Structural determination by negative-ion MALDI-QIT-TOFMSn after pyrene derivatization of variously fucosylated oligosaccharides with branched decaose cores from human milk

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We prepared neutral oligosaccharide fraction from milk of a woman (blood type A, Leb+) by anion-exchange column chromatography after the removal of lipids and proteins. Further fractionation was performed by means of Aleuria aurantia lectin-Sepharose column chromatography and reverse-phase HPLC after labeling with a pyrene derivative. This pyrene labeling allowed identification by negative-MALDI-TOFMSn analysis of 22 oligosaccharides with decaose cores, among which 21 had novel structures. Negative ions could not be produced from neutral oligosaccharides without labeling on MALDI. Mono-, di-, tri-, and tetrafucosylated decaose fractions contained three, nine, six, and four isomers, respectively. Our method enables easy determination of fucosylated structures on the N-acetyllactosamine branches of these isomers. On negative-MSn the fragment ions included several A and D ions, from which fucosylation on the branches could be elucidated. Other characteristic ions were also detected. Y-type cleavage at the reducing side of –3GlcNAc indicated the occurrence of type 1 chain. Specific fragment ions were produced from H, Lea, and Lex antigens. Linkage-specific exoglycosidase digestion confirmed the structures. The results indicate that the diversity of the oligosaccharides is due to combinations of type 1 H, Lea, Lex, and Leb/Ley on branched decaose cores. In typical oligosaccharides, 6-branches always consist of type 2 chain, while 3-branches, such as β and γ chains, are fucosylated type 1 chains. From the viewpoint of biosynthesis, the presence of fucosylation and type 1 chain may halt elongation of the N-acetyllactosamine and promote formation of branched structures.

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