Human monoclonal cryoimmunoglobulins. I. Molecular properties of IgG3 kappa (Jir protein) and the cryo-coprecipitability of its molecular fragments by papain.

The temperature-dependent precipitability of a monoclonal IgG3 kappa cryoimmunoglobulin (Jir) without known antibody activity is shown to be affected by various physico-chemical factors, such as protein concentration, pH value and NaCl concentration. The molecular properties characterizing this protein (carbohydrate and amino acid compositions, peptide constitutions and susceptibility to enzymatic proteolysis) are described. The cryoprecipitability of the protein was completely lost upon papain hydrolysis, and none of the isolated fragments, Fab-Fc, Fc, and Fab, showed any precipitating activity. In the cryo-coprecipitation assay using the 125I-labeled fragments, it was demonstrated that the association activity with intact Jir protein was still retained on the Fab-Fc and Fc fragments, but not on the Fab fragment. The evidence suggests that a specific interaction may be involved in the primary intermolecular association required to form the cryoprecipitate at temperatures below the critical point, and that one of the pairing sites resides on the Fc portion of the protein molecule.