Thermodynamic Stability of a κI Immunoglobulin Light Chain: Relevance to Multiple Myeloma

Immunoglobulin light chains have two similar domains, each with a hydrophobic core surrounded by β-sheet layers, and a highly conserved disulfide bond. Differential scanning calorimetry and circular dichroism were used to study the folding and stability of MM-κI, an Ig LC of κI subtype purified from the urine of a multiple myeloma patient. The complete primary structure of MM-κI was determined by Edman sequence analysis and mass spectrometry. The protein was found to contain a cysteinyl post-translational modification at Cys214. Protein stability and conformation of MM-κI as a function of temperature or denaturant conditions at pH 7.4 and 4.8 were investigated. At pH 4.8, calorimetry demonstrated that MM-κI undergoes an incomplete, cooperative, partially reversible thermal unfolding with increased unfolding temperature and calorimetric enthalpy as compared to pH 7.4. Secondary and tertiary structural analyses provided evidence to support the presence of unfolding intermediates. Chemical denaturation resulted in more extensive protein unfolding. The stability of MM-κI was reduced and protein unfolding was irreversible at pH 4.8, thus suggesting that different pathways are utilized in thermal and chemical unfolding.