Iris hybrid lectin (IRA)
A blood group type A disaccharide (GalNAc alpha 1-3Gal)-binding lectin has been purified from Dutch Iris (Iris x hollandica) bulbs. This lectin agglutinates both native and trypsin-treated rabbit erythrocytes, but not human erythrocytes, irrespective of blood group type. The lectin is a heterodimer consisting of two peptide chains (27 and 34 kDa) linked by disulfide bonds.
The carbohydrate binding specificity of the lectin was investigated by quantitative precipitation, hemagglutination inhibition, and precipitation inhibition assays. It is a Gal/GalNAc-specific lectin, with an extended carbohydrate combining site, which appears to be most complementary to GalNAc linked to the C-3 or C-6 hydroxyl group of galactose. As inhibitors, these disaccharides are approximately 30-60 times more potent than galactose and 4-8-fold more active than N-acetyl-D-galactosamine, whereas both the blood type A trisaccharide (GalNAc alpha 1-3[L-Fuc alpha 1-2]Gal) and the Forssman disaccharide (GalNAc alpha 1-3GalNAc) are noninhibitory, suggesting the importance of a free equatorial hydroxyl group at the C-2 position of the penultimate galactose for lectin binding; either an acetamido group or a fucosyl group at this position appears to cause steric hindrance, thus abolishing binding to the lectin.
H. T. Preis