Biochemical and immunological characterization of human opsonic alpha2SB glycoprotein: its identity with cold-insoluble globulin.

The relationship between human cold-insoluble globulin (CIg, plasma fibronectin) and the human serum opsonic alpha2SB glycoprotein was investigated using immunochemical and biochemical techniques. The two proteins appeared to have identical molecular weights by sodium dodecyl sulfate-polyacrylamide gel electrophoresis on 3.3% gels; have identical migration in the native state on 2.7 to 27% gradient polyacrylamide gels; and have a similar amino acid composition within the accuracy of analysis. Human serum demonstrates antigenic identity when diffused against monospecific antisera to both proteins confirming the presence of common antigenic sites on both molecules. Purified human serum opsonic alpha2SB glycoprotein and purified CIg also demonstrate antigenic identity when diffused against monospecific antiserum to either of the isolated proteins. Antiserum to both proteins also inhibits in vitro hepatic Kupffer cell phagocytic uptake of test particles. These results suggest the idenity of these two proteins and reveal a major physiological function for human plasma CIg. Thus, CIg may be important in the regulation of hepatic reticuloendothelial phagocytic activity and nonspecific systemic host defense. This process of systemic host defense has been shown to be depressed in patients following trauma, major surgery, burn injury, and during neoplastic disease, and, in part, mediated by a deficiency or depletion of the alpha2SB glycoprotein.