A method to obtain purified free light chain monomers and dimers from urine samples of patients with multiple myeloma

Abstract

Antibody light chains are synthesized in excess by plasma cells, and this excess can be secreted into biological fluids as dimers or monomers in various proportions. Structural differences between monomers or dimers of free light chains (FLC) can affect their biological functions and possibly their pathogenicity. They also may exhibit differential immune reactivity, perhaps explaining discrepant quantifications when measured by different immunoreagents. Having purified FLC monomers and dimers available can be useful for studying their properties. Here we propose a simple preparatory procedure to purify FLC monomers and dimers from urine samples of patients with plasma cell disorders. Two representative urine samples containing lambda or kappa FLC were loaded into a nonreducing sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). The gel strips containing separate monomers and dimers were excised, electroeluted, and the FLC recovered. The FLC were recovered from SDS-PAGE gel in sufficient amounts to be quantified by UV and two automated nephelometric assays immunochemical. The procedure was found to be simple, reproducible, and with a high yield, thus offering the opportunity to compare different assays. Not all urine samples are suitable for this procedure, but this approach allows for the purification of FLC monomers and dimers from many selected urine samples which maintain their oligomeric organization.

Keywords: Dimers; Free light chains; Monomers; Multiple myeloma.

Fig. 1

On the left: non-reducing SDS-PAGE of urine samples. The gel was stained with Coomassie. Sample A clearly shows monoclonal FLC only. Sample B also shows other proteins, with albumin being the most represented. On the right is the corresponding densitometric scan on both samples. The peak areas corresponding to dimers and monomers were calculated for both samples

Fig. 2

For both samples A and B, lane 1 shows eluted dimers and lane 2 shows eluted monomers; MW, reference molecular weights (the same as in Fig. 1). Comassie staining is shown above; Western blot is shown below. The apparent MW is around 20 kDa for monomers and between 37 and 50 kDa for dimers