About the Lectin

Saline extracts of potato tubers contain a potent hemagglutininin that is not inhibited by monosaccharides. Carbohydrate inhibition studies have shown that the lectin has an extended binding site that can accommodate up to four carbohydrate units. The lectin binds oligosaccharides containing β(1,4)-linked GlcNAc. The tetrasaccharide is approximately 20 times more potent an inhibitor than the disaccharide ¹ . STA also reacts with poly N-acetyllactosamine structures from N-linked oligosaccharides, albeit rather weakly ² . Nevertheless, it is useful for histochemical staining of these structures ³ . Many lectins do not react with sulfated sugars, however, STA does bind to chitin sulfates and keratan sulfates ⁴ . Purified STA is one of the most highly glycosylated lectins, composed of approximately 50% carbohydrate by weight. The heavily-glycosylated portion is a hydroxyproline-containing sequence homologous to the cell wall extensions, while the carbohydrate-binding region is rich in cysteine and glycine, and contains three hevein domains, such as occurs in chitinases and other N-acetylglucosamine-binding lectins ^5,6 . Antibodies to STA have been shown to have dual binding properties. One antibody will react with the non-glycosylated, carbohydrate binding region of STA, while another antibody will react with the glycosylated portion of the protein. The antibody which reacts with the glycosylated part of the lectin will not inhibit lectin induced agglutination; however, it will cross react with other lectins, such as DSA and LEA, that contain the same carbohydrate regions ⁷ . The antibody reacting with the carbohydrate binding region of STA does inhibit agglutination. Although potato tubers have been used for the preparation of STA, a lectin has been isolated from potato fruits using chromatofocusing ⁸ . With respect to carbohydrate specificity, carbohydrate content, molecular weight, and antibody cross-reactivity, the lectin purified from the fruits is identical to the tuber lectin. Chromatofocusing has shown that the two lectins differ with respect to their isoelectric point. The tuber lectins found in different varieties of potato may also differ with respect to their isoelectric point.

REFERENCES

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