Mapping an antibody binding site by nuclear decay

The study of proteins and their complexes by cleavage of polypeptide chains provides a new approach to understanding their behavior in solution. Using radionuclides as the potential .source for this bond-breaking energy, we have assessed the effects of nuclear decay on two antibody binding sites. Monoclonal antibody CHA255 specifically binds para substituted (S)-benzyl-EDTA[In] chelates. Allowing CHA255 to bind a chelate containing the radioisotope In-111, which decays by electron capture, results in chain cleavage at several points within the binding pocket of the antibody. Correlation with the anti body/hapten crystal structure supports cleavage at or near the residues that either hydrogen bond to the hapten or directly coordinate the metal. In exper iments with a different monoclonal antibody that binds an yttrium chelate. we have found similar results from the energetic 2.2 MeV beta decay of yttriuin-90. Accurate assignment of these cleavage sites will provide calibration of the short range over which different modes of nuclear decay act to cleave polypeptides backbones.