title: Lactoperoxidase-catalyzed iodination of human IgM. Differences between 7 S IgM and 19 S IgM and between cell surface 7 S IgM and serum 7 S IgM.
abstract: We have studied the lactoperoxidase-catalyzed iodination of human IgM and have measured the ratio of radioactivity incorporated into the mu chain to that incorporated into the L chain (i.e. the mu/L ratio). Both 7 S and 19 S IgM were examined. The ratio of radioactivity was found to be larger for 7 S IgM than for 19 S IgM for all four of the monoclonal IgM proteins examined. The data suggest that some tyrosines of the mu chain which are buried and not available for iodination in 19 S IgM become exposed on conversion of 19 S IgM to 7 S IgM. The mu/L ratio for the IgM found on the cell surface of RPMI 8392 cells was significantly smaller than the ratios for all of the five 7 S IgM proteins studied in solution. It appears, therefore, that a portion of the mu chain of the cell surface IgM of the RPMI 8392 cells is buried in the membrane.
mesh_terms: D002384:Catalysis; D006801:Humans; D007074:Immunoglobulin G / Q000032:analysis*; D007075:Immunoglobulin M / Q000032:analysis*; D007457:Iodine Radioisotopes; D007553:Isotope Labeling; D007784:Lactoperoxidase; D010544:Peroxidases; D011947:Receptors, Antigen, B-Cell / Q000032:analysis
chemical_list: D007074:Immunoglobulin G; D007075:Immunoglobulin M; D007457:Iodine Radioisotopes; D011947:Receptors, Antigen, B-Cell; D007784:Lactoperoxidase; D010544:Peroxidases