For antibodies both the amine and carboxylate groups are plentiful. For this reason, conjugation procedures that utilize these groups will cross link randomly to all parts of the antibody molecule. The distribution of amine and carboxylate groups on a three dimensional structure of an immunoglobin is nearly uniform throughout the surface topology. Conjugation to these functional groups can lead to random conjugation and hence, random orientation of the antibody often blocking the antigen binding site. Obscuring the binding site will decrease antigen binding activity in the conjugate. Site directed conjugation generally leads to conjugation chemistry that is successful at preserving the activity of the antibody. Conjugation through the sulfhydryl of a fragmented antibody is one such method of site directed conjugation. In this method the conjugation takes place at certain positions on the immunoglobin surface that is far from the antigen binding sites thus preventing blockage of these sites and preserving activity.
Sugar moieties present on the Fc region of the antibody can be oxidized under mild conditions using periodate to introduce reactive aldehyde residues which then can be subsequently conjugated to hydrazide containing liposomes to form a hydrazone linkage. One of the limitations to using this strategy is the necessity for the antibody molecule to be glycosylated. Antibodies of the polyclonal origin (antisera) are usually glycosylated and work well in this procedure, but other antibody preparations may not possess polysaccharides. Some monoclonals my not be post transitionally modified with carbohydrate after hybridoma synthesis. Recombinant antibodies grown in bacteria also may be devoid of carbohydrates. For information on aldehyde modified antibody, see the section under antibodies/antibody modification for conjugation.
Note: Before attempting this conjugation condition it is highly recommended to see if the antibody has carbohydrate especially, if the immunoglobin is of hybridoma or recombinant origin.