Core‐typing of O‐linked glycans from human gastric mucins

Abstract

Mucins from the pooled gastric juice of Lewis‐positive secretors were investigated to establish their glycosylation patterns with particular reference to the type and abundance of the glycan‐core structures. Following reductive β‐eliminination, the neutral glycan alditols from these mucins were fractionated by ion exchange and size‐exclusion chromatographies and subjected to structural analyses. It was possible to gain insights into the core sequences of the neutral O‐linked glycan alditols by matching (a) composition data from liquid secondary‐ion mass spectrometry of the native alditol fractions, (b) specific structural information on the core sequences by thin‐layer‐chromatography mass spectrometry of alditol‐derived neoglycolipids and (c) data from electron‐impact mass spectrometry of permethylated glycan alditols or their partially methylated alditol acetates. The predominant core structures detected among the neutral glycans representing about 77% (by mass) of the total carbohydrates released from gastric mucins were core 1, Galβ1‐3GalNAc (Ac, acetyl) and core 2, Galβ1–3(GlcNAcβ1–6)GalNAc in the approximate ratio 1:2. Core 3, GlcNAcβ1–3GalNAc, and core 4, GlcNAcβ1–3(GlcNAcβ1–6)GalNAc, were much less abundant (< 10%), while core 5, GalNAcα1–3GalNAc, core 6, GlcNAcβ1–6GalNAc, and a recently described sequence GalNAcα1–6GalNAc (core 7) were not detected. This investigation also addressed the question of the presence of the sequence Galβ1–6GalNAc which has been reported previously to occur as a core‐structure element in gastric mucins. This was greatly assisted by the availability of the authentic chemically synthetised disaccharide alditol which, when converted into a neoglycolipid after mild periodate oxidation, gives diagnostic ions in mass spectrometry and can be detected with high sensitivity. No evidence was found for the presence of this unusual sequence among the oligosaccharides in gastric mucins.