Mechanism of IgA-albumin complex formation that affects the fructosamine assay

We recently demonstrated glycation of monoclonal IgA and the presence of IgA-albumin complexes, but the mechanism of IgA-albumin complex formation was not clear. We isolated the IgA-albumin complexes from 5 IgA type M-proteinemia patients’ sera. To elucidate the mechanism of IgA-albumin complex formation, we performed the dissociation assay of IgA-albumin complexes, the identification of albumin binding sites of monoclonal IgA using immunoelectrophoresis, western blotting and chromatography technologies. In all patients with IgA type M-proteinemia, the IgA-albumin complexes were dissociated by treated with 2-mercaptoethanol (2-ME), but not by treated with a strong acid as acetic acid or NaCl of high concentrations. Moreover, when the purified monoclonal IgA containing IgA-albumin complexes was digested with the IgA protease from Neisseria gonorrhoeae, no macro-albumin was demonstrated. It seems probable that albumin is bound to the monoclonal IgA molecule by covalent disulfide bonds, and that the binding site of albumin is located in near the hinge region of IgA molecule and involve the free SH group thought to be present in the α-chain.