Anomer specificity of the 14 kDa galactose-binding lectin: A reappraisal.

A β-anomer preference among galactosides has been attributed to the S-type 14 kDa galactose binding lectin. Here the anomeric preference of this lectin from bovine brain (BBL) is reexamined using inhibition of lectin-mediated haemagglutination, binding of the lectin to dot-blotted glycoproteins and affinity electrophoresis of the lectin through polysaccharide-containing gels. 1·0-methyl α-D-galactoside was 8 times better inhibitor of BBL than the corresponding ß-anomer. The terminal galactose in bovine thyroglobulin (exclusively. α-linked) were also nearly 8 times more inhibitory than those in asialofetuin (exclusively ß-linked). The terminal α-galactose-containing endogenous glycoproteins of bovine brain were nearly 4 times better inhibitors of BBL than laminin. Removal of terminal α-galactose units by α-galactosidase fully abolished the BBL binding of thyroglobulin and endogenous glycoproteins. BBL was also sugar-specifically retarded by polyacrylamide gel containing guar galactommannan which bears only α-linked galactose. Data indicated that α-galactosides were sometimes better than their β-anomers in binding to BBL. The significance of this observation to the physiological role of galactose-binding lectins is discussed.

Physicochemical properties and binding-site amino acid residues of galactoside-binding protein of human placenta.

The galactose-binding lectin of human placenta has been purified to homogeneity by affinity chromatography on asialo-fetuin column. The protein, extractable from the tissue only with lactose is apparently membrane-bound. Molecular weight determination of native protein and subunit indicated a dimer of 13·4 kDa subunits. Inhibition of haemagglutination with various saccharides indicate that thiodigalactoside is the best inhibitor followed by lactose· However, p-nitrophenyl- and 1-O-methyl derivatives of galactose showed that α-anomers inhibited slightly better than β-anomer. Modification of amino acid residues indicated involvement of arginine, lysine and histidine residues at the saccharidebinding site. Cysteine residue modificatioin also abolished haemagglutinating activity. Amino acid composition of the lectin is also presented·