Naturally-ocurring low molecular weight IgM in patients with rheumatoid arthritis, systemic lupus erythematosus, and macroglobulinemia. II. Structural studies and comparison of some physicochemical properties of reduced and alkylated IgM, and low molecular weight IgM.

Some physicochemical properties of naturally occurring low molecular weight IgM (LMW IgM), IgM and their subunits obtained by reduction-alkylation were compared. On the basis of analytical ultracentrifugation, SDS polyacrylamide electrophoresis, analytical Sephadex chromatography, and heavy (H) and light (L) chain composition purified LMW IgM had an S20,W constant of approximately 8S, a molecular weight of 195,000 to 205,000 and a structural formula H2L2. Reduction and alkylation of IgM (IgM RA) by two techniques produces two components which could be distinguished by analytical ultracentrifugation, sucrose density ultracentrifugation, and gel filtration studies. IgM RA-cys had an estimated molecular weight similar to LMW IgM and IgM-2ME had physical properties similar to IgG. The molecular weights of H and L chains of IgM and LMW IgM were similar: 69,000 to 75,500 and 26,000 to 27,000, respectively. Unlike LMW IgM and IgM RA-cys, the less dense IgM RA reduced with 2ME dissociated into half subunits, and free H and L chains in SDS gel electrophoresis, indicating that it lacked intra-subunit disulfide bonds. J chain was not detected in LMW IgM or in the chromatographically purified reduced-alkylated subunits from a patient whose IgM contained J chain. Carbohydrate analyses of IgM, LMW IgM, and reduced-alkylated IgM products did not demonstrate differences in individual sugars or in the percentage of total carbohydrate. The different sedimentation and gel filtration properties of the two components obtained by reduction-alkylation of IgM could not be attributed to variations in peptide chain or carbohydrate content, rather they are probably dependent on conformational changes resulting from variable, reduction of intra-subunits disulfide bonds.