Biological process of carbohydrate attachment to ovalbumin

Dithiothreitol or glutathione, essential for amino acid incorporation into polypeptides in cell-free systems, inhibited glucosamine incorporation into ovalbumin, a glycoprotein containing about three residues of N-acetylucosamine and about five residues of mannose. However, the thiol was necessary for mannose incorporation into ovalbumin in cell-free system prepared from hen's oviducts. The two monosaccharides were incorporated into soluble ovalbumin (intracellular ovalbumin released from the polysomes, corresponding to ovalbumin A3 which contains no phosphate and probably no carbohydrate) present in cell sap which was prepared from homogenates of minced oviducts by ultracentrifugation. It was difficult to study the incorporation of the carbohydrates into ovalbumin in connection with synthesis of the protein using the crude cell-free system. The incorporation of glucosamine and mannose into various forms of ovalbumin was therefore studied using minced oviducts. It was concluded from the incorporation of the two radioactive monsaccharides and of amino acids into the extracellular, soluble and nascent protein fractions that all the carbohydrate moieties were not incorporated into the growing polypeptides bound to polysomes, but were incorporated into ovalbumin molecules released from polysomes after synthesis had been completed.