About the Lectin

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.¹

REFERENCES

1. Mo H, et al. J Biol Chem. 1994 Mar 11;269(10):7666-73.