The Lewis x epitope is a major non-reducing structure in the sulphated N-glycans attached to Asn-65 of bovine pro-opiomelanocortin

The N-terminal glycopeptide of pro-opiomelanocortin (POMC),designated as the 16K fragment, is highly conserved throughoutvertebrates from amphibians to mammals and is likely thereforeto have an important functional role. In this paper, we reportthe first structural characterization of N-glycans attachedto asparagine-65 of a 16K glycopeptide. The 16K fragment wasisolated from bovine pituitaries and the N-glycans were analysedusing fast atom bombardment mass spectrometry together withsugar and linkage analysis. Sulphated-N-acetylgalactosamine-cappedantennae, typical of the pituitary glycohormones, were presentin the major acidic components. The POMC oligo-saccharides aredistinct from those of the pituitary glycohormones because thesulphate is exclusively located on the 3-arm of biantennarystructures and, in addition, a significant proportion of themolecules carry the Lewis x epitope. It is probable that thesedifferences reflect the absence of a tripeptide motif in POMCwhich fully conforms to the criteria previously defined forthe recognition sequence for the N-acetylgalactosamine transferasethat is specific for the pituitary glycohormones [Smith andBaenziger (1992) Proc. Natl. Acad. Sci. USA, 89, 329–333].It remains to be seen whether the Lewis x epitope is involvedin selectin-mediated events, but previous studies suggest thatthe sulphated moieties are unlikely to play a major role inclearance. The Lewis x epitope is also present in the neutralN-linked oligosaccharides, together with a variety of otherantennae including a rarely found fucosylated GalNAc-GlcNAcstructure. FAB-MS glycoprotein Lewis x POMC sulphated-GalNAc