Hypercalcaemia due to calcium binding by a polymeric IgA kappa-paraprotein.

Total serum calcium concentration was raised in a 63-year-old lady with multiple myeloma and markedly elevated serum IgA kappa-paraprotein concentration. Symptoms of hypercalcaemia were absent, and serum ionized calcium was normal, suggesting calcium binding by the abnormal protein. This was demonstrated directly after isolation of the paraprotein and characterization of the calcium/protein interaction. After reduction of the paraprotein with mercaptoethanol, sodium dodecyl sulphate polyacrylamide gradient gel electrophoresis revealed two bands corresponding to light and heavy chains, but under non-reducing conditions the isolated paraprotein migrated in a series of bands, possibly representing polymeric forms of the basic immunoglobulin moiety.