Dual Specificity of Human Plasma Lactose‐Binding Immunoglobulin to Anomers of Terminal Galactose Enables Recognition of Desialylated Lipoprotein(a) and Xenoantigens

Human plasma lactose‐binding immunoglobulin ( LI g) isolated by affinity chromatography on lactose‐ S epharose was largely I g G with significant I g A and I g M contents. LI g‐mediated agglutination of desialylated human RBC was inhibited equally by the α ‐ and β ‐anomers of methyl galactoside. Recognition of either the terminal α ‐galactose ( TAG )‐containing glycans of bovine thyroglobulin or the N ‐acetyl lactosamine ( L ac NA c)‐terminating glycans of asialofetuin by LI g was inhibitable nearly as much by the α ‐galactoside melibiose as by the β ‐galactoside lactose. Melibiose covalently conjugated to protein and coated on polystyrene wells captured several times more LI g molecules than its lactose analogue. LI g binding to bovine thyroglobulin or rabbit RBC membrane proteins, both bearing TAG was substantially reduced by prior treatment of the proteins with α ‐galactosidase to remove TAG though enzyme‐treated glycans contained newly exposed L ac NA c moieties. Desialylated O ‐linked oligosaccharides, however, were no ligand for LI g. Unlike LDL , plasma lipoprotein(a) [ L p(a)] coated on polystyrene well and desialylated by neuraminidase was recognized by LI g through terminal L ac NA c moieties exposed by the enzyme on its apo(a) subunit. Further, same amount of added fluorescence‐labelled LI g formed significantly more immune complex with L p(a) in high L p(a) plasma than in low L p(a) plasma. Results suggest (1) possibility of a role for LI g in combating non‐primate molecules and cells bearing TAG moiety and (2) a mechanism for L p(a)‐mediated vascular injury as diabetes, infections and inflammations induce greater release of neuraminidase into circulation.