Identification and Properties of Glycated Monoclonal IgA That Affect the Fructosamine Assay

Serum fructosamine is used to monitor short-term (1–3 weeks) average glycemia (1)(2), but controversy remains whether it is influenced by the serum concentrations of albumin (3)(4), IgA (5)(6), or monoclonal IgA (7)(8). We recently demonstrated glycation of monoclonal IgA and the presence of IgA-albumin complexes (9), but the relationship between the glycation of monoclonal IgA and the IgA-albumin complexes was not clear.In this study, we measured immunoglobulin-albumin complexes, serum fructosamine, and the glycation of immunoglobulin in patients with monoclonal proteinemia and in patients with polyclonal hyper-IgA.We obtained serum samples from 40 patients with M-proteinemia with no history of diabetes and whose plasma glucose concentrations were <1.1 g/L. The M-protein was IgG in 17 of the patients [mean (SD), 29.2 (21.0) g/L], IgA in 13 [18.5 (16.4) g/L], and IgM in 10 [16.8 (11.3) g/L]. We also analyzed sera from 15 nondiabetic patients with hepatitis who had polyclonal hyper-IgA [5.1 (1.1) g/L] as controls.Serum fructosamine was measured at 37 °C in an automated analyzer (JCA-RX 10 Clinalyzer; Japan Electron Optics Laboratory) with reagents from Roche Diagnostics Corporation.Serum protein electrophoresis was performed on agarose gels. Immunoelectrophoresis was based on the method of Scheidegger (10). Fructosamine was detected on agarose gels by incubating the gel for 30 min at 37 °C with a 2× concentration of a fructosamine reagent.IgA was isolated from serum with use of a jacalin-agarose (Funakoshi Corporation) column (11) and 0.8 mol/L galactose to elute the IgA. The IgA fraction was dialyzed against 0.2 mol/L Tris-HCl buffer (pH 8.0), concentrated by ultrafiltration, loaded on a DEAE-Sephacel (Amersham Pharmacia Biotech) ion-exchange column, and eluted with a linear gradient of 0–0.5 mol/L NaCl in the 0.2 mol/L Tris-HCl buffer.Sera were separated by sodium dodecyl sulfate–polyacrylamide gel electrophoresis (SDS-PAGE), …