Purification and characterization of the two forms of human plasma α2HS-glycoprotein

The two forms of $\text{α}{}_2\text{HS-glycoprotein}$ were purified from Cohn fraction VI of normal human plasma and characterized in terms of their major chemical and physicochemical properties. Separation of these two proteins was achieved by chromatography on DEAE-cellulose at pH 4.4 followed by gel filtration through Sephadex G-100. The isoelectric points of the disc gel electrophoretically and immunochemically homogeneous glycoproteins were found to be at 4.1 and 4.7 and their apparent molecular weights, as determined by SDS-polyacrylamide gel electrophoresis, were shown to be 51 000 and 56 000, respectively. The amino acid compositions of both proteins were very similar, although differences, particularly in the arginine and histidine contents, were noted. The amino- and carboxyl-terminal amino acids were found to be the same for both proteins and were threonine and alanine, and valine and leucine, respectively, suggesting that both forms of this protein consist of two polypeptide chains. The total carbohydrate moiety of the relatively basic form (14%) proved to be comparable to that of the relatively acidic form (13%). More important, however, the sialic acid content of the latter was higher than that of the former. These results suggest that the difference between the two forms of $\text{α}{}_2\text{HS-glycoprotein}$ resides both in its carbohydrate and polypeptide moieties.