Immunoglobulin ϰ-chains. Comparative sequences in selected stretches of Bence-Jones proteins

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Abstract
Three type K Bence-Jones proteins have been fully reduced and carboxymethylated with high-specific-activity iodo[C14]acetate. A tryptic digest and a chymotryptic digest of each protein were fractionated on a Sephadex column and the radioactive peptides were purified by paper electrophoresis. All the proteins studied had 5 unique carboxymethylated cystelne sequences. Three of these were identical with the exception of a single substitution, and 2 had some variations. The common peptides could be placed in the C-terminal half of the molecule. The variations around the other 2 half-cysteine residues provided information about the nature of the variability of the primary sequence of immunoglobulin [kappa] -chains. The results are consistent with the hypothesis that the chains derive from a common ancestor by somatic mutation of a small number of genes or by gene doubling and selection in the course of evolution. The isolation of the N-terminal peptide in methionine-containing Bence-Jones proteins is also described.