Enzymic acylation of mucus glycoprotein with palmitic acid in rat submandibular salivary gland

The enzymic activity which catalyses transfer of palmitic acid from palmitoyl coenzyme A to mucus glycoprotein was found in Triton X-100 extracts of the microsomal fraction of rat submandibular and sublingual salivary glands. The acyltransferase activity of this fraction was 1.3-1.4 times greater in submandibular gland than in sublingual gland. Further subcellular fractionation of submandibular gland showed that the enzyme activity was associated with a Golgi-rich membrane fraction. Optimum enyzme activity for fatty acylation of mucus glycoprotein was at pH 7.4 using 0.5 per cent Triton X-100, 2mM dithiothreitol 25 mM NaF and 10 mM MgCl₂; higher concentrations were inhibitory. The apparent K_m of the submandibular microsomal enzyme for mucus glycoproein was 5.9 × 10^-7 M, and for palmitoyl-CoA, 3.3 × 10^-5 M. The ¹⁴C-labelled glycoprotein product of the reaction co-migrated on CsCl equilibrium, density-gradient centrifugation with submandibular mucus glycoprotein, and contained ester-bound palmitic acid. The fatty acyltransferase showed no activity with proteolytically-degraded glycoprotein; the acceptor capacity of reduced and S-carboxymethylated glycoprotein was only about 10 per cent lower than that of the intact mucus glycoprotein. This suggests that the acylation of salivary mucus glycoprotein with fatty acids occurs at its non-glycosylated, proteolysis-susceptible regions, and that the majority of these fatty acids are linked to the glycoprotein through hydroxyl esters.