Noncovalent association of heavy and light chains of human immunoglobulins. III. Specific interactions between VH and VL.

Mildly reduced monoclonal human IgM proteins have been cleaved at cysteinyl residues to give VH fragments after S-cyanylation with 2-nitro-5-thiocyanobenzoic acid. The noncovalent interaction between the VH fragments and autologous kappa-chains was studied by ultraviolet difference spectroscopy and circular dichroism. A bimolecular complex was formed with an association constant in excess of 10(7) M-1 at 23 degrees C. Complex formation was accompanied by burial of tryptophan and tyrosine side-chains. In contrast to the studies with autologous species, the VH fragments did not associate with heterologous kappa-chains as judged both by difference spectroscopy and gel filtration using radiolabeled VH fragments. This specificity in the association between VH and VL has been attributed to interactions contributed to by residues in the third hypervariable region of VH encoded by the DH and JH genes.