Purification and characterization of a galactose-specific lectin from the stems and leaves of Dolichos lablab (Indian lablab beans)

Seeds of the Indian lablab beans (Dolichos lablab) contain two distinct lectins which differ in their sugar specificities, viz. the glucose/mannose-specific lectin and the galactose-specific lectin. Stems and leaves of the three-week-old plants contain proteins that strongly agglutinate rabbit erythrocytes but not human erythrocytes, and the agglutinating activity is inhibited by galactose, its derivatives, lactose and N-acetylgalactosamine. However, the lectin does not bind to the conventional Sepharose-derivatized galactose or lactose gel and can be purified to homogeneity by ion exchange chromatography and gel filtration. The lectin eluted as a single symmetrical peak from a Biogel P-200 column with a molecular mass of 66 kDa. Purified stem and leaf lectin is a glycoprotein with 3% reducing sugar and is dissociated into two subunits in SDS-PAGE with molecular masses of 48 kDa and 20 kDa, respectively. The lectin cross-reacts with an antibody to the glucose/mannose-specific seed lectin, suggesting similar antigenic sites in these two lectins. Antibodies raised against the purified galactose-specific seed lectin show specific reactivity with the seed lectin as well as with the stem and leaf lectin, suggesting that they are related proteins.