Site-directed biotinylation of antibodies for controlled immobilization on solid surfaces

Site-directed biotinylation of antibodies at the hinge region was developed to immobilize antibodies in an oriented manner via biotin–streptavidin linkage. When intact antibody was biotinylated with maleimide-activated biotin after reduction, the reaction preferentially occurred at the sulfhydryl groups between the CH1 and the CL domains and, provided that the reagent concentration exceeded a certain level, at those between the CH2 and the CH2 domains at the hinge. Based on this result, we devised an approach in which free maleimide was added to compete with the activated biotin for the preferential sites between the CH1 and the CL domains. Since the smaller molecular size of free maleimide made it more accessible for the reaction than biotin, maleimide bound to the groups between the CH1 and the CL domains first and thus conceded the groups between the CH2 and the CH2 domains to biotin under optimal conditions. In an alternative approach, selective biotinylation at the hinge was also achieved by reacting activated biotin with F(ab′)2 fragment prepared by enzymatic cleavage. This result indicated that, when free of Fc, the hinge structure, which contains the functional groups, of the fragment was open, allowing easy access to the biotin derivative from the aqueous medium. Both site-directed biotinylation preparations were tested as capture antibodies in sandwich-type immunoassays and compared to whole antibody randomly biotinylated at amino groups on the molecule. Preparations of both the intact antibody and the F(ab′)2 showed consistently enhanced detection capabilities that were 2.6 and 20 times that of the control, respectively.