Identification of mucus glycoprotein fatty acyltransferase activity in human gastric mucosa

Co-translational processing and intracellular transport of rat salivary mucus glycoprotein

A preparation of peptidyl-tRNA from intact microsomes of mucin-synthesizing polysomes of sublingual salivary gland cells contained fatty-acylated galactosamine-free and galactosamine-enriched peptidyl-tRNA fractions, whereas trypsin-chymotrypsin treated microsomes yielded predominantly the acylated galactosamine-enriched peptidyl-tRNA complexes. Radioscanning and chemical analyses revealed that palmitate was substituted on all nascent peptides, except those shorter than 20 amino-acid residues. In contrast, the [35S]-methionine label was detected only on galactosamine-free peptides containing up to 70 amino acids. On SDS-polyacrylamide gel, the peptides released from galactosamine-enriched tRNA complexes separated into a multitude of bands ranging in size from 6000 to 60,000 dalton, whereas the total preparation afforded peptides ranging from 2000 to 60,000 dalton. Pulse-chase experiments, using radiolabelled methionine, palmitic acid and N-acetylgalactosamine, combined with chemical characterization of the radiolabelled fatty acids and carbohydrates from purified peptidyl-tRNA, confirmed that the N-terminal fatty acylation and the initial O-glycosylation with N-acetylgalactosamine are the co-translational processes taking place as soon as peptide is sufficiently large to be acylated, trimmed, and translocated to the luminal site of endoplasmic membrane.

Pathogenesis of peptic ulcer disease and gastritis: importance of aggressive and cytoprotective factors

Acid production is a major gastric function. Second messengers (cyclic AMP and calcium) are released when parietal cell membrane receptors (H2, muscarinic and gastrin) are stimulated. The second messengers then stimulate the 'gastric proton pump' to produce hydrogen ions. New evidence suggests that there is a unidirectional flux of hydrogen ions into the lumen induced by unique physical properties of mucus and a sodium gradient from lumen to serosa. Luminal hydrogen ions, bile salts, ingested drugs, and ingested alcohol are potential gastric epithelial toxins. The stomach's protective mechanism includes a well-defined mucus layer, an epithelial bicarbonate secretion, a tight epithelium, and a good nutrient blood supply. Endogenous prostaglandins partly control these mechanisms. Modern therapeutics are increasingly directed towards improving gastric cytoprotection.

Mucus glycoprotein fatty acyltransferase in patients with cystic fibrosis: effect on the glycoprotein viscosity

The presence of an acyltransferase activity which catalyzes the transfer of palmitic acid from palmitoyl coenzyme A to mucus glycoprotein has been demonstrated in the microsomal fraction of human rectal mucosa. The activity of this enzyme in the mucosa of patients with cystic fibrosis (CF) was found to be 3.5 times higher than that from normal individuals. The CF mucus glycoprotein in comparison to that of normal contained 1.3 times more associated lipids and 6 times more covalently bound fatty acids. The viscosity of the intact CF glycoprotein was 1.8 times higher than that of normal glycoprotein. Extraction of associated lipids led to 3-fold drop in the viscosity of CF glycoprotein and 5-fold drop in the case of normal glycoprotein. Further loss in the viscosity occurred following removal of the covalently bound fatty acids. The viscosity of such modified CF mucus glycoprotein was only about 10% higher than that of similarly treated normal glycoprotein.