Unique oligosaccharides of the gastric h,k-atpase

The β-subunit of the H,K-ATPase (HKβ) is predominantly exposed at the luminal aspect of the parietal cell. N-linked carbohydrate moieties on HKβ are of special interest 1) likely providing stability in the acidic environment, and 2) implicated as primary antigenic sites for chronic autoimmune gastritis. We have characterized structural features of HKβ oligosaccharides using high pH anion-exchange chromatography, matrix assisted laser desorption mass spectrometry with collision induced dissociation and carbohydrate-specific antibodies. For HKβ from rabbit, all 7 potential N-linked sequons are glycosylated: two with high mannose-type; the remainder with asialo-lactosaminetype oligosaccharides, which are ct-Gal capped, some with lactosamine repeats. The high mannose glycoforms are conserved among species, with Asn¹⁹³ specifically identified as an oligomannosidic site in rabbit and pig, suggesting some significance in HKβ folding or topology. Fucose residues were identified in the lactosamine-type oligosaccharides, being core-fucosylated in some species and branch-fucosylated in others. In most humans examined oligosaccharides of HK are branch-fucosylated and strongly positive for Le^y antigen (αFuc l→2 βGal l→4 [ctFuc l→3] GlcNAc-), but less positive for Le^x (βGal l→4 [aFuc l→3] GlcNAe-). Thus, HKβ lacks sialic acid in all species examined (rabbit, pig, frog, rat, mouse and human). Instead, more acid-stable sugars, like galactose and fucose, cap terminal oligosaccharide branches. The retention of essential oligosaccharide structures may have superceded selective pressures and become antigenic targets leading to autoimmune dysfunction.