Identification of α-fetoprotein-P4 sugar chain as α 2→6 monosialylated biantennary complex-type oligosaccharides with an exposed galactose on the mannose α 1→6 arm by two-dimensional extended agarose gel-lectin affinity electrophoresis

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Abstract
Erythroagglutinating phytohemagglutinin (E-PHA)-reactive α-fetoprotein (AFP)-P4 and E-PHA weakly-reactive AFP-P3 of sera from cord blood at term and from patients with hepatocellular carcinomas showed a mobility of monosialo-AFP in two-dimensional extended agarose gel-E-PHA affinity electrophoresis. Asialo-AFP had as high an affinity for E-PHA as that of monosialo-AFP-P4, while disialo-AFP, AFP-P2, revealed a negligible affinity for E-PHA. For Allomyrina dichotoma lectin (allo A), asialo-AFP had no affinity with a mobility of AFP-A1s, monosialo-AFP-P4 had the highest affinity with a mobility of AFP-A3, and monosialo-AFP-P3 had an intermediate affinity with a mobility of AFP-A2s. The affinity of AFP-P4 for E-PHA was reduced by galactosidase digestion, indicating the presence of galactose residues, one sialylated and the other exposed. From the known high affinity of E-PHA for biantennary complex-type oligosaccharides with a terminal β 1→4 galactose of the Man α 1→6 antenna and from that of allo A for an α 2→6 sialylated β 1→4 galactose of the Man α1→3 antenna, together with the nonbisected biantennary structure of AFP-P4 as deduced from its high affinity for concanavalin A, the sugar chain structure of AFP-P4, which increases frequently in hepatocellular carcinomas, was identified as α 2→6 monosialylated nonbisected biantennary complex-type oligosaccharides with an exposed β 1→4 galactose of the Man α 1→6 arm and with or without fucosylated core N-acetylglucosamine, and AFP-P3 as similar oligosaccharides with an exposed β 1→4 galactose of the Man α 2→3 arm.

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