Noncovalent association of heavy and light chains in Rana catesbeiana immunoglobulins.

The unreduced immunoglobulins (Ig) in the bullfrog, Rana catesbeiana, dissociate into two components when subjected to electrophoresis or molecular sieving in dissociating solvents. One of these components is monomeric light chain and the other is a disulfide-bonded complex of heavy chains. This unusual behavior has been observed with all classes of bullfrog Ig that have been isolated and characterized previously: a high m.w. Ig that resembles mammalian IgM and two antigenically distinct varieties of low m.w. Ig. Light chains, isolated from the high m.w. Ig by gel filtration in 8 M urea, 1 M acidic acid, were found to contain, on average, 5.7 residues of half-cystine. None of these residues were in the free sulfhydryl form nor were they blocked by half-cystine. Moreover, none was alkylated after mild reduction of the high m.w. Ig. These findings indicate that none of the light chain half-cystine residues participate in an interchain disulfide bridge, and that most of the light chains contain three intrachain bridges. This unusual pattern of disulfide bonding appears to be responsible for the noncovalent association of heavy and light chains in this species.