Electrophoretic isolation and partial characterization of a major secretory glycoprotein from the submandibular glands of the mouse.
After either cholinergic or adrenergic stimulation of the submandibular glands of the mouse, a major protein of the incubation medium could be isolated by electrophoresis, designated the AM2 protein. About 5 per cent of the secreted proteins and 2.4 per cent of the secreted protein-bound sialic acid was recovered as the purified AM2 protein. The AM2 protein appeared to be electrophoretically pure in 7.5% polyacrylamide gel both at pH 8.9 and at pH 4.3. In sodium dodecyl sulfate-electrophoresis the molecular weight was estimated to be about 80 000 for the major component and about 40 000 for the minor component. By isoelectric focusing the isoelectric point has been determined to be 4.7. The amino acid analysis indicated Glx, Asx, Leu and Ala as the major amino acids, comprising 15.0, 10.6, 9.2 and 9.1 per cent of the amino acid residues, respectively. The ratio of the acidic amino acids and their amides (Glx plus Asx) to the basic amino acids (Lys plus Arg) was 2.2. The sugar analysis showed that the AM2 glycoprotein consists of 17.3 per cent of carbohydrate, with as major carbohydrate component glucosamine. The molar ratio of the sugars was Man : Gal : Glc : GlcNH2 : sialic acid = 2.3 : 1.0 : 4.7 : 9.8 : 2.9. Galactosamine could be detected as a trace component and fucose was not detectable.