Interaction of mistletoe toxic lectin-I with sialoglycoproteins

The binding properties of mistletoe toxic lectin-I (ML-I) with sialo-N- and O- glycans were investigated by quantitative precipitin and precipitin inhibition assays. Human α₁-acid glycoprotein reacted strongly with ML-I, precipitating over 82% of the lectin nitrogen tested, while the precipitability of its asialo product decreased by 30%. Native fetuin precipitated 50% of the ML-I added, and its reactivity was reduced by 20% after desialylation. On the contrary, the poor reactivity of rat sublingual sialoglycoprotein with ML-I increased substantially after removal of sialic acid and completely precipitated the lectin added. The glycoprotein-lectin interactions were inhibited by NeuAcα2→3/α2→6Galβ1→4Glc and/or Galβ1→4Glc(NAc) residues. From the above results, it is concluded that ML-I is specific for sialic acid. However, sialic acid of some O-glycans also acts as masking molecule as the precipitability of rat sublingual and bovine submandibular glycoproteins with ML-I increased after desialylation.