Tracking reduction-induced molecular changes in pathological free light chains by SV-AUC.

Multiple myeloma is a blood cancer characterized by plasma cell proliferation and excessive production of monoclonal proteins, often leading to renal complications and other forms of organ damage. A set of nine immunoglobulin free light chain (FLC) samples purified from urine of multiple myeloma patients was subjected to sedimentation velocity analysis. Aim of the study was to track changes of the oligomerization state of each FLC while triggering reduction-induced aggregation into larger structures. Sedimentation velocity experiments, combined with further techniques sensitive to structural changes, were performed to determine the degree of FLC oligomerization in each patient sample under different experimental conditions. Structurally, the FLC monomers are stabilized by two intramolecular disulfide bonds, while covalent dimerization occurs through an unpaired C-terminal cysteine residue. Incubation with the reducing agent TCEP cleaves intra- and intermolecular disulfide bonds, destabilizing both monomers and dimers. Remarkably, different incubation times revealed that destabilized dimers do not dissociate into stable monomers but instead accumulate directly into oligomers and higher-order aggregates. In addition to larger aggregates, fragments with sizes around 1 S were detected with increasing TCEP incubation time. This fragmentation behavior was consistent among FLCs originating from the immunoglobulin kappa variable 1-33 gene (IGKV1-33). Sedimentation velocity-based characterization of FLCs can provide insights into the relationship between their stability and aggregation capacity. An understanding of this relationship is crucial for the development of therapeutic strategies to prevent renal complications associated with monoclonal gammopathies such as multiple myeloma.