Isolation and characterization of a serine-threonine-rich galactoglycoprotein from normal human plasma.

A serine-threonine-rich galactoglycoprotein was isolated in homogeneous form from pooled normal human plasma. After precipitation of the major portion of the plasma proteins by Cohn's low temperature-low salt-ethanol procedure, the proteins and glycoproteins remaining in the supernatant solution of Cohn Fraction V were concentrated with the aid of DEAE-cellulose. Fractional elution from this ion exchange resin yielded a crude preparation of the galactoglycoprotein which was purified subsequently by gel filtration through Sephadex G-100. Homogeneity of the protein was established by several criteria of purity including disc polyacrylamide gel electrophoresis and COOH-terminal amino acid analysis. The major physicochemical constants of the macromolecule including its molecular weight of 81,000 were measured. The chemical composition of this protein was found to be unusual: the total carbohydrate moiety accounts for 76% of its weight and consists of 23% sialic acid, 23% total neutral sugar, including 20% galactose, 1% fucose, and almost equal amounts of GalNAc and GlcNAc totalling 27%. The polypeptide moiety accounting for 26% of the weight of the glycoprotein distinguishes itself by a high content of serine and threonine.