The isolation and characterization of rat sublingual mucus-glycoprotein

A purified glycoprotein, designated RSL-major, was isolated from the rat sublingual gland by means of the procedure of Tettamanti and Pigman. It was found to be homogeneous by analytical ultracentrifugation, to have a mol. wt. of 2.2 x 10 6, and to contain 81% (w/w) of carbohydrate, which consists mainly of sialic acid, 2-acetamido-2-deoxy- D-glucose, 2-acetamido-2-deoxy- D-galactose, and D-galactose in the molar ratio of 1.4:1.4:1.0:1.5; small amounts of fucose and mannose [1.2 and 2.8% (w/w), respectively] were also present. The sialic acid residues were resistant to the action of V. cholerae neuraminidase. This resistance was completely abolished by removal of the O-acetyl groups contained in the sialic acid. The sialic acid in RSL-major appeared to be a mixture of N-acetyl-4- O-acetyl- and N-acetyl-4,7(8)-di- O-acetylneuraminic acids. The carbohydrate to protein attachment of RSL-major was shown, by alkaline β-elimination reaction, to consist of an O-glycosyl linkage between 2-acetamido-2-deoxy- D-galactosyl residues in the oligosaccharide chains and seryl and threonyl residues in the protein core. The average oligosaccharide, contained in RSL-major, was postulated to be a heptasaccharide. A second material, designated RSL-minor, and also isolated from the rat sublingual gland, was obtained as a mixture of glycoprotein(s) and hydroxylapatite gel, and was not purified further.