Partial characterization of five glycoprotein fractions secreted by the human parotid glands

Abstract

Isoelectric focusing of human parotid saliva in gradient pH 7–10 gave a major glycoprotein fraction at pH above 10 (p I > 10 fraction), a minor p I 9.5 fraction, and low amounts of glycoproteins isoelectric around pH 9 (p I 9 fraction). Subsequent gel filtration (Bio-Gel P-100) separated the p I > 10 glycoproteins into three subfractions (I, II, III), whereas the p I 9 and 9.5 fractions appeared as excluded peaks. The three p I > 10 subfractions and the p I 9.5 fraction were subjected to equilibrium centrifugation in the ultracentrifuge. The p I 9.5 fraction was heterogeneous (mol. wt 30,000–70,000), whereas the three p I > 10 subfractions appeared more homogeneous. Their molecular weights were 18,000, 11,500 and less than 10,000 for I, II and III, respectively. The carbohydrate content decreased by increasing p I of the glycoproteins, and ranged from above 50 to below 5 per cent of the total glycoprotein weight. The main monosaccharides in the fractions were identified as glucosamine, glucose, galactose, mannose and fucose. Amino acid analyses showed that, in all glycoprotein fractions, proline, glycine and glutamic acid (at a ratio 2:1:1) accounted for 70–80 per cent of the total residues, and the basic residues (lysine and arginine) for a further 10–12 per cent. The fraction of p I > 10 had a higher lysine/ arginine ratio than the fractions of lower pI. The similarity in amino acid composition indicates that all these glycoproteins are related.