[The primary structure of a crystalline monoclonal immunoglobulin kappa-type L-chain, subgroup I (Bence-Jones protein Rei.); isolation and characterization of the tryptic peptides; the complete amino acid sequence of the protein; a contribution to the elucidation of the three-dimensional structure of antibodies, in particular their combining site (author's transl)].

Bence-Jones protein Rei. was isolated from the urine of a plasmacytoma patient by precipitation with ammonium sulfate and purified by ion exchange chromatography. The determination of the molecular weight showed that the protein was present in its monomeric form. The preparation cotained a minor fraction, which consisted of the dimer of the variable parts and was characterized by its ability to crystallize. X-ray data with a resolution on the atomic level could be achieved and allowed, together with the known primary structure, the construction of a three-dimensional model, which also gave an insight into the antigen binding site. It consists of the hypervariable regions, which form a pocket 15 A in diameter lying between the two monomers at the ends of the molecules. Sequence studies were performed with tryptic peptides which were purified by chromatographic procedures and aligned in homology to other kappa-chains. The protein belongs to subgroup I on the basis of its characteristic amino acid sequence and follows a highly regular pattern of amino acid exchanges. Its sequence is in agreement with an evolutionary origin of antibody variability.