A Calcium Binding IgG Myeloma Protein

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Abstract
A Ca binding Ig[immunoglobulin]G was isolated and purified by column chromatography from serum of a myeloma patient with asymptomatic hypercalcemia. The myeloma IgG, characterized as an IgG .kappa., revealed a normal-sized H chain of 56,000 daltons and a L chain of 31,000 daltons. Another population of IgG separated and purified from the same patient''s serum did not bind Ca and had a normal 26,000 dalton L chain. Ca 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. L and H chains were obtained by reacting the Ig with dithiothreitol and iodoacetamide followed by Sephadex G-100 chromatography. Ca binding activity was proved to be associated with Fab IgG fragment. Preparates containing Fc, H or L chains did not bind Ca in vitro.