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·