Structure of Immunoglobulin D: Evidence for the Absence of an Extra Disulfide Bridge in the Fc Fragment

Molecular weight determinations of immunoglobulin D suggest the presence of an extra region in the δ chain. In an attempt to locate this region, an IgDλ myeloma protein (Gur), was digested with trypsin for 4 min at 56 ° and the Fc fragment isolated by ion exchange chromatography. N-terminal analysis of this fragment showed a heterogeneity in the site of splitting by trypsin. The Fc was digested with pepsin and trypsin and the cysteine containing peptides isolated by a two dimensional paper high voltage electrophoresis (diagonal map) at pH 3.5. Further purification of these peptides was carried out by HVE at pH 6.5 and 2.1 and their amino acid composition and partial sequence were determined.

Only five cysteic acid peptides were obtained, one corresponding to the inter heavy-heavy bridge and the other four, to intra chain bridges. This finding would exclude the possibility of an extra « classical domain » in this region. The position of these peptides in the δ chain has been arranged based on homology with the other classes of immunoglobulins.