Carbohydrate binding properties of th Dolichos biflorus lectin and its subunits

Equilibrium dialysis studies on the binding of the Dolichos biflorus lectin with [14C]methyl alpha-D-GalNAc showed that the lectin has two combining sites/molecule and an intrinsic association constant at 3 degrees C of 4.2 X 10(3) liters mol-1. Binding studies on individual fractions (II, IV, and VII) of the lectin that differ in their chromatographic properties on concanavalin A-Sepharose gave association constants for methyl alpha-D-GalNAc of 2.2 X 10(3) liters mol-1, 3.2 X 10(3) liters mol-1, and 3.2 X 10(3) liters mol-1, respectively. Molecular exclusion chromatography of iodinated Subunits I and II of the lectin, as well as sedimentation velocity studies of the noniodinated subunits, showed that the isolated subunits form aggregates in aqueous solution. Aggregates of subunit I were capable of agglutinating blood type A erythrocytes, precipitating blood group A + H substance, and binding to blood group A + H substance in an affinity electrophoretic system. Aggregates of subunit II exhibited none of these binding properties and did not inhibit the ability of the intact lectin to agglutinate type A erythrocytes. Affinity electrophoresis of subunit I showed that it has an association constant for N-acetyl-D-galactosamine similar to that of the intact lectin. The results suggest that it is subunit I that is primarily responsible for the carbohydrate binding properties of the lectin.