Binding of mammalian beta 2-microglobulin by glycoproteins in fish serum

The results demonstrate the presence in cod serum of beta 2-microglobulin (beta 2m)-binding molecules. Upon fractionation on Sephadex G-200, the bound beta 2m appears mainly in the void volume, but also as a minor peak with the apparent size of albumin. The complexes show affinity to Con A-Sepharose and Lens culinaris lectin-Sepharose, respectively, indicating that they contain glycoproteins. Because of the high molecular weight of the beta 2m-containing complexes they can be separated from unbound beta 2m by polyethyleneglycol (PEG-6000) precipitation, thus allowing rapid analysis. These beta 2m-binding molecules exhibited size- and charge-homogeneity when separated by gel filtration (Sepharose 4B) and by ion-exchange chromatography (DEAE-cellulose), respectively. The binding is temperature-dependent. At 37 degrees C, maximum binding is reached after about 2 hr. The dissociation is considerably slower, complete dissociation taking about 2 days. According to Scatchard analysis, the association constant is of the order 2 X 10(9)/M. The binding is sensitive to denaturating agents and high salt concentrations. Optimum binding is seen at neutral pH and the beta 2m-binding activity is heat-labile at 50 degrees C. The binding of heterologous beta 2m by cod serum can be used as a cross-reactive assay specific for the detection of beta 2m. Whereas unlabelled human, guinea-pig, and rat beta 2m all give similar inhibition, higher concentrations of rabbit beta 2m are needed for the same degree of inhibition.