Complexes of alpha 1-microglobulin and monomeric IgA in multiple myeloma and normal human sera.

alpha 1-Microglobulin (alpha lm), a glycoprotein heterogeneous in charge, was reported to occur both as a 31-kilodalton (kd) monomer [low mol. wt alpha lm (LMW-alpha lm)] and as polymers or complexes formed with other plasma proteins including IgA [high mol. wt alpha lm (HMW-alpha lm)]. The present study was designed to characterize HMW-alpha lm in normal human serum and in myeloma sera. The following sera were selected: five IgG, 16 IgA and four Bence-Jones protein myelomas. alpha lm was identified by specific monoclonal antibodies in competitive radioimmunoassay and solid-phase ELISA. HMW-alpha lm was found to be associated almost exclusively with monomeric IgA and possibly in very small proportion with dimeric IgA. Ever in cases of predominantly dimeric IgA myelomas, alpha lm was associated with the monomeric form of the monoclonal IgA. The molar ratio of HMW-alpha lm to monomeric IgA never exceeded 3.5% and it was estimated to range from 0.5 to 1.4% in normal serum. No association with other proteins than IgA and no alpha lm polymers were found in IgA myeloma. Two types of HMW-alpha lm-IgA complexes were found: (a) those that were dissociable into IgA and LMW-alpha lm after mild reduction, and (b) those which were dissociated only after complete reduction of the complexes into IgA and an 88-90-kd component bearing alpha lm but no IgA epitopes. It was concluded that either of the two molecular species of alpha lm bearing common epitopes, with apparent mol. wts of 31,000 and 88,000-90,000, respectively, could form stable complexes with monomeric IgA. The association is likely to be performed through disulfide bridges. Nearly all the 88-90-kd but only a small proportion of the 31-kd component is associated with IgA.