Human IgA1 half-molecules: clinical and immunologic features in a patient with multiple myeloma

Abnormal IgA1 half-molecules consisting of one heavy and one light chain were found in a patient (N.N.) with typical multiple myeloma. The serum and the urine of this patient contained both 7.0S and 3.9S IgA myeloma proteins. The IgA half-molecules (3.9S) were found to have a molecular weight of 59,000 daltons and were composed of one alpha1 chain of about 40,000 daltons and one light chain of 22,000 daltons. Furthermore, enzymatic degradation suggested that the alpha chain of the N.N. half-molecules had a large deletion in its Fc portion. We suggest that its heavy and light chains were probably bound noncovalently, since the interchains connecting the heavy and light chains of these IgA half-molecules were easily dissociated with 1% SDS and 8 M urea. Cytologic studies identified at least two types of myeloma cells, and it is possible that half-molecule IgA production might result from mutation among the myeloma cells producing whole-molecule IgA.