The binding of the Lewis-a human blood group determinant by two hybridoma monoclonal anti-Lea antibodies.

The combining sites of two hybridoma monoclonal antibodies with specificities for the Lewis-a human blood group determinant, beta-D-Galp-(1----3)-[alpha-L-Fucp-(1----4)]-beta-D-GlcpNAc-(1---- , were probed by use of a wide range of inhibitors of the reaction between an 125I-labelled artificial Lewis-a antigen and the antibodies under the conditions of a solid-phase radioimmunoassay. Amongst the inhibitors examined were the eight possible monodeoxy derivatives of the trisaccharide methyl glycoside. As was previously found for other antibodies and lectins, the results indicated that, in each case, a cluster of polar groups provide a key polar interaction with the protein. The further involvement of large lipophilic regions of the trisaccharide was also indicated. These regions are expected to provide an essentially nonpolar interaction for complex formation, and appear to include intramolecularly hydrogen-bonded hydroxyl groups. Although the features of the trisaccharide that are recognized are similar in kind, the patterns are very different. For example, in the case of antibody AH8-34, the key polar group is provided by OH-2 and OH-3 of the beta-D-Galp group, whereas for the other antibody (CF4-C4) OH-3 and OH-4 of the beta-D-Galp group and OH-4 of the alpha-L-Fucp group provide the key polar interaction. It appears that, for AH8-34, the trisaccharide is accepted with OH-2 of the beta-D-Galp group and OH-3 of the alpha-L-Fucp group intramolecularly hydrogen-bonded to neighboring proton acceptors. For CF4-C4, such intramolecular hydrogen-bonding appears to involve OH-4 and OH-6 of the beta-D-Galp group.