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Synthesis of selectively radiolabeled hexasaccharides for the determination of enzymatic regioselectivity
Authors:J. C. McAuliffe  M. Ujita  M. Fukuda  O. Hindsgaul
Affiliation:(1) The Burnham Institute, 10901 North Torrey Pines Rd., San Diego, CA 92122, USA;(2) National Institute for Basic Biology, Okazaki, 444-8585, Japan
Abstract:Poly-N-acetyllactosamines provide backbone structures for functional modifications such as sialyl Lewis X. To understand how the biosynthesis of poly-N-acetyllactosamines is regulated, two branched oligosaccharides of the structure Galbeta1,4GlcNAcbeta1, 6(Galbeta1,4GlcNAcbeta1,2)-Managr1,6Manbeta-octyl 1 and 2 were synthesized in which one of the terminal galactose units was selectively radiolabeled. Hexasaccharides 1 and 2 were assembled from the chemically synthesized pentasaccharide precursors GlcNAcbeta1,6(Galbeta1,4GlcNAcbeta1,2)-Managr1,6Manbeta-octyl3 and Galbeta1,4GlcNAcbeta1,6(GlcNAcbeta1, 2) - Managr1,6 Manbeta-octyl 4 respectively, through treatment with UDP-1-[3H]-Gal and beta1,4 galactosyltransferase. Compounds 1 and 2 were subsequently incubated with UDP-GlcNAc and the UDP-GlcNAc: Galbeta1-4Glc(NAc)beta1,3-N-acetylglucosaminyltransferase (i-GlcNAc transferase) resulting in a partial conversion to a mixture of heptasaccharides which were purified by HPLC. The branch selectivity of the addition of N-acetylglucosamine to compounds 1 and 2 was then characterized by endo-beta-galactosidase digestion of the heptasaccharides, followed by isolation of the resultant pentasaccharides on C18 reverse-phase silica cartridges. Comparison of the amount of radiolabel to a control reaction lacking endo-beta-galactosidase indicated the favored site of GlcNAc addition to be the lower beta1,2-branch over the beta1,6-branch by a 3:1 ratio.
Keywords:Glycosyltransferase  N-acetyllactosamine  biosynthesis  radiolabel
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