A calcium binding IgG myeloma protein.

A calcium binding IgG was isolated and purified by column chromatography from serum of a myeloma patient with asymptomatic hypercalcaemia. The myeloma IgG, characterized as an IgG kappa, revealed a normal sized heavy chain (56 000 dalton), and a light chain of 31 000 dalton. Another population of IgG separated and purified from the same patient's serum did not bind calcium and had a normal 26 000 dalton light chain. Calcium binding activity in vitro is optimal at pH 8.0, and reaches its maximum after 3 h of 45Ca myeloma IgG incubation. Cleavage of the purified IgG by trypsin yielded peptides which were further isolated by column chromatography and characterized as Fab and Fc fragments. Light and heavy chains were obtained by reacting the immunoglobulin with dithiothreitol and iodoacetamide followed by Sephadex G-100 chromatography. Calcium binding activity was proved to be associated with Fab IgG fragment. Preparates containing Fc, heavy or light chains did not bind calcium in vitro.