Macroglobulins

The majority of plasma globulins have a molecular weight in the range of 150.000 to 200.000. A small proportion, less than 4 to 5 per cent of the total serum proteins, are very large size molecules, with molecular weights in the neighborhood of 1,000,000. These large proteins are referred to as macroglobulins. If plasma is centrifuged at ultrahigh speeds, three classes of proteins can be differentiated as a result of differences in the rate of sedimentation (see the section on ultracentrifuge separations).

The macroglobulins constitute the heaviest, S19. class, and they include 50 to 70 per cent of the alpha-2 globulins and 10 to 30 per cent of the gamma globulins. The normal level present in plasma is about 0.20 gm./100 ml., ranging from 0.07 to 0.43 gm./100 ml. In a very small number of individuals, considerably higher levels may be found. In primary macroglobulinemia, first studied in detail by Waldenstrom, the level of S19 proteins is well over 15 per cent of the total serum proteins. Macroglobulinemia is a metabolic disease, characterized by weight loss, and susceptibility to infection, and involves the entire reticuloendothelial system. The  serum electrophoretic pattern is similar to that found in multiple myeloma: Bence-Jones proteins are occasionally found, and some of the macroglobulins behave like cryoglobulins, with resultant impaired blood flow in the extremities. Secondary macroglobulinemia is more common, and is associated with a large number of different disease entities. The level of macroglobulins is always under 15 per cent. Secondary macroglobulinemia is found in patients with leukemia, lymphosarcoma, and rheumatoid arthritis. In myelomatosis, the concentration of S19 proteins is usually normal, or only slightly elevated.

Macroglobulinemia is best diagnosed by ultracentrifugation of a serum specimen. The peak demonstrating the presence of the high molecular weight IgM globulins shows up quite clearly. Isolation of the macroglobulins can also be achieved by gel filtration on Sephadex-200, using either column or thin-layer chromatography. Immunoelectrophoretic patterns can provide supportive evidence, the increased level of macroglobulins being demonstrated by the presence of heavy precipitant arcs with anti-IgM antisera.

 The Sia test is a simple qualitative test for macroglobulins based on the fact that the IgM globulins have decreased solubility in water that contains a low concentration of salts. In one procedure, 0.10 ml. of serum is added slowly down the sides of a test tube into a solution of 5.0 ml. of 0.01 M phosphate buffer, pH 7.1. If macroglobulins are present at levels over 0.7 gm./100 ml. a flaky, birefringent precipitate is formed, which dissolves on the addition of a spatula tip of NaCl. Normal, myeloma, and arthritis sera give no precipitation, and lipemic sera only a hazy opalescence. Malaria and kala-azar sera produce a positive result.