Antibody active sites and immunoglobulin molecules.

In order to obtain detailed information about the relationship between structure and function in antibody molecules, a method called affinity labeling has been devised to attach chemical labels specifically to amino acid residues in the active sites of antibody molecules. With antibodies to three different haptens, highly specific labeling of the active sites has been achieved. Tyrosine residues on both heavy and light polypeptide chains have been labeled in a molar ratio close to 2:1, and labels on the two chains are equally specific to the active sites. Peptide fragmentation studies of the labeled chains of one antibody system have shown that: (i) within 25 amino acid residues of the labeled tyrosine on either chain, substantial chemical heterogeneity exists among different antibody molecules of the same specificity; and (ii) the labeled peptide fragments from both chains are very similar in physicochemical characteristics, including average size, heterogeneity, and unusual hydrophobicity. These experimental results have led us to the view that a particular region of the heavy chain and a particular region of the light chain are utilized to construct the active sites of the three different antibodies, differences in specificity arising from chemical perturbations in these two regions. Correlated structural studies of affinity-labeled antibodies and of the homogeneous light chains (Bence Jones proteins) and heavy chains produced in multiple myeloma may permit the identification of these special active-site regions. The view that active sites of different specificity are chemical perturbations of a particular region of the antibody molecule has a possible close analogue in enzyme systems, particularly among the esterases. The marked chemical similarities we have observed between the active site regions of heavy and light chains indicate to us that chemical homologies, but not identities, exist between the chains. This is reinforced by recently obtained amino acid sequence data which reveal homologies between the two chains near their carboxyl-terminals. These results indicate that the structural genes which code for the synthesis of heavy and light chains are related, presumably having arisen from some common ancestral gene during evolution. This conclusion strongly suggests that both heavy and light chains determine antibody specificity, and has important implications for the still-unknow mechanisms of antibody biosynthesis.