Purification and characterization of N‐glycolylneuraminic‐acid‐specific lectin from Scylla serrata

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Abstract
A sialic‐acid‐binding lectin with specificity for N‐glycolylneuraminic acid (NeuGc) was purified from the hemolymph of the marine crab Scylla serrata by affinity chromatography using thyroglobulin‐coupled agarose. The binding specificity of Scylla lectin distinguishes it from other known sialic‐acid‐specific lectins found in Limulus polyphemus and Limax flavus, which show a broader range of specificity for sialic acids. The molecular mass of the purified lectin is about 55 kDa. Under reducing conditions (SDS/PAGE), it resolved into two subunits of 30 kDa and 25 kDa. NeuGc inhibited hemagglutination activity of the purified lectin at a concentration as low as 0.6 mM, whereas N‐acetylneuraminic acid (NeuAc) even at a concentration of 100 mM, failed to inhibit hemagglutination. This finding was supported by potent inhibition of hemagglutination by bovine and porcine thyroglobulins, which contain a NeuGcα2‐6Gal as terminal component of oligosaccharide residues. Neither glycoproteins (glycophorin NN; porcine submaxillary mucin), which contain NeuAcα2‐3Gal/GalNAc and NeuAcα2‐6GalNAc, nor human acid glycoprotein, which contains NeuAcα2‐3/α2‐6 Gal, or colominic acid, a sialopolymer with NeuAcα2‐8NeuAc, inhibited the lectin activity. The specificity of the lectin for NeuGc appears to account for the fact that it agglutinates rabbit and mice erythrocytes, but not human A, O, AB, rat or chicken erythrocytes, which contain NeuAc. The inability of the lectin to agglutinate erythrocytes (horse) that prominently express NeuGc could be due to O‐acetylation of NeuGc. In support of this, bovine submaxillary mucin, which contains O‐acetylated NeuGc inhibited the hemagglutination of the lectin better after removal of O‐acetyl groups by base treatment. The unique specificity of Scylla lectin is of diagnostic potential for human cancer tissues expressing NeuGc, since NeuGc is not found in normal human tissues.

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