THE LEWIS-X EPITOPE IS A MAJOR NONREDUCING STRUCTURE IN THE SULFATED N-GLYCANS ATTACHED TO ASN-65 OF BOVINE PROOPIOMELANOCORTIN

The N-terminal glycopeptide of pro-opiomelanocortin (POMC), designated as the 16K fragment, is highly conserved throughout vertebrates from amphibians to mammals and is likely therefore to have an important fun ctional role. In this paper, we report the first structural characteri zation of N-glycans attached to asparagine-65 of a 16K glycopeptide. T he 16K fragment was isolated from bovine pituitaries and the N-glycans were analysed using fast atom bombardment mass spectrometry together with sugar and linkage analysis. Sulphated-N-acetylgalactosamine-cappe d antennae, typical of the pituitary glycohormones, were present in th e major acidic components. The POMC oligosaccharides are distinct from those of the pituitary glycohormones because the sulphate is exclusiv ely located on the 3-arm of biantennary structures and, in addition, a significant proportion of the molecules carry the Lewis x epitope. It is probable that these differences reflect the absence of a tripeptid e motif in POMC which fully conforms to the criteria previously define d for the recognition sequence for the N-acetylgalactosamine transfera se that is specific for the pituitary glycohormones [Smith and Baenzig er (1992) Proc. Natl. Acad. Sci. USA, 89, 329-333]. It remains to be s een whether the Lewis x epitope is involved in selectin-mediated event s, but previous studies suggest that the sulphated moieties are unlike ly to play a major role in clearance. The Lewis x epitope is also pres ent in the neutral N-linked oligosaccharides, together with a variety of other antennae including a rarely found fucosylated GalNAc-GlcNAc s tructure.