer) with similar liposomes containing cerebroside sulfate
(galactosylceramide I sulfate) (CBS), suggesting that it
mediates a carbohydrate-carbohydrate association between these two
glycolipids. In order to determine if such an association occurs, the
noncovalent complexes formed on addition of calcium chloride to GalCer
and CBS in methanol were examined by positive and negative ion spray
mass spectrometry. Monomeric Ca complexes of both
lipids were observed. In addition, Ca also caused
oligomerization of GalCer. Oligomerization of CBS anion was not seen,
but dimers would not have been observed, as they would be neutral.
However, Ca caused heterotypic complexation of GalCer
and CBS. Although these heterotypic complexes were of low abundance in
methanol compared with the other monomeric and homotypic oligomeric
positive ions formed at low declustering potentials, the heterotypic
dimer
[GalCer CBS Ca -H] had the greatest stability of all oligomers formed and was the
only one to survive at high declustering potentials. Na did not cause oligomerization of GalCer in methanol indicating
that the complexes of GalCer with Ca are not caused
by van der Waals interactions between the lipid moieties. GalCer and
CBS are present in high concentrations in myelin. This
Ca -mediated carbohydrate-carbohydrate interaction,
which can bridge apposing bilayers, may be involved in adhesion of the
extracellular surfaces of the myelin sheath.
INTRODUCTION
Calcium-mediated interactions between cell surface carbohydrates
have recently been implicated as a basis of cell recognition and
adhesion and have therefore been the subject of increasing interest (1, 2, 3, 4, 5, 6, 7, 8) .
Carbohydrate-carbohydrate interactions between free sugars and
polysaccharides have been known for some time(9, 10, 11, 12, 13, 14) and
have more recently been investigated among glycolipids in lipid
bilayers(1, 2, 3, 6, 15, 16, 17, 18, 19) .
The list of glycolipids that have been shown to participate in divalent
cation-mediated carbohydrate-carbohydrate interactions includes the two
major myelin glycolipids glactosylceramide and cerebroside sulfate
(galactosylceramide I sulfate)(6, 18) . In
central nervous system myelin these two glycolipids comprise 27% (by
weight) of the total myelin lipid(20) . Thus an interaction
between these two glycolipids across apposing membrane surfaces might
play a role in the formation of the compacted myelin membrane. x-ray
crystallography of a number of divalent cation complexes of simple
carbohydrates has provided detailed information concerning the
structure of these complexes (cf. (9) and (10) for
reviews). NMR and infrared spectroscopy have complemented this
information(11, 12) . However, the complex-forming
properties of calcium with carbohydrates attached to lipid moieties
remain largely unexplored. Most of the evidence for complex formation
so far comes from liposome aggregation or lipid binding studies using
solid phase presentation of the lipids, either bound to a solid support
or in liposomes. In addition, Fourier transform-infrared spectroscopy
was used to provide information about the structure of the complex of
Ca with digalactosyldiacylglycerol and the groups on
the carbohydrate that chelate with the divalent cation in a membranous
environment in the presence of water(19) . The polyvalent
nature of presentation in lipid bilayers or on a solid support
increases the affinity of the interaction. The difficulty of observing
the divalent cation-carbohydrate interactions in solution (especially
in aqueous solution) given their weak nature and the solubility
characteristics of lipid-bound carbohydrates makes the study of such
interactions rather challenging. Electrospray ionization mass
spectrometry (ESI-MS) ( )is a relatively new technique, which
can detect the presence of a complex in a solvent in which it is
soluble(21, 22, 23, 24, 25) .
The soft ionization conditions employed allow the transfer of complexes
present in solution to the gas phase with minimal
decomposition(26) . ESI-MS has recently been used to detect the
divalent cation-mediated complexation of the carbohydrate of some
glycolipids, resulting in the homotypic and in some cases heterotypic
oligomerization of these lipids in methanol(21) . This
technique was successful at detecting Ca oligomerization of the Le oligosaccharide(21) , while H-NMR failed to
detect metal binding or evidence of oligomerization of the free sugar
in water(27) . In this paper we present evidence for the
noncovalent association among galactosylceramide, Ca ,
and the anion of cerebroside sulfate in methanol solution using
electrospray ionization mass spectrometry. Positive ion scans showed
that Ca caused homotypic oligomerization of GalCer
and that it bound to monomers of CBS anion to form a singly charged
positive ion. Dimers of CBS anions with Ca form a
neutral species and are not detected. However, Ca caused heterodimerization of GalCer and CBS anion in methanol.
Although the heterotypic complexes were of low abundance compared with
the others at low declustering potentials, the heterotypic dimer had
greater stability than any other complex.
EXPERIMENTAL PROCEDURES
All reagents and solvents used were either analytical grade
or high pressure liquid chromatography grade. Calcium chloride
(CaCl 2H O) was purchased from Fisher
(Fairlawn, NJ). Methanol was from Caledon (Georgetown, Ontario,
Canada), stearic acid from Fluka (Switzerland),
1- -D-galactosyl sphingosine (psychosine) from Sigma, and
oxalyl chloride from Aldrich.
Synthesis of LipidsGalactosylceramide was synthesized
from psychosine by reaction with stearoyl chloride (prepared from
stearic acid and oxalyl chloride) in tetrahydrofuran in the presence of
aqueous sodium acetate(28) . The product was purified by column
chromatography on silica gel and finally precipitated from a small
volume of chloroform/methanol (90/10) by the addition of excess
acetone. The identity of the final product was established as
1-O-galactosyl-2-N-stearoyl sphingosine by thin layer
chromatography, differential scanning calorimetry, H NMR
spectroscopy, and mass spectrometry.Galactosylceramide I sulfate was synthesized from lysosulfatide (psychosine sulfate)
prepared from bovine brain sulfatide and stearoyl chloride as described
previously(29) . The structure of the product was verified as
above. H NMR showed that it was in the
NH salt form (data not shown). However,
the NH adduct was not detected in the
presence of excess Ca , indicating that the
Ca completely displaced it from the lipid. When it
was converted to the Ca salt form and examined by
ESI-MS in the absence of excess Ca , it formed the
Na salt form due to exogenous Na present in the system. Therefore, it was necessary to study it in
the presence of excess Ca .
Mass Spectral AnalysisStock solutions of the two
lipids were prepared at concentrations of 550 nmol/ml (GalCer) and 385
nmol/ml (CBS) in methanol. A stock solution of
CaCl 2H O in methanol was prepared at a
concentration of 6.8 µmol/ml. Aliquots of these solutions were
appropriately mixed and made up to the requisite volume to give the
final concentrations required for mass spectral analysis. Insolubility
of the lipids in water or methanol/water solutions precluded their
study in these solvents.Electrospray mass spectra were acquired on
a Perkin-Elmer Sciex API III triple quadrupole mass spectrometer. Both
positive and negative ion scans were performed. Concentrations of the
lipids were usually kept at 20-50 nmol/ml and the Ca concentration at 200 nmol/ml for optimum spectral quality. For
CID experiments, higher concentrations of the lipids were used for
greater signal strength. The solutions were introduced at the flow rate
of 5 µl/min.
RESULTS AND DISCUSSION
Binding of Galactosylceramide to
Ca The complex-forming property of galactose
with Ca in crystalline form has been well
documented(30) . In the crystal structure, Ca is coordinated to five hydroxyl groups, contributed by three
-D-galactose molecules, and to three water molecules.
Since each galactose can provide only one or two hydroxyl groups for
Ca binding, single isolated galactose moieties have a
low affinity for Ca in aqueous solution. However, the
Ca -galactose complex may be more stable in methanol
due to the lower tendency of Ca to associate with
methanol than water(31) . In GalCer, the -conformation of
the sugar and the presence of the ceramide aglycone may also modify the
binding to Ca . The positive ion ESI mass spectrum of
GalCer (monoisotopic mass 727.6) in the presence of excess
Ca (Fig. 1) indicates that in addition to the
monomeric complex ion of GalCer with Ca ([GalCer Ca ] ),
which is found in high abundance, the lipid forms several oligomers as
well, of the general formula
[nGalCer Ca ] .
This behavior is similar to that of
Le -lactosylceramide(21) . A small amount of the
Na complex is also present at m/z =
750.6.
Figure 1:
Positive ion spray mass spectrum of
GalCer (50 nmol/ml) with Ca (500 nmol/ml) in
methanol. The declustering potential was 80
V.
It is reasonable to assume that the oligomers of GalCer are
formed by interaction of the carbohydrate head groups through
coordination with Ca . As will be discussed later, the
relatively higher stabilities of the oligomers, especially the dimer
compared with the monomer, lend some support to this suggestion, since
the coordination of hydroxyl groups from more than one carbohydrate
moiety with the cation has been shown to be the preferred arrangement
in the crystal structures of the Ca complexes of many
carbohydrates including galactose. It should also be noted that in the
presence of Na , the monomeric ion
[GalCer Na ] was the
primary ion present in the spectrum with only trace amounts of a
dimeric species,
[2GalCer Na ] (not
shown). There was no evidence of any higher oligomers containing
Na . As will be seen later, Na appears
to have a high affinity for GalCer to form the monomeric ion, but even
in the absence of Ca , its ability to promote
oligomerization is negligible. This supports the conclusion that the
oligomers of GalCer formed in the presence of Ca for
the most part are not due to Van der Waals interaction between the
ceramide moieties. The negative ion spectrum of GalCer in the presence
of excess calcium chloride on the other hand showed predominantly a
peak due to a monomeric ion at m/z = 762.4 (not shown),
which was identified as
[GalCer+Cl ] . It
persisted at declustering potentials of -80 to -180 V.
Binding of Galactosylceramide I Sulfate to
Ca In contrast to GalCer, CBS (monoisotopic
mass 807.6) in the presence of excess Ca , under the
same conditions as above, gives a single adduct with Ca corresponding to the formula
[CBS Ca -H] (m/z = 846.6) as evident from the positive ion
spectrum shown in Fig. 2. It is of lower intensity than that of
the complex ions of GalCer. Neutral dimeric species of CBS anion of the
type [2CBS Ca -2H], although
possible, would not be detected. The only ion detected in a negative
ion scan was [CBS-H] in high
abundance at m/z 806.4 (not shown). Negatively charged trimers
of the type
[3CBS Ca - H] have too high an m/z ratio to be detected by the mass
spectrometer used.
Figure 2:
Positive
ion spray mass spectrum of CBS (46 nmol/ml) with Ca (500 nmol/ml) in methanol. The declustering potential was 80
V.
Complex Formation between GalCer and CBS in the Presence
of Ca In order to detect
Ca -mediated complex formation between GalCer and CBS,
solutions containing the two lipids at a concentration of 20 nmol/ml
each and Ca at a concentration of 200 nmol/ml in
methanol were used. The positive ion ESI mass spectrum of the mixture
was acquired under similar conditions as for the individual lipids. Fig. 3shows a representative spectrum at a declustering
potential of 80 V. In addition to the Ca adducts of
the individual lipids as described above, there are hetero-oligomers of
the two lipids detectable in the spectrum at m/z =
1170.6 ([2GalCer CBS 2Ca -2H] ), 1211.0
([GalCer 2CBS 2Ca -2H] ),
and 1574.2
([GalCer CBS Ca -H] ).
The combined intensities of the three peaks containing
CBS GalCer Ca positive ions were
approximately 30% of the intensity of the
[CBS Ca -H] peak. The relative intensities of the peaks due to the
hetero-oligomers were constant at concentrations of each lipid of
5-50 nmol/ml. A hetero-oligomer containing GalCer plus two CBS
anions and only one Ca would not be detectable since
it would be neutral. The negative ion scan of the mixture of the two
lipids in the presence of calcium chloride contained only two peaks due
to [GalCer Cl ] and
[CBS-H] (not shown). It did not show
any peaks resulting from the association of the two lipids. This result
is not surprising in view of the fact that the negative ion spectra of
the individual lipids showed no Ca -bound ions.
Figure 3:
Positive
ion spray mass spectrum of GalCer (20 nmol/ml) plus CBS (20 nmol/ml)
plus Ca (200 nmol/ml). The declustering potential was
80 V.
The Effect of Declustering Potential on the Stability of
the ComplexesThe declustering potential was varied from
+50 to +180 V in order to alter the collisional energy and
thus the amount of complexes and oligomers. For these experiments the
concentration of GalCer was kept at 20 nmol/ml, while that of CBS was
increased to 50 nmol/ml. This gave a relatively more intense peak at m/z = 846.6, corresponding to
[CBS Ca -H] , but
had little effect on the intensity of any of the other peaks. With an
increase in declustering potential, the various Ca adducts of the two lipids disappear. Surprisingly, at 180 V (Fig. 4) the only Ca adducts still remaining
are [CBS Ca -H] (m/z = 846.6), the homodimer of GalCer (m/z = 747.8), and the heterodimer
[GalCer CBS Ca -H] (m/z = 1574.6). The former two ions are
considerably reduced in intensity. As the declustering potential is
increased from 50 to 180 V, the first Ca complex of
GalCer to disappear from the spectrum is the monomer
[GalCer Ca ] , and the
last to survive in vestigial amounts is the dimer
[2GalCer Ca ] . This
may allow a qualitative comparison of their stabilities to be made, the
monomer being the least stable, the dimer the most stable, and other
oligomers somewhere in between.
Figure 4:
Positive ion spray mass spectrum of GalCer
(20 nmol/ml) plus CBS (50 nmol/ml) plus Ca (200
nmol/ml) at a declustering potential of 180
V.
Although almost all of the
Ca adducts of GalCer dissociated at a declustering
potential of 180 V (Fig. 4), there is a high intensity peak at m/z = 750.6 due to the retention of the Na adduct. The new peaks at 408.2 and 348.2 are fragment ions, as
confirmed by parent ion scans, indicating that under the conditions of
the experiment significant fragmentation of the lipids occurs. The
ratio of intensity of the heterodimer to that of the CBS monomer
increases with increase in declustering potential as shown in Table 1. While it is not clear whether there is actually an
increase in the abundance of the heterodimer ion with increase in
declustering potential, since the intensity of the monomer shows a
gradual decrease, the conclusion that the heterodimer
[GalCer CBS Ca -H] is the most stable of all the Ca adducts of
either lipid, despite its low abundance, seems inescapable.
The low
abundance of this complex in methanol contrasts with liposome
aggregation studies, which indicated that the
Ca -mediated heterotypic aggregation of GalCer with
CBS anion was greater than the Ca -mediated homotypic
interaction of either. The polyvalent nature of liposomes compared with
the monovalent lipids in solution may contribute to the greater
heterotypic interaction of liposomes. This difference in behavior may
also be due to the relative stability of the Ca adducts of GalCer in methanol solution compared with water due to
the lower ability of Ca to chelate methanol compared
with water(31) . The greater solubility of galactose and other
sugars in methanol containing dissolved calcium chloride than in pure
methanol, in which they are virtually insoluble, is well documented (32) and also indicates the formation of
Ca -galactose complexes in agreement with the ESI-MS
results.
CID Analysis of the
[GalCer CBS Ca -H] ionCollision-induced decomposition of the heterodimer of
GalCer and CBS was accomplished by selecting the ion at m/z = 1574.2 with the mass analyzer and allowing it to collide
with argon. The resulting fragments were analyzed in the positive ion
mode. Fig. 5shows the spectrum obtained. The noncovalent
dissociation of the complex should result in the formation of both
components of the complex, viz. the cationized species of
GalCer and CBS. The peak at m/z = 846.6 corresponding
to the ion [CBS Ca -H] is formed by the loss of GalCer (mass = 727.6) from the
parent ion. No intact GalCer ions are found in the spectrum, but the
ion at m/z 408.4 is its decomposition product. The collisional
energy thus appears to be entirely utilized for covalent decomposition
in preference to noncovalent decomposition of the complex. The
occurrence of predominantly covalent dissociation of complexes under
CID conditions has been interpreted as the result of the tightness of
the binding of the components(21) . Thus in addition to
confirming that the complex with m/z = 1574.2 is a
heterodimer of GalCer and CBS with Ca , the CID
experiment also corroborates the finding that this complex is
relatively stable, since it undergoes mainly covalent decomposition.
However, it is interesting to note that it is the GalCer component of
the complex that appears to undergo decomposition while the CBS is
unaffected. Thus based on the results of both the CID experiment and
the relative tendency of different complexes to dissociate in response
to increasing declustering potential, it can be concluded that the
heterodimeric complex
[GalCer CBS-H Ca ] is more stable than the calcium complex of either lipid alone.
Figure 5:
CID mass
spectrum of the heterodimeric complex
[GalCer CBS Ca -H] (m/z = 1574).
ConclusionsIn this study we have demonstrated
binding of Ca to each of GalCer and the anion of CBS
individually. We have also demonstrated the existence of a specific
interaction between GalCer and CBS anion mediated by Ca as suggested by the divalent cation-mediated aggregation of
phosphatidylcholine/cholesterol liposomes containing GalCer with
similar liposomes containing CBS(6, 18) .
Oligomerization of GalCer, but not of CBS anion, by Ca also occurred in methanol. However, the low degree of aggregation
of GalCer-containing liposomes by Ca compared with
the greater degree of aggregation of CBS liposomes with GalCer
liposomes suggests that in water this homotypic interaction must be
much weaker than the heterotypic interaction and/or that the
heterotypic interaction is stronger in water than in methanol. The
observation of the heterotypic complex ion
[GalCer CBS Ca -H] in the ESI mass spectrum even under conditions harsh enough to
dissociate all other intermolecular clusters points to the specificity
of this interaction and stability of the complex.
FOOTNOTES
- *
- The costs of publication of this article
were defrayed in part by the payment of page charges. This article must
therefore by hereby marked ``advertisement'' in
accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
This study was supported by a grant from the Medical Research Council
of Canada (to J. M. B.).
- §
- To whom
correspondence should be addressed: The Research Institute, Hospital
for Sick Children, 555 University Ave., Toronto, Ontario, Canada, M5G
1X8. Tel.: 416-813-5919; Fax: 416-813-5022.
- (
) - The
abbreviations used are: ESI-MS, electrospray ionization mass
spectrometry; GalCer, galactosylceramide; CBS, acid form of cerebroside
sulfate (galactosylceramide I
sulfate); CID,
collision-induced decomposition; Le , Lewis X determinant
(Gal 1 4-(Fuc 1 3)-GlcNAc).
ACKNOWLEDGEMENTS
We thank Dr. Henrianna Pang and Mary Cheung of the
Carbohydrate Research Centre, University of Toronto for carrying out
the ESI-MS measurements and for helpful advice.
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