Abnormal bilirubin binding to human serum albumin in a patient with unusual myeloma immunoglobulin G.

The binding of bilirubin was analysed in the serum of a patient with unusual myeloma. This patient presented an acquired Von Willebrand's disease due to the presence of a ternary complex formed by one molecule of myeloma immunoglobulin G and two molecules of albumin (Harlé et al. La Presse Médicale 19, 1661-1664 (1990)). The binding characteristics of bilirubin--number of sites (n) and association constants (k)--were estimated in the serum of the proband, and compared with those of purified human serum albumin and pooled normal human serum. The data suggest that complex formation between albumin and immunoglobulin induces abnormal bilirubin binding in the patient's serum, which becomes apparent when the bilirubin/albumin ratio exceeds 1.3. This abnormal binding may be due to conformational changes of the albumin molecule in the complex. The greatly increased number of lower affinity binding sites for bilirubin is certainly due to the abnormally high concentration of myeloma immunoglobulin G in the serum of the patient.