Sites of “Spontaneous” Degradation of IgD

When 10 IgD myeloma proteins were examined by SDS-polyacrylamide gel electrophoresis (SDS-PAGE), the δ-chains of each protein showed multiple banding; the average extrapolated m.w. for the two major bands was 63,900 ± 735 and 60,550 ± 675 daltons. When compared to δ-chains, only ε-chains and not σ-, α- or µ-chains showed a multiple banding pattern. Double reciprocal plots of the observed m.w. versus varying acrylamide gel concentrations demonstrated that of all the Ig heavy chains δ-chain mobility was most variable. Digestion of δ-chains with yeast carboxypeptidase caused an initial release of methionine and proline, followed by leucine, tyrosine, valine, and serine. Carboxypeptidase A did not release any amino acids, whereas carboxypeptidase B released fractional molar quantities of both lysine and arginine. Therefore, the native δ-chain probably has a Pro-Met-COOH carboxy-terminal sequence analogous to the Pro-Gly-COOH sequence of δ-chains. A portion of the δ-chains may have undergone degradation near the COOH-terminus by an enzyme with tryptic like specificity, thus explaining the nominal release of basic residues by CPB as well as the multiple forms of the δ-chain seen by SDS-PAGE analysis. Tryptic Fc but not Fab or Fd fragments of IgD showed multiple banding by SDS-PAGE analysis which provides further evidence that the COOH-terminal region of the δ-chain is responsible for the apparent heterogeneity. The NH2-terminal sequence of Fc fragments isolated from IgD that had undergone “spontaneous” fragmentation during isolation was analyzed. The fragments formed by serum enzymes showed multiple sequences in contrast to Fc fragments generated by trypsin which gave a singular NH2-terminal sequence. Three cleavage sites were deduced for lysyl and arginyl residues 6, 8 and 12 residues NH2-terminal to the common half-cystine residue which participates in the inter-heavy chain disulfide bond. These data demonstrate that the δ-chain is highly susceptible to degradation both near the COOH-terminus and in the inter-Fd-Fc region by plasma enzymes of trypsin-like specificity.