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Structure of the sugar chain of alphafetoprotein purified from a human yolk sac tumor and its reactivity with concanavalin A.

We have attempted to determine the carbohydrate moiety of human alphafetoprotein (AFP) produced by a yolk sac tumor. AFP was obtained from the cystic fluid of human yolk sac tumors grown in nude mice and was purified using an immunoadsorbent column coupled with monoclonal anti-AFP antibody. Then, the carbohydrate chain of the purified AFP was quantitatively released from the polypeptide chain. The resulting oligosaccharide was labeled and, by sequential exoglycosidase digestion in combination with methylation analysis and periodate oxidation, the structure was determined to be: Sia alpha 2----6Gal beta 1----4GlcNAc beta 1----2Man alpha 1----6 (GlcNAc beta 1----4) (Sia alpha 2----6Gal beta 1----4GlcNAc beta 1----2Man alpha 1----3) Man beta 1----4GlcNAc beta 1----4 (Fuc alpha 1----6) GlcNAcOT Compared with the known structure of the sugar, chain of human hepatic AFP, it was found that the sugar chain of yolk sac AFP contained an additional sugar, N-acetylglucosamine (bisect GlcNAc) linked to the beta-mannose. In the light of recent knowledge, this result indicates that the Concanavalin A (Con-A) binding site of the sugar chain is blocked by this GlcNAc in human yolk sac AFP. This fact forms the basis for the clinical use of the Con-A binding test to determine the origin of AFP in patients.