Urinary proteins of patients with myeloma, prepared by precipitation with ammonium sulphate, have been separated by gel filtration on Sephadex G-100 after reduction and aminoethylation. Many specimens separated into a major peak of Bence Jones protein and into minor peaks of albumin, a protein tentatively identified with heavy chain and a smaller molecular weight protein corresponding to the variable portion of the corresponding Bence Jones protein. The Bence Jones protein purified by gel filtration was analyzed by electrophoresis and by peptide mapping after tryptic digestion. The peptide maps of 24 type K and 20 type L Bence Jones proteins were compared.

A set of common peptides was identified in the peptide maps of the Bence Jones proteins of the same type; the common peptides of type K proteins were completely different from the common peptides of type L proteins. The patterns of distinctive peptides was compared; no similarities were found between distinctive peptides of type K and of type L proteins. Some similarities were observed in the distinctive peptides of proteins of the same type. The similarities involved in many cases peptides containing cysteine, whereas similarities in other peptides were limited. This observation suggested that the amino acid sequence around the cysteines of the variable NH2-terminal half of the Bence Jones proteins may show less variability than other sequences. A few proteins of the same type differed in all their distinctive peptides, an indication that multiple amino acid differences exist between individual Bence Jones proteins.

The genetic mechanisms responsible for the variability in the amino acid sequence of the NH2-terminal half of the light chains of immunoglobulins are discussed in view of the results of the comparison by peptide mapping of the Bence Jones proteins.

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