Structural features of the carbohydrate units of plasma glycoproteins.

Abstract

For the characterization of the type and relative abundance of different carbohydrate structures in plasma glycoproteins, rat whole plasma was subjected to exhaustive proteolytic digestion and the resulting glycopeptides were fractionated into structurally district groups by chromatography on concanavalin‐A‐Sepharose followed by mild NaOH/NaBH4 treatment and gel filtration. The most abundant fraction was found to be the biantennary transferrin‐type glycopeptides (71% of recovered carbohydrate) followed by triantennary fetuin‐type structures and a smaller proportion of more branched structures (together 24%), neutral mannose‐rich chains (2%) and O‐glycosidic chains (3%). Structural similarity of the two major fractions to the transferrin and fetuin glycopeptides was supported by similar elution volumes in gel filtration, similar behaviour in concanavalin A affinity chromatography and similar sugar composition and substitution pattern of sugars in methylation analysis.The glycopeptide pattern differs significantly from that observed previously for brain, kidney, liver or erythrocytes, where the more branched structures predominate, and which also contain higher amounts of the neutral mannose‐rich chains. As indicated by methylation analysis, the plasma glycoproteins are characterized by carbohydrate units terminating in N‐acetylneuraminic acid. The level of terminal galactose, N‐acetylglucosamine and fucose residues was low. This finding is in correlation with the described occurrence of binding‐activities in liver and other tissues which remove glycoproteins having these sugar terminals from the circulation.