Lymphoblastoid cell adhesion mediated by a dimeric and polymeric endogenous β‐galactoside‐binding lectin (galaptin)

Glutaraldehyde‐polymerized human splenic galaptin, a β‐galactoside‐binding lectin, was demonstrated to have enhanced hemagglutinating and asialofetuin binding activity relative of native dimeric galaptin when these lectins were present in solution. The polymerized lectin consisted primarily of 2‐, 4‐ and 12‐membered species after reductive alkylation. Both forms of galaptin bound, at 4 °C, to saturable B lymphoblastoid cell surface receptors. Estimates obtained by Scatchard analyses, with the binding data expressed in terms of 14.5 kDa subunit molarity, were 5 × 10⁷ binding sites/cell with affinity constant K_a = 2.2 × 10⁵ M for dimeric galaptin and 17 × 10⁷ binding sites/cell with K_a = 3.4 × 10⁵ M⁻¹ for polymeric galaptin. Both forms of galaptin adsorbed to polystyrene with high efficiency; however, only plastic‐adsorbed polymeric galaptin mediated adhesion of lymphoblastoid cells. Cell adhesion was inhibited by lactose. Plastic‐adsorbed polymeric galaptin bound asialofetuin more efficiently than dimeric galaptin. Asialofetuin binding was inhibited 65% and 30–50% by lactose for plastic‐adsorbed polymeric and dimeric galaptin, respectively, Native fetuin bound to the adsorbed dimeric galaptin in a lactose‐insensitive manner. These data indicate that cell surface receptor‐galaptin interaction is carbohydrate specific whereas polystyrene‐adsorbed galaptin may demonstrate protein‐protein interactions with soluble ligands.