Several polylactosamine-modifying glycosyltransferases also use internal GaINAcβ1-4GlcNAc units of synthetic saccharides as acceptors

Abstract

The Ga1NAcβ1-4GlcNAc determinant (LdN) occurs in some human and bovine glycoconjugates and also in lower vertebrates and invertebrates. It has been found in unsubstituted as well as terminally substituted forms at the distal end of conjugated glycans, but it has not been reported previously at truly internal positions of polylactosamine chains. Here, we describe enzyme-assisted conversion of LdNβ1-OR oligosaccharides into GlcNAcβ1-3GalNAcβ1-4GlcNAcβ1-OR. The extension reactions, catalyzed by human serum, were modeled after analogous β3-GlcNAc transfer processes that generate GlcNAcβ1-3Galβ1-4GlcNAcβ1-OR. The newly synthesized GlcNAcβ1-3GalNAc linkages were unambiguously identified by nuclear magnetic resonance data, including the appropriate long-range correlations in heteronuclear multiple bond correlation spectra. The novel GlcNAcβ1-3′LdN determinant proved to be a functional acceptor for several mammalian glycosyltransferases, suggesting that human polylactosamines may contain internal LdN units in many distinct forms. The GlcNAcβ1-3′LdN determinant was unusually resistant toward jackbean β-N-acetylhexosaminidase; the slow degradation should lead to a convenient method for the search of putative internal LdN determinants in natural polylactosamine chains.