An Atypical Immunoglobulin

A protein of the Ig G family has been isolated from the serum of a patient with a tentative diagnosis of a plasma cell neoplasm. The protein has a lower sedimentation constant (5.4) and a lower molecular weight (125,000) than normal immunoglobulins of the G family. The protein has heavy-chain determinants of type G and light-chain determinants of the κ-type. Heavy and light chains have been prepared by reductive cleavage followed by gel filtration. The heavy-chain preparation is homogeneous in starch gels in acidic buffer containing urea but has a faster mobility than normal Ig G heavy chains. The light-chain preparation is resolved into two components in electrophoresis, and both have slower mobility than normal Ig G light chains. The heavy- and light-chain preparations cross react with normal Ig G heavy and light chains in immunodiffusion analysis. Sedimentation equilibrium studies suggest that both the heavy and light chains have lower molecular weights than their normal counterparts.