Mucus glycoprotein of human saliva: differences in the associated and covalently bound lipids with caries

A high molecular weigh mucus glycoprotein has been isolated from submandibular saliva of caries-resistant and caries-susceptible individual by a procedure involving fractionation on Bio-Gel P-100 and A-50 columns followed by equilibrium density-gradient centrifugation in CsCl. The purified caries-resistant mucus glycoprotein displayed a buoyant density of 1.50 and accounted for 9.5% of the dry weight of caries-resistant saliva. The caries-susceptible mucus glycoprotein representd 14.1% of the dry weight of caries-susceptible saliva and gave a buoyant density of 1.43. Both glycoproteins exhibited similar protein and carbohydrate content, but the caries-resistant mucus glycoprotein contained 28.7% less associated lipids and 3-times less covalently bound fatty acids than the caries-susceptible mucus glycoprotein. The associated lipids were represented by neutral lipids, glycolipids and phospholipids, whereas the covalently bound fatty acids consisted mainly of hexadecanoate, octadecanoate and docosanoate. Extraction of associated lipids caused the caries-resistant glycoprotein to band in CsCl gradient at the density of 1.54 and caused the caries-susceptible glycoprotein to band at the density of 1.52. A further shift in the buoyant densities occurred following removal of the covalently bound fatty acids, and both glycoproteins banded at the density of 1.57. While the intact caries-resistant and caries-susceptibel glycoproteins were susceptible to proteolysis by pronase, the lipid-rich caries-susceptible glycoprotein was degraded to a lesser extent. Extraction of associated lipids increased the degradation of both glycoproteins, but the caries-susceptible glycoprotein still remained 25% less susceptible. However, the susceptibility to pronase of the delipidated and deacylated caries-resistant and caries-susceptible glycoproteins was essentially identical. The caries-resistant and caries-susceptible mucus glycoproteins also differed in susceptibility to peptic degradation. The apparent K_m values for intact caries-resistant and caries-susceptible glycoproteins were 10.5 · 10⁻⁷ M and 8.1 · 10⁻⁷ M, while the values for the delipidated and deacylated caries-resistant and caries-susceptible glycoproteins were 13.0 · 10⁻⁷ M and 12.4 · 10⁻⁷ M. The results suggest that the differences in the content of associated lipids and covalently bound fatty acids are responsible for the different physicochemical characteristics of caries-resistant and caries-susceptible salivary mucus glycoproteins, which may be determining falctors in the resistance to caries.