SPECIFICITY IN THE COMBINATION OF FD FRAGMENTS WITH L CHAINS TO FORM HAPTEN-BINDING SITES

Abstract

Light chains and Fd frag-ments can be separated completely in propionic acid and then recombined to form Fab fragments with antibody activity. In recombinationa correct alignment of the Fd fragments and the L chains occurs to give a competent antibody site, just as occurs with the recombination of separated heavy and light chains of the antibody; thus the Fc fragment is not required for correct alignment. Fd fragments of antibody alone show very low binding activity toward the specific hapten. The combination of Fd fragments and light chains also requires that both components come from antibody from the same rabbit in order to give binding sites. Whenthey are derivedfrom different rabbits producing antibody against the same antigen, they still give Fab fragments as shownby immunoelectrophoresis but do not have competent binding sites. The subunits of the papain digest fraction, FabI, FabII, have the capacity to cross-combine to form active Fab fragments with competent binding sites. FdI from FabI combines with LII chains from FabII to give the composite (FdI-LII) with good binding activity. The composite of (FdII-LI) has good binding activity. The composites from the 2 types of antibody molecules yielding different Fab fragments have antibody activity although heretofore these molecules have appeared to be different on the bases of chromatography and amino acid analysis. There is also a preferential combination of the Fd fragments to combine with the correct L fragments to give binding sites since this combination takes preference over the combination of Fd fragments of antibody with light chains of normal globulin (or of light chains of antibody with Fd fragments of normal globulin).