Isolation, characterization, and subunit structures of multiple forms of Dolichos biflorus lectin

Area of Science:

• Biochemistry
• Molecular Biology
• Plant Science

Background:

• Lectins are proteins with specific carbohydrate-binding properties.
• Dolichos biflorus lectin (DBL) is a well-known plant lectin with applications in blood group typing.
• Previous studies suggested DBL homogeneity, but detailed heterogeneity analysis was lacking.

Purpose of the Study:

• To isolate and characterize the Dolichos biflorus lectin.
• To investigate the potential heterogeneity of DBL.
• To compare the biochemical and functional properties of different DBL forms.

Main Methods:

• Isolation of DBL using affinity chromatography on immobilized hog blood group A + H substance.
• Fractionation of DBL using concanavalin A-Sepharose chromatography.
• Characterization using electrophoresis (PAGE, SDS-PAGE), isoelectric focusing, sedimentation equilibrium, immunodiffusion, and amino acid analysis.

Main Results:

• DBL was isolated and initially appeared homogeneous but was resolved into two electrophoretic forms (A and B) by concanavalin A-Sepharose chromatography.
• Form B (12% of sample) did not bind to concanavalin A, while Form A bound and was eluted with methyl alpha-D-glucopyranoside.
• Carbohydrate analysis revealed variations in mannose and N-acetylglucosamine content between fractions of Form A, indicating heterogeneity.
• Both separated A and B forms exhibited similar molecular weights, antigenic properties, amino acid compositions, NH2-terminal (alanine) and COOH-terminal residues (leucine/valine).
• Both forms showed specific agglutination of type A human red blood cells and similar reactivity with hog blood group A + H substance.
• SDS-PAGE indicated that both forms are tetrameric, composed of subunits with molecular weights around 26,000-26,500 Da.

Conclusions:

• Dolichos biflorus lectin exists as at least two electrophoretically distinct, active forms (A and B) in dry seeds.
• These forms share significant structural and functional similarities but differ in carbohydrate composition and concanavalin A binding.
• DBL heterogeneity, previously masked, is revealed through advanced chromatographic techniques.