A Sialic Acid-free β1-Glycoprotein of Normal Human Plasma

Abstract

Abstract A hitherto unknown β1-glycoprotein was isolated from a subfraction of Cohn Fraction VI of pooled normal human plasma. For the isolation and purification of this very soluble globulin, precipitation with zinc ions at pH 5.8 in the presence of ethanol, chromatography on diethylaminoethyl cellulose at pH 5.0, and gel filtration through Sephadex G-100 were employed. The resulting preparation appeared monodisperse on ultracentrifugation and paper, starch gel, and disc electrophoresis at pH 8.6. The major physichemical and chemical properties of this β1-glycoprotein were determined. The molecular weight was 31,000. The isoelectric and isoionic points of this globulin were found to be at pH 4.4 and 4.9, respectively. The conformations which this protein assumed in pH 5.8 sodium acetate buffer, 50% aqueous 2-chloroethanol, and 7 m urea are completely reversible as judged by optical rotatory dispersion measurements. This β1-glycoprotein contains 30% carbohydrate, consisting of galactose, mannose, fucose, and glucosamine. Most striking is the lack of sialic acid. Its carboxyl-terminal amino acid proved to be asparagine, while the amino-terminal amino acid did not react with Sanger's reagent. Although monodisperse by a series of criteria, this plasma protein resolves into three components on starch gel electrophoresis near its isoelectric point, an observation that was interpreted as being indicative of variants. The apparent content of this glycoprotein in pooled plasma is approximately 2 mg per liter.