The kinetics of synthesis and secretion of immunoglobulin by bone marrow cells from 32 patients with multiple myeloma (17 IgG, 8 IgA, 7 Bence Jones only) and two patients with benign monoclonal gammopathy have been compared. Optimal conditions have been defined for quantitatively measuring the incorporation of radioactive leucine into intracellular and secreted protein and immunoglobulin.
In cells continuously labeled with [3H]-leucine the rate of protein synthesis in the intracellular lysate was constant over the 24-hr-culture period. The amount of labeled intracellular immunoglobulin increased linearly during the initial 2 to 4 hr and then remained constant, indicating saturation of the intracellular pool of immunoglobulin. Labeled protein and immunoglobulin were secreted linearly after an initial lag phase of 2 to 8 hr. The major part of the secreted protein was identified as the myeloma protein and was proportional to the percentage of myeloma cells in culture; this was particularly the case for IgG and Bence Jones myeloma patients. By linear regression analysis the amount of myeloma protein secreted by the cells was directly proportional to the amount synthesized within the cell and in addition correlated with the synthesis and secretion of total protein. These neoplastic plasma cells synthesize and secrete myeloma protein as a major product and appear to be comparable to normal antibody-producing plasma cells.